RADIOACTIVITY

among points of special interest that have arisen since 1910 may be mentioned the prep- aration of metallic radium by mme. curie and debierne by electrolysis of a radium salt with a mercury cathode. radium resembles metallic barium, melts at about 700°c, and is rapidly attacked when exposed to the air. radium.—the atomic weight of radium (q.v.) was found by mme. curie to be 226-45, using for the purpose about o-4 gr. of pure radium chloride. a careful redetermination by henig- schmid with about one gr. of radium gave a value 225-9, and is probably correct to 1 in 1,000. an international radium standard consisting of about 22 milligrammes of pure radium chloride has been prepared by mme. curie (q.v.), and is preserved in the bureau international des poids et mesures at sevres, near paris. sec- ondary radium standards have been issued to all governments who wished to purchase them. these have been calibrated by y-ray methods both in vienna and paris, and are supposed to be correct within 1 in 200. the purchase and sale of radium have generally been conducted on certificates given in terms of this international standard. the wide use of radium for therapeutic purposes, and its high cost—from {15 to £30 per milligramme element—have led to close search for uranium deposits. the amount of radium in an old mineral is always proportional to its content of uranium in the ratio of 3-3 parts of radium by weight to 10,000,000 parts of uranium. consequently, an old mineral containing 1,000 kgm. of uranium should contain 330 milligrammes of pure radium. initially several grammes of radium were separated from the urani- nite deposits in joachimsthal, bohemia, and some of the material, which was the property of the austrian govt., was generously loaned to representative workers in radioactivity in england. a part of this radium is in the charge of the radium institute of vienna, which is specially devoted to radioactive investigations. the increasing demand for radium led to the working of low- radiation—radioactivity grade ores, containing on an average only about 2% of uranium oxide, of which there are extensive deposits in colorado and other parts of the united states. until 1922 the greater part of the world supply of radium was produced in america, but in that year very rich deposits of uranium ore, containing about 20 times as much radium as the american ore, were found in the belgian congo, and the american production practically ceased. large quantities of radium were employed by the allies during the world war for night compasses, gun-sights, etc. the radium is mixed with phosphorescent zinc sulphide to form a paint which becomes continuously luminous but, owing to the destruc- tion of the zinc sulphide by the rays, this luminosity gradually decays. radium emanation.—the atomic weight of the radium ema- nation is now known to be 226-4= 222, as was inferred earlier. this was confirmed by direct weighing with microbalance by ramsay and gray. the radium emanation has proved of great service not only in radioactive researches but also in therapeutic work. the radium salt is dissolved in an acid solution and the emanation is pumped off with the large quantity of hydrogen and oxygen liberated by the action of the radiations on water. after sparking the mixture, the emanation can be purified by condensation with liquid air. a very intense source of 8 and y radiation can be obtained by introducing the purified emanation into fine capillary tubes. such emanation needles have been widely used for therapeutic purposes, while the use of very thin-walled tubes provides a powerful line source of a- rays. the 6 and ¥ activity of such tubes rises to a maximum aboyt four hours after introduction of the emanation, and then decays with the period of the emanation, viz., 3:85 days. the quantity of emanation liberated from one gramme of radium is called a curte and from one milligramme a miflicurte. the quantity of radium emanation in a tube can be accurately determined by comparison of its y-ray activity with that of a radium standard, since the pene- trating y-rays, both from the radium and the emanation in equi- librium, arise mainly from the same product radium c. as regards other radioactive substances, large quantities of mesothorium have been obtained as a by-product in the separation of thorium from monazite sands. this substance, which is half transformed in about 6-7 years, emits only 8-rays, but gives rise to radiothorium and subsequent products which emit a-rays and pene- trating 8-and y-rays. asa source of powerful 6 and y radiation, this substance is very analogous to radium and can be obtained in about the same concentration. since radium and mesothorium are isotopic clements, they are always scparated together. most commercial sources of thorium contain also uranium and radium, and con- sequently radium is always separated with the mesothorium and in relative amount depending upon the proportion of uranium to thorium in the mineral. since mesothorium has a radioactive life short compared with radium, it commands a smaller price. the amount of mesothorium is standardised by comparison of its y-ray effect with a radium standard. mme. curie separated the polonium from several tons of pitch- blende and obtained an exceedingly active preparation of a few milligrammes, but was unable to obtain it in a pure state, although several of its spectrum lines were detected. it was hoped by this experiment to decide whether polonium was transformed directly into lead, but this was found difficult to establish owing to the presence of impurities with the very small quantity of polonium. the three ty pes of radiation, known as the a-, 8-, y-rays, emitted by radioactive substances are analogous in many respects to the types of radiation observed when a discharge passes through a vacuum tube, but are of much more penetrating character. it may be noted here that for the electrons in a vacuum tube to attain the velocity of the swift 8-rays from radium, a potential difference of at least 2,000,000 volts would have to be applied. the very penetrating y-rays are identical in all respects with x-rays of very short wave-length. in- tense y-rays are only observed in substances which emit swift 8 particles, and apparently owe their origin to the rearrangement of the nucleus after the emission of the 8 particle. to produce x-rays as penctrating as the y-rays, about 2,000,000 volts would have to be put on the discharge tube. : the a-rays, shown in 1903 by rutherford to consist of a stream of positively charged atoms projected with high velocity, are now known to consist of charged atoms of helium which are projected with velocities of about 10,000 m. per second. while the majority of products break up with the expulsion either of an a particle or 2 swift 8 particle, in a few cases no detectable radiation was observed. such products were at first called ‘‘ rayless’’ products, but the sequence of chemical properties, discussed later, shows that a 8 particle must be liberated but at too low a speed to detect with cer- tainty. actinium is now the only example of such a product. a number of new products have been discovered, particularly in the uranium and actinium series. the results are included in the table of radioactive elements which will be found on the following page. radioactivity table of radioactive elements range atom-| atom- cm. a- element ic ic t rays | rays wt. | no. (15° and 760 mim.) uranium-radi- um series uranium i, 238-18 2 | 4:5 x10° yr. a 2-50 uranium x, 234 90 | 23-8 days bly 8 uranium x2 234 qi i-t5 min. b, ¥ : uranium z 234 gi 6-7 hr. b uranium ii. 234 92 | about 2 x10® yr. a. 2-90 uranium y 230 go | 24-6 hr. b , (3 %) tonium 230 go | aboutgox1otyr., a 3-07 radium 226 88 [,700 yr. a 3°52 radium emanation 222 86 | 3°85 days a. 4:16 radium a 218 84 | 3-05 min. a a‘75 radium 3 . [214 82 | 26-8 min. 8, ; radium c . [214 83 19-5 min. a, 8, y| 6-94 radium d . [210 82 16 yr. b, ¥ radium e 210 83 4°85 days b, ¥ : radium f . ero 84 136-5 days a 3°83 (polonium) radium g (end- product ura- nium-lead) 206 82 thorium series. thorium . ; \2a2-t || 90° | 2:2 x10 4517 a 2:72 mesothorium i, | 228 88 6°7 yr b, y - mesothorium 2 | 228 89 | 6-2 hr. b, ¥ ae radtethorium 228 go igo yr. a. 3:87 thorium x 224 88 | 3:64 days a 4°30 thorium emanation 220 86 54 sec. a 5:00 thorium a 216 84 o-i4 sec a 5°70 thorium b 212 82 10:6 hr. b, ¥ - thorium c 212 83 | 60 min. a ‘ree 8-60 thorium cc’. | 208 81 3-2 min. b, y i thorium d (end- product tho- rium-lead) . | 208 82 actinium series. protoactintum . | 230 qi about 10% yr. a. 3°31 actinium . . | 226 89 20-vr. b eo radioactinium. | 226 go 19 days a 4°6 actinium x 222 88 11-2 days a, 4°26 actinium emanation 218 86 | 3:9 sec. a. 5:6 actinium a 214 84 ‘o02 sec. a 6:3 actinium b 210 82 | 36 min. ne d - actinium c 210 83. | 2-16 min. a 5-15 actinium c” . | 206 81 4°76 min. b, y actinium d (end- product actini- um-lead) 206 82 in the table t is the time-period of a product or the time re- quired for the product to be half-transformed. it will be seen that the value of t, which is a measure of the relative stability of atoms, varies between 2-210! years (thorium) and -oo2 second (actinium a). the atomic weights and atomic numbers of uranium, radium, uranium-lead, thorium, thorium-lead have been directly determined. the atomic weights and atomic num- bers of the others are deduced on the assumption that the expul- sion of an a particle (helium atom) of charge 2 and mass 4 lowers the atomic number of the succeeding element by two units and the atomic weight by four. the expulsion of a £ particle raises the atomic number by one unit, but it is not supposed to influence the atomic weight to a detectable degree. branch products.—in the great majority of cases each of the radioactive elements breaks up in a definite way, giving rise to one a or § particle and to one atom of the new product. un- doubted evidence, however, has been obtained that in a few cases the atoms break up in two or more distinct ways, giving rise to two or more products characterised by different radioactive properties. a branching of the uranium series was early de- manded in order to account for the origin of actinium. while the latter is always found in uranium minerals in constant proportion 277 with the uranium, boltwood showed that the activity of the actinium with its whole series of a-ray products in a uranium mineral was much less than that given by a single a-ray product of the main radium series. the head of the uranium series is be- lieved to be uranium y, the branch product of the uranium series first observed by antonoff. the branching is supposed to occur in the product uranium n2, 3% going into the actinium branch and the other 97° into the main uranium series. the atomic weights of actinium given in the table are calculated on this basis. the more recently discovered product protoactinitum is the hitherto missing link between uranium y and actinium. the most striking cases of branching occur in the “ c ” prod- ucts of radium, thorium and actinium, each of which breaks up in two or more distinct ways. in the case of radium c, a new substance called radium c” was obtained by recoil from a nickel plate coated with radium c. this product emitted only g-rays and had a period of 1-4 minutes. fajans estimated that the amount of the product was only xo of that of radium c. to account for these results the following scheme of transforma- tion has been proposed :— radium c* b i-4 min. o- ge radium c ° | 19-5 min. pv 6 ‘. 22 o : radium c’ a : : ——— radium d — etc. 107° sec. end where in the main branch a 8 particle is first expelled, giving rise to radium c’, which emits an a particle. the reverse process is assumed to take place in the other branch. radium c’, which emits a swift a particle, has an exceedingly short period of trans- formation, which has been measured approximately by jacobsen, and found to be about ro seconds. it is uncertain whether the radium c” branch ends alter the expulsion of a 8 particle. the resulting product is an isotope of lead like radium d in the main branch, in the case of thorium c, two sets of a particles are observed, one-third of the total number having a range of 4-8 cm. and the remainder 8-6 cm. here, as in radium c, the main series goes by a §-ray change to the c’ product. in actinium c, which also shows a dual transformation, marsden found that 99-84% of the c atom change with the emission of an a particle of 5-15 cm. range to the c” product, the remaining 0-16 % going to c’, which gives a particles of 6-4 cm. range. rutherford and wood found that the thorium c group emitted another set of @ particles of the great range of 11-3 cm., but of very small amount, about an of the total number. this result suggests that thorium c is even more complex than has been supposed. evidence has been obtained of similar long range a particles from radium c and actinium c. it is quite possible that a close examination of radioactive substances may reveal other examples of such complex methods of transformation, for, after the violent ex- plosion that occurs during the breaking-up of an atom, more than one state of temporary equilibrium may be possible for the resid- ual atom. relation between range of a-rays and period of trassforma- tion.—we have seen that each a-ray product emits a particles of characteristic velocity which have a definite range m air. it was early observed that there appeared to be a connection be- tween the period of transformation of a product and the velocity of the a particles emitted. the shorter the period of transforma- tion, the swifter is the velocity of expulsion of the a particle. this relation was brought out clearly by the measurements of geiger and nuttall, where it was shown that if the logarithm of the range was plotted against the logarithm of a, the constant of transformation, all the points lay nearly on a straight line. a similar result has been observed for the thorium and actini- um products. this relation, when carefully tested by geiger appears to be only a rough approximation. it is still of great interest as indicating a possible relation between the stability 273 of the radioactive nucleus and the velocity of the expelled helium atom. this relation has proved very useful in forming estimates of the period of transformation of ionium and other substances before the results of more direct determinations were available. from this relation also the change which gives rise to the swift a particle of radium c was believed to be exceedingly rapid. this has been confirmed by the detection of radium c’ and meas- urement ofits period. a similar conclusion is drawn for the prod- uct emitting the very swift a particles from thorium c. chemistry of the radioclements (see chemistry).—apart from uranium and thorium, and a few special cases like radium and polonium, the radioactive products of short life exist in too small quantity to examine by the ordinary chemical methods; but by the use of the radioactive method of analysis, it was possible to form some idea of their chemical behaviour. certain very interesting points soon came to light. soddy found that the two elements, radium and mesothorium, although quite dissimilar in radioactive properties, were chemically so identical that it was impossible by chemical methods to separate one from the other. other cases of this kind had long been suspected, viz., thorium and radiothorium, thorium and ionium and radium d and lead. he named such inseparable elements isotopes, since they appeared to occupy the same place in the periodic classification of the elements. (see isotrores.) following the chemical study of the radioelements by soddy, fleck and von hevesy, an important generalisation connecting the chemical properties of the radioelements was announced independently in 1913 by russell, fajans and soddy. after the expulsion of an a particle from a radioactive substance, the re- sulting product shifts two places in the direction of diminishing mass when the clements are arranged in families according to the mendeleeff classification. _ the expulsion of a 8 particle causes a shift of one place in the opposite direction. for example, by the loss of an a particle from ionium of group iv.,the resulting product, radium, belongs to group ii., while the loss of another particle gives rise to the emanation which occupies the group o, and soon. by this meth- od the chemical properties of all the known radioelements can be predicted from a knowledge of the radiations emitted from the products. this generalisation can be viewed from another im- portant standpoint. from the work of moseley, the properties of an element are defined by the atomic number which is be- lieved to represent the resultant positive charge on the nucleus. the loss of an a particle of mass 4, carrying two positive charges, lowers the atomic number by two units, while the emission of a b particle raises it by one unit. on looking through the table of the racdioelements on preceding page it will be seen that many of them can be grouped under the same atomic number. these represent the raclioactive isotopes of which some of the more important are given below, preceded by the atomic numbers:— 81. thallium (204), thorium c’’ (208), actinium c” (206). ' 82. lead (207), uranium-lead (206), thorium-lead (208), radium d (210), thorium b (212), radium b (214), actinium b (210). 83. bismuth (208), radium e (210). i 4. ‘gat (210), thorium a (216), radium a (218), actinium 21 86. radin emanation (222), thorium emanation (220), actinium emanation (218). 88. radium (226), thorium x ium a (230), 90. thorium (232), radiothorium (228), ionium (230), uranium x (234), uranium y (230), radioactinium (226). < (224), mesothorium ij (228), actin- it will be seen that many of the radioactive elements are iso- topic with known chemical elements. these radioactive isotopes differ not only in atomic weight but also in radioactive proper- ties. the isotopes of lead are of special interest as they include the end-products of the uranium, thorium and actinium series— a question that will be discussed more fully later. it is of interest to note that polonium, actinium and protoactinium are new types of chemical elements which have no counterpart among the ordinary inactive elements. i transformation of uranium.—lin 1900 the late sir w. crookes found that the 6-ray activity of ordinary uranium could be re- radioactivity moved by a single chemical operation and concentrated in an active residue. this is due to the separation of the product ura- nium x, of period 24 days, which emits b-and y-rays. a complete analysis of the transformations of uranium has been a matter of much difficulty. boltwood showed that the a-ray activity of uranium was about twice as great as that of a corresponding a-ray product in the uranium-radium series, indicating that uranium contained two successive a-ray products. this was confirmed by geiger, who showed that the a-rays from uranium consisted of two groups with ranges 2-5 and 2-9 cm. respectively. these two a-ray substances, called uranium j. and uranium ii. are isotopic, atomic weights 238 and 234 respectively. the latter, whose period is estimated at about 2,0c0,cco years, exists in relatively very small quantity compared with uranium i. fol- lowing the generalisation connecting the radiations and chemical properties of the series of radioclements, fajans predicted the presence of a new product with properties analogous to tantalum, and promptly succeeded in isolating it experimentally. the new product uranium xn» sometimes called brevium, has a period of i‘1§ minutes and emits swift b-rays. the scrics of changes is thus:— uri-ur x-uur x--ur i-ionium. we have seen that antonoff discovered another 6-ray substance called uranium y, separated with uranium x,, which has a period of 24:6 hours. this exists in too small quantity to be in the main line of succession, but is to be regarded as a branch product of uranium x, and is believed to be the first element of the subsid- jary actinium series. rutherford and geiger found the number of « particles emitted per gramme of uranium per second to be 2-37 104%, from this the period of uranium is calculated to be about 6,000,000,000 years. thorium—the first product observed in thorium was the emanation of period 54 sec., and this gives rise to the active de- posit, which has been shown to consist of at least four successive products called thorium a, b, c, c”. the emanation, after the emission of an a particle, changes into a product of very short life emitting a-rays. its period was found by geiger and moseley to be about yp second. the succeeding product, thorium b, emits only weak b-and y-rays with a period of 10-6 hours, chang- ing into thorium c of period one hour. we have seen that thori- um c breaks up in a complex way, emitting three distinct groups of particles. thorium c’” is readily separated from c by the meth- od of recoil. it emits penetrating b-and +y-rays with a half-period of three minutes. the active deposit as a whole decays ultimately with the period of thorium b, viz., 10-6 hours. | : a special interest attaches to the product thorium x, first separated by rutherford and soddy, since experiments with it laid the foundation of the general theory of radioactive trans- formations. a close analysis of thorium has led to the discovery by hahn of a number of other important products. when the thorium x is separated from a thorium mincral or old thorium preparation, there appears with it another product called mesothorium i, of period 6-7 years, which is. transformed with the emission of weak 8-rays into mesothorium 2, of period six hours, which emits swift 8 particles and penetrating y-rays. this changes nto an a- ray product, radiothorium, of period two years, which is trans- formed into thorium x, radiothorium is an isotope of thorium, while mesothorium i is an isotope of radium. the radiothorium can readily be sepa- rated from a solution of mesothorium and obtained in a concen- trated form. mesothorium when first separated would show a very weak activity, but in consequence of the growth of its sub- sequent product radiothorium, its activity would increase for several years. after reaching a maximum it would ultimately decay with the period of mesothorium, viz., 6:7 years. actinium.—actinium of period about 20 years is believed to emit weak §-rays changing into radioactinium, an a-ray product of period 19 days, first separated by hahn. this changes into actinium x, an a-ray product of period 11 days, discovered by godlewski. then follows the actinium emanation of period 3-9 radioactivity sec., which gives rise to four further products named actinium a, b, c, c”. actinium a has the shortest life of any product whose rate of transformation has been directly determined. its period, as determined by geiger and moseley and fajans, is -o02 second. after emitting an a particle, a changes into b, a product of period 36 minutes emitting weak 8- and y-rays, analogous to thorium b. actinium c of period 2-16 minutes undergoes a com- plex transformation, giving rise to two distinct groups of a particles. the main branch gives rise to actinium c” of period 4°8 minutes, which is readily isolated by the recoil method. actinium c”, which emits 8- and y-rays, is analogous in all re- spects to thorium c”. in the above discussion on branch products it has been shown that the parent of actinium, called protoactinium, has been re- cently isolated by hahn and soddy. this substance emits a- rays and has an estimated period of 10,000 years. we have seen that the actinium series is believed to have its origin in a dual transformation of uranium x. the first branch product, repre- senting about 3° of the total, is believed to be uranium y, a b- ray product of period one day. this is directly transformed into protoactinium. _ while very active preparations of actinium have been made, it has not been found possible to separate it entirely from the rare earths with which it is mixed. protoactinium exists in much larger amounts and should be ultimately obtained in a pure state. end-products of the transformations (re-stated)—after the radioactive transformations have come to an end, each of the ele- ments uranium, thorium and actinium should give rise to an end or final product, which may be a known element or an unknown element of very slow period of transformation. since the expul- sion of an a particle lowers the mass of the atom by four units, and there are cight a-ray products, the atomic weight of the end atom should be 238—8x4=206. the atomic weight of radium by this rule should be 238—3x4= 226, a result in good accord with experiment. the atomic weight of the end-product of ura- nium is close to that of lead, viz., 207, and boltwood early sug- _ gested that lead was the end-product of radium. since in old minerals the transformations have been in progress for intervals measured by millions of years, the end-product should collect and be an invariable companion of the radioelement. boltwood showed that lead is always present in old radioactive minerals, and in amount to be expected from their uranium content and geologic age. this problem has been definitely attacked in the light of the chemical generalisation already given. it was clear from this that the end-products of uranium, thorium and actinium should all be isotopes of lead but with atomic weights 206, 208 and 206 respectively. in other words, uranium-lead if uncontaminated with ordinary lead should show a smaller atomic weight than ordinary lead (207), while thorium-lead should give a higher value. by the work of richards, soddy and henigschmid, these conclusions have been definitely confirmed. the lowest value for uranium-lead is 206, and the highest for thorium-lead 207-7. ' since any admixture with ordinary lead tends to give a value nearer 207, these results may be considered as a definite proof of the nature and atomic weight of the end-products. in min- erals containing both uranium and thorium the atomic weight of the mixture of the isotopes will depend on the relative amounts of these two elements and their relative rates of transformation. in unaltered minerals the determination of the amount of lead coupled with its average atomic weight allows us to determine the amount of uranium-lead even if some ordinary lead be pres- ient. in this way it should be possible to make a reliable esti- mate of the age of selected minerals and thus indirectly the age of the geologic strata (see geology). the amount of helium in the mineral gives a minimum estimate of its age, for, except in the most compact minerals, some of the helium must undoubt- edly escape. nature and properties of the a- -rays (re-stated) —although the ‘a-rays from active substances are of small penetrating power compared with the 8- or y-rays, they are responsible for most of the energy evolved by radioactive substances and contribute 279 most of the ionisation. rutherford showed in 1903 that the a- rays were deflected in a powerful magnetic and electric field and consisted of positively charged particles projected with high velocity. from the first it seemed probable that the a particle was an atom of helium, and this was subsequently confirmed in a number of ways. the value of e/m—the ratio of the charge on the particle to its mass—and the velocity can be determined from observations on the deflection of the pencil of rays by a magnetic field and electric fields. in this way rutherford and robinson showed that the a particle, whether from the radium emanation, radium a or c, gave a value of e/m=4820 e.m. units, while the electrochemical value of e/m=4826, assuming that the mass of the helium atom is 4-00 and that it carries two unit positive charges. the magnitude of the charge carried by each particle was measured by regener and rutherford and geiger, and found to be twice that carried by the electron. the velocity of the a particles expelled from radium c (of range 7:06 cm.) was found to be 1-92x109 cm. per sec., or about 75 the velocity of light. from this result the velocity of expulsion of all a particles can be calculated from the relation found by geiger, that v3= kr where v is the velocity of the particle and r its range in air. the evidence indicates that the a particles from active products are in all cases atoms of helium. the a particles from a given product are all emitted with constant veloc- ity, which is characteristic for that product. we have already mentioned that the velocity of expulsion appears to be connected with the period of transformation of the element. the laws of absorption of the a particle were first worked out by bragg and kleeman. on account of its great energy of motion, the a particle travels in nearly a straight line through the gas, produc- ing intense ionisation along its track. the efiects produced by the a particle, whether measured by ionisation, phosphorescence or photographic action, vanish suddenly after the a particle has traversed a definite amount of matter. this definiteness of the end of the range of the a particle of given velocity is remarkable. the range of the a particle is usually expressed in terms of cms. of air traversed at 15°c. and 760 mm. pressure. on account of its great energy of motion the effect due to a single a particle can be detected in a variety of ways. sir wilham crookes first noted that the a-rays produce scintillations when they fali on a screen of phosphorescent zine sulphide. it is now known that each of these scintillations is due to the impact of a single a particle. the number of scintillations can be counted with the aid of a suitable microscope, and this method has proved of great utility in many investigations. scintillations due to a- rays are observed in certain diamonds, but they are usually not so bright as in zine sulphide. kinoshita has shown that a single a particle produces a detectable effect on a photographic plate. when the a-rays fall on a plate nearly horizontally the track of the a particle is clearly visible under a high-power microscope. by the expansion method developed by c. t. r. wilson, the track of the a particle through the gas is made visible by the con- densation of the water on each of the ions produced. in a similar way the track of a 6 particle can be easily shown. the photo- graphs of these trails bring out in a striking and concrete way not only the individual existence of a and £ particles, but the main effects produced in their passage through matter. | properties of b- and y-rays (re-stated).—we have seen that the @ particles, which are emitted by a number of radioactive products, consist of swift negative electrons spontaneously jib- erated during the transformation of active matter. the velocity of expulsion and the penetrating power of @-rays vary widely for different products. for example, the rays from radium b are much more easily absorbed by matter than the swift b-rays from radium c. moseley showed that in the case of these two products each disintegrating atom gave rise usually to one 8 particle. there is undoubtedly a close connection between 8- and y- rays, and swift @-rays are usually accompanied by penetrating y-ravs. for example, radium c, which emits very swift f-rays, some of which reach a velocity more than 0-98 of the velocity of light, gives rise to the most penetrating y-rays observed in the uranium-radium series. there is one very notable exception, viz., 280 radium e, which emits swift 8 particles but weak y-rays. gray has shown that §-rays in passing through matter give rise to y- rays, and that these in some cases correspond to the characteristic x radiations observed by barkla. the absorption of the y-rays has been determined by the electrical method. radium b has been found to emit several groups of y-rays which differ widely in penetrating power. the greater part of the rays from radium c consist of penetrating y-rays which are exponentially absorbed by matter. the ionisation in an electroscope falls off according to the equation i/ib=etm*, where d is the thickness of matter traversed and y the coefficient of absorption. when lead is used as an absorbing material the value of w=o-5 for the most pene- trating y-rays from radium c. the absorption coefficient for different kinds of matter is roughly proportional to the density, indicating that the absorption depends only on the mass of mat- ter traversed. the detailed study of the b-ray spectra discussed below has led to the conclusion that the y-rays are characteristic radiations emitted from the radioactive nucleus in its rearrangement after | the ejection of the 8 particle. these radiations from the nucleus in their passage through the atom excite the characteristic radia- tions of the external electronic system. thus the total y radia- tion of a radioactive atom consists not only of the y-rays from the nucleus, which are generally far more penetrating than any type of radiation observed in an x-ray tube, but also of the characteristic x-rays of the atom. | rutherford and wooster have re-examined the spectrum of the y-rays from radium b by reflection from rock-salt and calcite crystals. they found that the lines of the characteristic “ l” spectrum corresponded to an atom of number 83, while the atomic number of radium b is 82. the y radiation must therefore be emitted after the ejection of the 8 particle of disintegration and the change of charge of the nucleus. this conclusion has been confirmed by independent methods by black, ellis and wooster, and lise meitner. the wave-length of the most penetrating y- rays is much too short to resolve or detect by the crystal method. in order to excite such rays in an x-ray tube potential] differ- ences of the order of 2,000,000 volts will be necessary. when the f-rays from a product like radium b or radium c are bent by a magnetic field and fall on a photographic plate, a kind of magnetic spectrum is obtained. superimposed on the continuous spectrum due to particles of all velocities (between certain limits) certain sharp lines are observed, each of which represents a definite group of 8-rays which are emitted at the same speed. the velocity corresponding to each line in the spec- trum has been determined for a number of f-ray products by hahn and lise meitner. the magnetic spectrum of radium b and radium c was examined in detail by rutherford and robin- son, and more than 50 lines were observed, representing 8 par- ticles projected over a wide range of velocity. the appearance of these lines in the spectrum appears to be connected with the emission of y-rays and is believed to be due to the conversion of the energy of the y-ray of definite frequency into the energy of an electron according to the quantum relation. when a thin layer of absorbing material is placed over the source, the primary b-rays diminish in velocity and the lines become broad and dif- fuse. at the same time, however, new groups of 6-rays are formed by the conversion of ‘y-rays into b-rays in passing through the absorbing material, and these give well-marked bands on the photographic plate, occupying very nearly the same position as those due to the primary 8-rays before absorption. the study of these primary and excited 8-ray spectra by ellis, meitner and others has given important results on the conversion of -rays into b-rays, and has furnished accurate measurements of the wave-lengths of the y-rays emitted by several products. production of helium (q.v.).—since the particle is an atom of helium, all radioactive matter which emits a particles must pro- duce helium. this has been found to be the case for every a-ray product that has been examined. the rate of production of helt- um by radium in equilibrium has been measured with accuracy by dewar, boltwood and rutherford. in terms of the inter- national radium standard, the rate of production of helium by radioactivity one gramme of radium in equilibrium with its three a-ray prod- ucts has been found to be 164 cu. mm. per year. this value is in excellent accord with that calculated from the rate of emission of a particles, viz., 163 cu. millimetres. the rate of production of helium by the radium emanation, ionium and polonium has been found by boltwood to be in fair agreement with calculation. soddy has observed the production of helium by purified urani- um, while strutt showed that the rate of production of helium in uranium and thorium minerals was in perfect accord with calculation. strutt has made a systematic examination of the amount of helium present in many minerals and rocks which contain minute quantities of radium, and has utilised the results to estimate the age of the geological deposits. on account of the tendency of the helium to escape from minerals in the course of geologic ages, this method gives only a minimum estimate of the age of the mineral, except in the case of very dense and compact specimens. the measurement of the lead content should ultimately prove a more reliable method of estimating the age. heat emission by radioactive matter.—as was stated earlier, there is no doubt that the evolution of heat by radium and other radioactive matter is mainly a secondary phenomenon, resulting largely from the energy of the absorbed radiation. since the particles have a large kinetic energy and are easily absorbed by matter, all of these particles are stopped by the radium itself or by the envelope surrounding it, and their energy of motion is transformed into heat. the evolution of heat from any type of radioactive matter is thus proportional to the energy of the ex- pelled a particles, together with the energy of the 8b- and y-rays absorbed in the envelope. the energy supplied by the recoil of the radioactive atom after the expulsion of an a particle is about 2°% of the energy of the a particle. these conclusions have been confirmed by the measurements of rutherford and robinson, who found that each of the a-ray products gave a heating effect proportional to the energy of the a particle and absorbed f-and y-rays. the emanation and tts products when re- moved from radium were responsible for three-quarters of the heating effect of radium in equilibrium. the heating effect of the radium emanation, radium a and radium c, decayed at the same rate as their activity. from their measurements they found that the total heating effect of radium in equilibrium surrounded by sufficient ma- tcrial to absorb the a-rays was 134-7 grammes-calories per hour per gramme. of this, 123-6 grammes-calories were due to the a particles, 4°7 to the f-rays and 6-4 to the y-rays. the energy of the b-and y- rays comes from radium b and radium c, but on account of their great penetrating power it is difficult to measure the y-energy with accuracy. the results, however, show that the energy of the y-rays is even greater than that of the §-rays, and the two together are equal to about 28 % of the energy of the « particles from radium c, measurements have been made of the heating effect of radium, uranium and thorium, and of uranium and thorium minerals. in each case the evolution of heat is of about the magnitude to be expected from the energy of the radiations. radioactivity of ordinary muatter.—apart from the wellknown radioactive elements of high atomic weight, only two other elements have been shown to exhibit radioactivity to a detectable degree, viz., potassium and rubidium. campbell showed that these elements emit only b-rays and in amount small compared with uranium. this property appears to be atomic, but no evidence has been obtained of any subsequent changes. if the 8 particle comes from the nucleus of the atom, potassium should be transformed into an isotope of calcium, and rubidium into an isotope of strontium. radium and thorium have been found to be distributed, but in very minute amount, in the surface rocks and soil of the earth. the emanation from the soil diffuses into the atmosphere and causes a small ionisation which can be readily measured. a penetrating y radiation, no doubt due to the presence of radium and thorium in the earth’s crust, has been observed near the earth’s surface, but becomes very small over a lake or the sea. brptriograrhy.—mme. m. curie, tratte de radioactivite, 2 vol. (1910); sir e. rutherford, radioactive substances and thetr radia- tions (1913); s. meyer and e. v. schweidler, radioaktivitet (1916); f. soddy, chemistry of the radioelements, parts i. and ii. (1914-5). see also under ‘ radioactivity " in the annual reports of the chem- ical society. (e. ru.) > radio receiver radio receiver (see also broadcasting; wireless telegraphy and telephony).—the ideal radio broadcast receiver would be one so sensitive that any desired station could be picked up and its performance reproduced with perfect acous- tic fidelity, in any desired sound volume, to the exclusion of all other signals and disturbances, with substantially no effort in operation or maintenance on the part of the owner. such a perfect machine can only be approximated in some respects by existing receivers. from the standpoint of the listener five qual- ities of commercial broadcast receivers may be distinguished and used in rating particular types of sects as to merit, namely: (a) sensitivity; (b) selectivity; (c) quality of reproduction; (d) magnitude of undistorted power output; (¢) convenience of opera- tion and maintenance. (a) sensitivity —a receiver must be several hundred times more scnsitive, 7.e., capable of extra radio frequency amplifica- tion in that ratio, if it is fed from a small loop rather than an outdoor antenna. this follows from the fact that the loop picks up a relatively minute amount of energy from the passing wave. the loop is more convenient and adds somewhat to the selec- tivity of the set, as for maximum signal strength, it must point in the direction of the station to be received, and at right angles to this orientation the signal js a minimum. it is not unusual for radio receivers to have a voltage ampli- fication in the thousands, corresponding to an energy amplifica- tion of the order of tens or hundreds of millions. at present there is no standard rating of reccivers according to sensitivity or am- plifying power. roughly, the amplifying power depends on the number of vacuum valves, not necessarily in the geometrical ratio which one might expect, however, for one set may utilise its stages of amplification more effectively than the other, with the final result depending on an intricate concatenation of design factors. the radio salesman’s rating is even more indefinite, as he refers to a receiver as having a normal range of so many hundred or thousand miles, with reference toa transmitting station of given power. in practice, inordinate, sensitivity is useless, inasmuch as nothing is to be gained by amplifying a signal below the ‘‘ noise level ’’ at the location of the receiver. (b) selectivity. —like amplifying power, selectivity with re- spect to frequency may be numerically expressed. the waves of transmitting stations are measured in metres of wave-length, corresponding in a fixed relation to oscillating frequency in kilocycles per second. one station may transmit at a mean wave-length of yoo metres, corresponding to a frequency of 750 kc. per sec., while another is set at a wave-length of 450 metres (666 kilocycles). in order to respond to either of these transmitters the receiver is tuned to the appropriate frequency. the ability to hear the performance of one without picking up an appreciable signal from the other depends on the width of the band of frequencies, likewise expressed in kilocycles, which the receiver will admit at any given setting. dr. alfred n. goldsmith has proposed the following scale for expression of receiver selectivity :— | (1) poor selectivity: equal signals become inaudible 80 kc. off tune. (2) good selectivity: equal signals become inaudible 30 kc. off tune. (3) very good selectivity: equal signals become inaudible 1o ke. off tune. (1) excellent selectivity: equal signals become inaudible 5 kc. off tune. condition (4) oversteps the limit of desirable selectivity, if the factor of quality of reproduction is to be kept in sight. the wave radiated by the broadcasting station is not confined to a single frequency, but has itself a certain width, normally about to ke., which cannot be clipped by the receiver without losing some of the essential component vibrations of speech or music. the receiver must accordingly admit a band ro ke. in breadth, cor- responding to zero audibility 5 kc. off tune, if serious distortion is to be avoided, and where interference conditions permit a 20 ke. receiver admittance is to be preferred. in other words, close juxtaposition of frequency bands of broadcasting stations 281 tends to impair quality of reception by(z) forcing receiver de- signers in the direction of excessive sharpness of tuning; and (2) interference between stations in spite of (1). (c) quality of reproduction.—the human ear responds, by no means with equal facility, to air waves between certain limits of frequency, ranging from a few oscillations per sec. to an upper limit as high as 20,000 vibrations per sec. in some individuals. actually this is a greater range than required, and an organ responding to from 10 to 10,0c0 cycles will take in all the funda’ mental tones and essential overtones of speech and music. this is about the width of the acoustic spectrum which the best broad- casting stations and the best receiving sets try to reproduce. to a greater or lesser extent, however, they tend to lose the end frequencies—those of very low and very high pitch. when the low notes are dropped out, the result is “ tinny’’—high-pitched, shrill, mechanical, lacking in body. if the high notes are dis- criminated against, the output of the set is “ drummy ”’— jacking sharpness, intelligibility, and characteristic timbre of voice and orchestralinstruments. intheintermediate band which is transmitted there may be over- or under-emphasised fre- quencies, which stand out unnaturally or are lacking in the final result. all these lapses may occur in either the amplifying sys- tem or the electroacoustic converter (telephone receiver or loud speaker) of the receiving system, as well as in the corresponding portions of the transmitting equipment. a properly designed system is “ flat,” 2.e., indiscriminatory, over a sufficiently wide auditory band. at the present time, a receiver and reproducer covering a band of from 60 to 6,000 vibrations per sec. is considered satisfactory, and does indeed afford a reproduction close enough to the original to satisfy critical taste. (d) magnitude of undistorted power output.—the output of a radio receiver, in terms of a fraction of a watt of sound energy, is a finite quantity and overloading reveals itself by distortion of the acoustic output, a distortion distinct from that caused by unequal amplification of different audio frequencies, as dis- cussed under “ quality of reproduction,” but just as important if pure reproduction is required. overload distortion causes a characteristic splitting or cracking of strong notes and em- phasised syllables, sometimes to the point of unrecognisability. harmonics or partial tones, not present in the original, are generated in the receiver. the accompaniment in music, and extraneous noises such as microphone hiss at the transmitter, atmospheric disturbances, etc., are brought up out of proportion, since the thermionic machincry of the recciver is ‘“ saturated ” for the passages which were louder in the original performance. the fact that must be kept in mind is that the vacuum valves used in receiving sets, particularly the last or output tube, have a definite and limited capacity for undistorted reproduction. a small valve, drawing moderate energy from the receiver power supply, can only deliver a few hundredths of a watt to the loud speaker—enough to fill an ordinary room comfortably. if greater volume is desired, larger tubes, with adequate power supply, must be employed. some of the amplifier-loud speaker combina- tions have tubes as large, and consume as much power, as small broadcasting stations. by this means an undistorted reproduc- tion as loud as the original is rendered available. (e) convenience of operation and maintenance —in the matter of convenience of operation the present trend is toward reduc- tion of the number of controls. some early models of multi-tube receivers carried from six to 1o controls, but the problem of tuning them proved too much for the average unskilled listener. the average number of tuning controls at present is probably three, with some auxiliary handles which do not require frequent adjustment. for local station selection, however, “‘ uni-control ”’ accomplished by turning a single knob or drum,has beendeveloped. for reception of distant stations finer adjustments, involving auxiliary or “* vernier ” controls, are generally required. a “‘ vol- ume control” for setting the loudness of the received signal at the desired level is necessary in addition to the tuning selectors. the best modern receivers show an increasing intricacy of internal equipment, with a surprising development of simplicity of control in inverse ratio. 282 the principal maintenance problem in the radio receiver is in connection with the power supply, and this disappears where power for the receiver is derived from the electric-light mains. the crystal, in the case of crystal receivers, and tubes, in tube receivers, require occasional renewal. for the rest, continuity of operation depends jargely on the reliability of the manu- facturer and the effectiveness of his testing and inspection arrangements. in the mechanism of the receiver proper there is nothing to wear out. crystal sets types of receivers —the simplest of broadcast receivers is the crystal type, which consists of a single tuned circuit including the antenna and ground, with the crystal, bridged across the tuning inductance or a portion thereof, delivering audio frequency energy to the telephones. except in a few instances where the listener is located a frac- tion of a mile from an extremely powerful broadcasting station, a crystal set is incapable of giving a loud speaker signal. in these isolated cases a long antenna in combination with a crystal of large surface and current-carrying capacity, such as carborundum, may be used to operate a loud speaker. usually the crystal set feeds a pair of telephones at low or moderate volume. the pos- sibility of distortion in such a simple device is necessarily far less than in valve reccivers. various crystals are used in radio reception. the most com- mon is galena (lead sulphide), with a fine wire, generally called a “‘ cat-whisker,” resting lightly on one surface. this combina- tion is sensitive and fairly stable, but a jaris apt to interrupt reception, and the frequent readjustment necessary is one of the outstanding defects of the crystal receiver. some pieces of the ore, however, abound in sensitive spots and require relatively little attention on the part of the listener. ‘“ fixed ” crystals, adjusted at the factory, have been devel- oped, and in some instances hold a fairly good adjustment for months. the process consists in sealing the wire and crystal together, and sometimes in the use of a multiplicity of contacts. the sensitivity is usually less than that of a good crystal used with an adjustable contact. no matter how carefully the tuning circuits are designed, the resistance of a crystal, being low compared with the input impedance of a tube, prevents sharpness of tuning. hence the crystal receiver is scarcely of use in congested radio neigh- bourhoods. it fills the need for a cheap, easily handled piece of equipment within 25 m. of a station, and where the operator is content to listen on head phones. in the british islands the crystal still holds a prominent position, in that distances from stations are less and the demand for the more realistic reproduc- tion of large tube receivers appears to be more moderate than in the united states, where the crystal outfit has fallen into a posi- tion of commercial unimportance. tube receivers (1) non-regenerative—the non-regenerative single tube re- ceiver is more selective but a rarity. it is only slightly more sensi- tive than a good crystal. in many instances a non-regenerative rectifying tube is used in multi-tube receivers. (2) regencrative—the principle of regeneration in tube cir- cuits is described under wireless telegraphy and telephony (g.v.). in receiving circuits regeneration affords an incxpensive form of radio frequency amplification. a single tube, regenerat- ing, serves both as a rectifier and very effective radio frequency amplifier. at a wave-length of 4oo metres, which is a common value in the broadcast band of many countries, a single tube with maximum regeneration, short of full oscillation, gives about as much signal as two radio frequency tubes and a straight rectifier, coupled through broadly tuned transformers with regeneration substantially suppressed. a common form of receiver is one in which the antenna tuning circuits are followed by the regenerative tube and one or two stages of audio amplification. this suffices for local (10-20 m. from ao-s kw. station) reception on loud speaker, and reception radio receiver of distant (up to1,500 m.) stations on telephones, or faintly on the loud speaker, when two audio stages are employed, under average conditions. if regeneration is carried far enough the radio frequency amplification secured is very sizable, but there is a tendency to impair quality of tone by cutting off the side- bands. the result is a characteristic muffled sound. by somewhat detuning the circuit just before the detector, it is possible to lose grave tones, on the other hand, and to “ equalise ” the signal to a degree (7.e., to correct for loss of high frequencies). a similar cffect may be present in some super-heterodyne receivers, as de- scribed below. one of the salient disadvantages of the simple regencrative receiver lies in its propensity to oscillate and radiate waves which cause howls in nearby receivers. this may be obviated by means of a “ blocking ” or one-way tube ahead of the detector. another drawback is that sensitivity is secured more or less at the ex- pense of quality, and that for high sensitivity careful adjustment is necessary to ensure that it remains tuned to the desired frequency. | (3) radio frequency cascade-amplification.—the difficulty of controlling regeneration was at first one of the principal obstacles to the construction of receiving sets with several stages of radio frequency amplification. regeneration occurs through the coupling or interaction of the grid and plate gnput and output) circuits of a tube. even if these circuits are not deliberately coupled, the natural internal coupling between the closely adjacent grid and plate of each tube, when several radio frequency staves are used, is enough to set the receiver into oscillation, with the usual resultant squeals, distortion, etc., and methods were developed whereby these internal capacitive couplings could be balanced out or “ neutralised.”’ in the usual form the receiver designed on this principle has two steps of radio frequency, a rectificr tube and two stages of audio frequency. however, by careful shielding (enclosure in metal boxes) of individual stages still further to reduce stray couplings, it is possible to employ three or even four radio frequency stages. ralanced receivers—also called ‘‘ neutrodynes ’’—~are nearly always operated on open antennae, although in one or two cases they have been built for sufficient amplification for use on a loop. the number of handles is generally three, for the antenna and two stages of radio frequency. one type of neutralised receiver having six tubes, with three stages before the detector, requires only two knobs, one for the antenna and the other for the three radio frequency stages. another type of balanced receiver has been built, using five dry-cell tubes, including two stages of radio frequency without regeneration, controlled re- generation in the detector and two stages of audio. another type of radio frequency receiver controls oscillation by biasing the grids of the radio frequency tubes positively, thus introducing losses which tend to brake the regenerative action and to suppress oscillation. the usual number of tubes 1s five; an outdoor an- tenna is desirable, and, unless carcfully handled, the recciver may fall into oscillation and radiate strongly. (4) intermediate frequency amplification: the super- hetero- dyne.—in the super-heterodyne receiver the incoming wave is mixed with that of a local oscillator, producing, after rectification, a lower frequency which is the difference of the two component frequencies, and which is more readily amplified than the oscilla- tions received directly from the station. this “‘ intermediate frequency ” is subjected to one to three stages of amplification, rectified a second time, producing telephone currents, which are then amplified at tone frequency in the usual manner. besides the facility of tuned radio frequency amplification at the intermediate level, which is commonly 50,000 cycles per sec., the super-heterodyne method has a peculiar advantage in de- velopment of selectivity. when two waves of different frequency enter this type of receiver, the percentage difference in fre- quency, and hence the separability, are much greater after heterodyning than before. for example, let the local oscillator have a frequency of 500,000 cycles per second. two waves are admitted to the set, of 530,000 and 550,000 cycles respectively. the percentage difference in frequency is less than 4%. after radio receivers piate i. fic. 1. a portable 6-tube super-heterodyne; loop antenna, batteries, and loud speaker, all self-contained. photograph shows the outfit open, ready for operation, with addition of a battery box permitting use of larger cells for other than portable service. usual american wave-length range. dry cell tubes. dual control for tuning. fic. 2. cone type loud speaker. fic. 3. 8-tube super-heterodyne of the desk type, completely self-contained with exception of loud speaker. (legs and battery chamber omitted from this picture.) single control for tuning local stations; dual control for distance reception. loop antenna on top; batteries in cabinet. dry cell tubes, with power valve output. wave-length range 200-550 metres. fic. 4. (internal view of fig. 8.) shows shielding of radio stages and gang condenser for parallel tuning of circuits. two tuning controls; usual american wave-length range. indoor or outdoor antenna. fic. 5. a 6-tube receiver (three stages radio frequency; detector; two stages of audio amplification) with single dial tuning. usual american wave-length range. storage battery tubes; loud speaker operation. indoor or outdoor antenna—preferably latter. fic. 6. internal view of a 5-tube receiver combining neutralised radio frequency amplification, regeneration and audio frequency amplification after detection. this set is built by home constructors according to plans furnished by a radio periodical. may be used with indoor or outdoor antenna and will operate loud speaker. storage battery operation. usual american wave-length range. fic. 7. cone type loud speaker with power amplifier en- closed. operates on 60-cycle alternating current on first audio stage of radio receivers, giving extremely high output without distortion. fic. 8. a cabinet model receiving set, with built-in loud speaker and space for all batteries required and for storage battery charger. six storage battery tubes, the last being a power tube for increased output. circuit includes three stages of neutralised radio frequency, detector, and two stages of audio amplification. (for internal view, see fig. 4.) fig. 9. combination electric talking-machine and super-heterodyne radio receiver, employing a cone type loud speaker which may be switched at will from the phonograph to the radio output. completely self-contained and operates without batteries on alternating current, 120 volts, 60 cycles. power tube output for both radio and phonograph reproduction. loop antenna in one of doors, with usual directional effect. turntable of talking-machine is electrically driven. american wave- length range. ( figs. 1, 3, 7, radio corporation of america; figs. 2, 4,8 stromberg-carlson telephone mfg. co.; fig. 5, atwater kent mfg. co.; fig. 6, doubleday, page & co.; fig. 9, victor talking machine co. ) pate it. radio receivers ae no} fue f.eneph? sat oe fic. 1. two-valve regenerative receiver capable of operating a loud speaker within 15-20 m. of a normal broadcast transmitter. wave-length range, 250-5,000 metres, depending on the coils plugged in. may be used with dry or storage battery tubes. one of the valves is a high-frequency amplifier and detector; the other a transformer coupled note frequency amplifier. intended for use with outdoor antenna. fic. 2. a crystal set, wave length range 250-2,000 metres, suitable for reception on head telephones within 20 m. of an ordinary broadcast station and 100 m. from a high-power transmitter. intended for use with outdoor antenna. fic. 3. horn type loud speaker. fic. 4. a seven-valve super-heterodyne receiver, operated on a frame antenna, without ground connection. two frames are provided, one covering the 250-550 metre wave-length range, the other 1,000-2,000 metres. storage battery operation, last two valves being of the power type. designed for loud-speaker output. fic. 5. four-valve receiver with one stage of radio frequency, detector, and two stages of audio frequency amplification; wave-length range of 150-4,000 metres. ’akes storage battery tubes with power valve in output. plate battery contained in base. intended for use with outdoor antenna and loud speaker; head telephones for distant stations. photograph shows appearance open. (figs. 1,2, 3 and 4, burndept wireless, ltd. fig. 5, falk, stadelmann & co., lid.) radiotherapy and rontgenology heterodyning and rectification, the two intermediate frequencies ire 30,000 and 50,000 respectively, a percentage difference of 66%. it is then very simple to discriminate between the two waves by means of an intermediate frequency amplifier tuned to, say, 50,000 cycles. in that case the 530,000 cycles station will be the one suppressed. inasmuch as the local beating frequency is variable, the other station may be suppressed with equal ease, and the 530,000 cycle wave admitted. the super-heterodyne is really a fixed frequency receiver, with means of converting the wave of any desired station to that frequency. super-heterodyne receivers average eight tubes, operate readily on a small loop, with one or two tuning controls and may be made completely portable and self-contained. they constitute the most expensive class of radio receiver, and afford the maximum selectivity and amplification of any type as yet developed. luning \fechanisms.—in the united states, with a relatively narrow band of wave-length (200-550 metres) to be covered, the usual form of tuning is with a fixed inductance and variable con- denser. in british practice, with a 200-1,700 metre band in england, if the high power daventry station is to be included, and continental telephone channels up 1o 4,c00 metres, it is necessary to vary the inductance as well, in order to cover wave- length ranges of this order. hence sets employing inductance taps, interchangeable lattice-work coils and the like devices, are more popular in british broadcast reception. reflexing.—it is possible to use one or more tubes in a radio receiver for both radio and audio frequency amplification. for example, given two valves, the first may be used as a radio frequency amplifier, the second as a rectifier and the audio currents sent to the first tube again, with the low frequency out- put of the same feeding the telephones. ‘tube number one is then said to be “ retlexed.” in this way thermionic valves may be employed economically, but the application of the method is limited if good selectivity, quality of reproduction and stability are desired. as a rule, if retlexingis resorted to, only one tubein a receiver is so employed, although sets have been constructed with three tubes reflexed, andl the term has been used to denote a separate receiver classification. . combinations of types——the characteristics of the forms of sets described above may be combined in various ways. for example, a super-heterodyne receiver may utilise a stage of neutralised radio frequency amplification before the hetero- dyne operation, or reflexing, regeneration and neutralisation may be combined. power supply.—vacuum tubes may be classified according to filament consumption, whether intended for storage or dry battery operation. the filament of a storage battery tube will consume about one watt, as compared with o-2 w. for a dry battery valve. three or four of the latter may be run on three- standard dry cells. the storage battery tubes, being larger, yield more amplification per stage. in general, for moderate volumes dry cell tubes will give equivalent service to storage battery tubes. when the filament power is derived from a chemical source, plate power is usually drawn from small dry battery blocks. for very loud reproduction, it is necessary to resort to power tubes with energy supply from the alternating current mains, through suitable transformers, rectifiers, and “ smoothing out ”’ filters for converting the bi-directional voltages into a direct or uni-directional flow. the same devices may be used for smaller outfits as well, in place of batteries. choice of a receiver.—the choice of a receiver is determined largely by the capital to be invested, as well as the operating conditions to be met. for example, if the prospective buyer is unable to erect an outdoor antenna, because of a landlord’s prohibition or other reason, and insists on receiving distant sta- tions, it will probably be necessary for him to purchase a super- heterodyne receiver operating on a loop. but if the location is in the country, with an efficient outdoor antenna available, and no broadcasting stations sufficiently near to require first-rate selectivity in reception, a cheaper receiver may be considered. again, a receiver required to give strong, undistorted reproduc- 283 tion, to fill the ballroom of a residence, for example, will neces- sarily be more costly than one which only serves a few persons in a small room. | as in other fields, if one is dealing with a reputable firm the returns depend roughly on the amount expended. a sane pro- ceilure is to invest first in a small set of one or two tubes, and to become familiar with radio conditions before buying a larger outfit. home construction.—instead of purchasing receivers, many listeners buy the parts and assemble them at home. with the increasing intricacy and decreasing cost, per unit of value, of radio equipment, it is scarcely worth while for amateurs to assemble their own receivers. ‘the product of extensive research, pro- fessional design, systematised manufacture and inspection, and large scale production, is likely to be superior to the results of untrained individual initiative, and not very much more ex- pensive, even if the expenditure of the home-builder’s time is not charged. for those listeners who wish to construct their sets, out of a desire to give rein to their creative energy, the best advice is to build equipment recommended by some well-known radio periodical possessing a laboratory where ideas are thor- oughly tried out before being allowed to get into print, and to follow closely the directions given. in all such enterprises changes are made at the changer’s risk. in the nature of radio design, a single substitution or slight alteration in wiring will sometimes entircly spoil the performance of a sct. brbpliograpny.—e. h. armstrong: “a new system of short wave amplification,” and discussion following, proceedings of the institute of radio engineers, vol. 9, no. 1 (feb. 1921); frank conrad: “ radio receiving equipment,’ proceedings of the institute of radio engineers, vol. to, no.6 (dec. 1922}; j. c. warner: “ recent devcl- opments in high vacuum receiving ‘lubes, radiotrons, model lv-199 and model uv-201a, proceedings ef the institute of radio engineers, vol. 11, no. 6 (dec. 1923); l. a. hazeltine: ‘ tuned radio frequency amplification with neutralization of capacity coupling,” q.s.7. (april 1923); ie. 1. armstrong: ‘ the super- hetcrodyne-—its origin, development and some recent improve- ments,’ proceedings of the institute of radio engineers,vol. 12, no. § (oct. 1924); c. r. wanna: “ design of telephone receivers for loud speaking purposes,’’ proceedings of the institute of radio engineers, vol. 13, no. 4 (aug. 1925); john f. dreyer, jr., and ray h. manson: ** the shielded neutrodyne receiver,” proceedings of the institute of radio engineers, vol. 14, no. 2 (april 1926); walter van b. roberts: “‘ how radio receivers work,’ doubleday page & co.; alfred n. goldsmith: “ highlights of radio broadcasting,’”’ wireless age (dec. 1923, march, june and aug. 1924), and reprinted in pamphlet form by radio corporation of america. (c. dr.)