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    "source_title": "Encyclopaedia Britannica (1926)",
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    "chunk_id": "1926:vacuum tubes:041ab1f2bedf",
    "title": "VACUUM TUBES",
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    "verified_text": "the beauty and the interest of the phenomena attending the passage of electricity through gases at low pressures have engaged the attention of physicists for a gencration, and a rich harvest of discoveries of the greatest importance has resulted. the discovery of the electron by j. j. thomson and of the proton by rutherford have given us the basis of the modern theory of electricity and of the structure of the atom (q.v.). thestudy of the electric discharge tube has further given us, or materially advanced, the x-ray tube, the wireless valve, the electric lamp, the cathode-ray oscillograph, the vacu- um flask, mercury vapour and neon lamps, etc. moreover the requirements of vacuum tube workers have led to the most profound improvements in the methods and technique of pro- ducing high vacua, more particularly during the present century. vacuum pumps.—the various types of high-vacuum pumps are mostly designed to operate after a preliminary exhaustion to a few millimetres of mercury by a “ backing ” pump. for this purpose a water-jet pump is sometimes suitable although a piston or rotary oil pump is generally to be preferred. some types of rotary oil pump are capable of pressures as low as zoo mm. mercury and are rapid in action. it is important to protect them from water vapour. the tepler and sprengel pumps are now little used on account of their slowness of action. this remark applies also to the earlier types of rotary mercury pumps. dewar’s discovery in 1904 of the absorbent properties of coconut charcoal when immersed in liquid air was a conspicuous advance. the process is very rapid for certain gases and is capable of affording extreme vacua, but the necessity for liquid air restricts its gencral utility. the molecular pump of gaede followed in 1913. holweck has brought out a new design. this depends for its action on the fact that in confined spaces at low pressures the collisions of a molecule with the boundary walls are much more frequent than with other molecules. if one of the walls is in rapid motion it follows that the molecules tend to take up the direction of this motion. the principle takes practical shape in the form of a grooved cylinder revolving at high speed within a cylindrical casing, the clearance being about 7s mm. or less. the pump is capable of the highest vacua, but is expensive and is occasion- ally attended by mechanical troubles inherent to the design. gaede and langmuir pumps.—but the method which has displaced almost all others for most purposes is that underlying gaede’s mercury-vapour diffusion pump (1915), of which lang- muir’s condensation pump (1916) may be regarded as an inde- pendently developed and valuable modification. a variety of types are now available constructed of glass, silica or steel. in all these a blast of mercury vapour, arranged to entrain mole- cules of the gas to be pumped, is directed tangentially across a vacuum tubes water-cooled surface. the energy of the “ reflected ” mercury molecules is too small and non-directive to interfere with the direct blast. the gas is accordingly passed on to the backing pump and the mercury vapour is condensed and used again. the highest vacua (10°§ mm. or less) are attainable under suitable conditions, gases and vapours being pumped cqually well. the speed of pumping of some types is very high. backing pressures range from a few millimeters to 34; mm. of mercury, depending on the type. | it should be added that, if full advantage is to be taken of the high speed of mercury-vapour pumps, wide-bore (about 1 in. diameter) connecting tubes should be used throughout and the backing pump should be fully capable of keeping pace. a prac- tical convenience with such pumps is the provision of an adjusta- ble air leak (e.g., by a needle valve) for maintaining a suitably high pressure in the discharge tube. if the presence of mercury- vapour in the vacuum tube is objectionable, a trap of liquid air or metallic potassium should be interposed. in the manufacture of electric lamps and wireless valves the final stages of evacuation are effected by making use of the “cleaning up ” properties of red phosphorus, magnesium, etc. the material is deposited on the filament or adjacent supports, and when vapourised by heating results in the reduction of the pressure to a low limit. if it is desired to maintain permanently the highest vacuum in a sealed vacuum tube, it is essential to heat the tube and its enclosures to as high a temperature as pos- sible while the exhaustion is proceeding. electric discharge in vacuum tubes.—~the potential differ- ence required to excite an electric discharge in a vacuum tube depends greatly on the conditions, e.g., the nature and pressure of the residual gas, the size and shape of the tube, the size shape, material and disposition of the electrodes. if the cathode can be heated and if it is coated with onc of the alkaline earths, the excit- ing voltage may be only so volts or less at suitable pressures. this is owing to the elimination of the cathode-fall of potential. if the electrodes are both cold, it is necessary to apply much higher voltages—some hundreds or thousands, such as are yielded by an induction coil or, preferably, some source of con- stant high potential. ‘che phenomena presented are very varied and of a complexity which has even now been only partly cleared up. though negative ions may play some part, the current passing through the tube is, in general, carried almost entirely by electrons and positive ions. their interdependence and es- sentially different nature lead to the existence of space charges within the tube and surface charges on the walls, which markedly influence the type of discharge. pressure ejffects—if the pressure is relatively high—say 10 mm. of mercury—the discharge consists of a collection of sinu- ous and irregular luminous streamers. at lower pressures—say 1mm. to #4 mm. of mercury—the tube normally shows the following appearances: (a) the anode glow—a thin film of light over the surface of the anode; (b) the cathode glow—a thin film of light over the surface of the cathode, bounding which is the non-luminous crookes dark-space, beyond which again is a second cathode glow bounded in turn by the ill-defined and somewhat transient faraday dark-space; (c) the positive col- umn, sometimes continuous, sometimes striated, occupying the main body of the tube and extending from the anode glow to the faraday cdark-space. it has been shown that the value of the axial electric field is not at all uniform along the tube, but varies widely as we pass from one clectrode to the other. at the anode there is a fall of potential, the extent of which depends only on the current and the nature of the residual gas and not on the nature of the anode. the anode fall appears to be due to an accumulation of electrons near the anode. | | cathode fall.—it is at the cathode that the electrons or cathode rays originate by positive ray bombardment. schuster, wehnelt and others showed long ago that an obstacle placed in the crookes dark-space cast a shadow both ways, as the ap- proaching positive rays and the resulting electrons were simul- taneously cut off. the electric field at the boundary of the dark- vaida-voevod—valparaiso space is small but increases linearly with distance from the dark- space boundary until it reaches a high value at a point very close to the cathode where it drops suddenly. the speed of the cathode rays normally corresponds with but little loss to the potential difference between the cathode and the boundary of the dark-space—the so-called ‘“ cathode fall.”” the cathode fall depends on the nature and pressure of the gas, the nature and area. of the cathode, and on the current density. the curiously sharp luminous boundary of the dark-space runs parallel to that of the cathode and is supposed to represent a region of ionisation by the cathode rays, together with recom- bination resulting in luminosity. it is inferred that, in general, there is a positive space-charge in the dark-space, while very close to the cathode is a negative space-charge due to the pro- duction of electrons by positive ray bombardment. the first cathode glow yields a line spectrum and stark has shown that it shows the doppler effect by reason of the rapid motion of the luminous particles towards the cathode. the width of the crookes dark-space is normally somewhat larger than the mean free path of an electron through the gas. the width depends on the current density and on the gas pres- sure, it increases as the pressure falls (f. w. aston, proc. roy. soc., london, 1911) and is frequently used as a measure of the vacuum prevailing. the positive column may present a uniform column of light or it may be striated, depending on the conditions. the dis- tance between the striations increases as the pressure of the gas is reduced and the diameter of the tube is increased. the electric field in the positive column is fairly small, the conductivity being high. if the positive column is uniform the field is uniform; when the positive column is striated the field is periodic, being greater in the bright parts of the striation than in the dark. the field is very small in the faraday dark-space, as in the adjacent cathode glow. in the case of the continuous positive column the luminosity is believed due to ionisation and subsequent recom- bination, produced by a stream of electrons which owe their ionising speed to space and surface charges in the tube. the striations are regarded as due to the insertion, so to speak, of a number of faraday dark-spaces. the phenomena of the positive column are complicated and appear to be intermittent even with a steady exciting potential, pulses of positive ions and electrons being apparently ejected from the electrodes in opposite direc- tions with different velocities. whiddington (prec. camb. pil. soc., 1924) has shown that periodic flashes of luminosity move from anode to cathode with a speed and frequency which increase as the gas pressure is reduced. these flashes do not show the doppler effect. with higher rarefactions—say 75 to 140 mm.—the positive and negative glows lose shape and luminosity, and the crookes dark-space grows at the expense of all clse. as its boundaries reach the walls they begin to fluoresce with an appearance well known to users of gas x-ray tubes. the exhaustion may ulti- mately be pressed until it becomes impossible for a discharge to pass in a tube with cold electrodes. it was witha suitably designed discharge tube that j. j. thomson first demonstrated the existence of electrons. by using a perforated cathode and allowing the positive rays to stream through, j. j. thomson and later aston made measurements of the masses of these positively charged atoms of gas and obtained results which have thrown striking light on the question of the isotopes (q.v.) of a considerable number of elements. incidentally, the positive rays are responsible for two well-known effects with discharge tubes—the pulverisation and “sputtering” of the cathode, and the bombardment and disintegration of the glass walls round the cathode. in the case of coolidge x-ray tubes and wireless valves the vacuum is so high that the positive rays play no effective part and the discharge phenomena are considerably simplified. further, if a leyden jar discharge is employed for a vacuum tube in which an appreciable amount of gas still remains, the discharge passes as a uniform continuous column stretching from anode to cathode without interruption. 925 biplrograpity.—j. j. thomson, conduction of electricity through gases (1893); f. dushman, “ eligh vacuum,” general electrical review (1920); l. dunoyer, vacuum practice (1926); g. w. c, kaye, ‘ production and measurement of iligh vacua,’’ journal royal soc. of arts (1926). (g. w.c. kb.) vaida-voevod, alexandre (1871- ), rumanian statesman, was born at olpret (transylvania) in 1871. he studied medicine in vienna and practised for many years at karlsbad (karlovy vary). at the age of 28 he joined the national rumanian party of transylvania and from 1906-18 sat in the hungarian parliament at budapest, where he waged a permanent fight in defence of the rumanians in transylvania. on the collapse of the central powers in the autumn of 1918, dr. vaida-voevod was instructed by the newly formed “ ru- manian national committee,” to voice their claims in the hun- garian parliament. invoking the right of self-determination he denied the right of the hungarian govt. to speak in the name of the rumanians of transylvania and claimed that the latter should have their own representative at the peace conference. soon after, the rumanian national committee took adminis- trative control of transylvania and delegated dr. vaida-voevod to represent the rumanians of transylvania and the banat at the peace conference in paris. | in ro1g dr. vaida-voevod was appointed prime minister and minister for foreign affairs of rumania and in the latter capacity went to paris and london early in the following year to negotiate with the french and british governments the recog- nition of the union of bessarabia with rumania. this he suc- ceeded in doing in march 1920 when the rights of rumania over bessarabia were recognised by the supreme economic council, and afterwards ratified by the treaty of london oct. 28 1920. dr. vaida-voevod resigned in the same year and afterwards sat in parliament as one of the leaders of the national rumanian party. vail, theodore newton (1845-1920), american capital- ist, was born in carroll co., o., july 16 1845. educated at the morristown academy, n.j., for two years he studied medicine. meanwhile he had become interested in telegraphy. in 1868 he became station agent and telegraph operator on the union pacific at pine bluffs, wyoming. then he was appointed clerk in the railway mail service, and his efficiency led to his being made in 1873 assistant superintendent of railway mail service, rising in 1875 to general superintendent. in 1878 he was made general manager of the american bell telephone company. in 1885 he resigned and was elected president of the newly organ- ised american telephone and telegraph co., which in 1900 acquired the american bell telephone company. in 1887 because of ill-health vail retired. | during a visit to south america he studied traction problems, and in 1896 installed an electric railway system in buenos aires, and later introduced telephone systems in many south american cities. in 1907 he was again induced to accept the presidency of the american telephone and telegraph company. when this company in 1910 bought control of the western union tele- graph co., vail was made president of the latter also, and intro- duced many changes, including ‘ night letters”? at reduced rates. when in 1914, as the result of a threatened suit by the government, the western union was again segregated, vail remained president of his old company. after the taking over of the wires in aug. 1918, by the government as a war measure, he was appointed advisor by the postmaster-general. when the wires were returned in 1919 to private ownership he was elected chairman of the board of directors of the amcrican telephone and telegraph company. he died in baltimore april 16 1920. see a. b. paine, zn one man’s life (1921). valparaiso, chile (see 27.865) has a population of about 185,- ooo. its modernisation may be said to date from the earth- quakes of 1906-7. from ashes and debris streets were widened and paved, better business structures gradually arose and modern sewerage provided a cleaner city. a striking feature of the newer valparaiso is the increased height of the buildings, many being five or more stories. harbour improvements, begun 926 in 1912, represent an outlay of several million pounds. the great depth of the water retards progress, but two breakwaters, building outward from opposite shores of the bay, have attained sufficient length to protect shipping. eventually 220 ac. of water space will offer protection. in 1923 vessels began to dock at modern piers; a mile of semi-circular stone seawall, scores of new warehouses, modern cargo-handling machinery serve annual imports of 700,000 and exports of 400,000 tons. the city’s civic improvements are striking and beautiful. the chilean naval academy buildings and fine residences are located on heights overlooking the bay. improved roads connect the upper and lower city. there are numerous passenger ele- vators. the suburban resort, viria del mar, is noted for its splendid buildings, amusements, flowers and society. a railway 117m. to santiago, electrified in 1924-5, relieves port congestion. from valparaiso good highways are gradually extending inland and to the capital. all classes of motor cars are multiplying. public utilities, advertising and homes are using electricity to an increasing degree. valparaiso province has about 600 manu- facturing enterprises. factories in or near the city are producing railway cars, clothing, furniture, wagons, shoes, soaps, perfumes and other classes of goods formerly imported. vambery, armin (1832-1913), hungarian oricntalist and traveller (see 27.876), died at budapest sept. 15 1913. vancouver, british columbia (see 27.883), the fourth city of canada and its chief pacific port, had an estimated population of 230,000 in 1923 (the suburbs of north and south vancouver and point grey included). the population of vancouver proper was 117,217, including about 10,000 orientals, in 1921. the increasing oriental trade and the shipping of wheat to europe by way of the panama canal have revolutionised the status of the port in recent years. in 1912 there were no shipments of grain or flour, but in 1924 53,240,500 bu. of wheat passed through the harbour. in 1912, 9,382 vessels entered, and in 1922 16,641 with a larger tonnage than that of montreal in the same year. the wheat storage capacity of the port is now very large, and the whole crop of alberta can be handled. the export trade is particularly active when the great lake ports are closed by ice. trade is increasing rapidly also in other directions, espe- cially in the export of lumber. harbour improvements include two large piers, one built by the canadian pacific railway and one by the harbour commis- sioners, which increase the capacity of the port for coastal ship- ping by 35%. twenty-four sea-going vessels can now be berthed at once, and further works are in hand. the port is a terminus of the canadian pacific and canadian national railway systems and, of over 50 steamship lines. industrial development is also proceeding rapidly, especially on granville island, a large block of reclaimed land near the business district. south vancouver is a separate municipality, mainly industrial, with a population of 32,182 in 1924. north vancouver, on the north shore of bur- rard inlet, nearly two miles from the centre of the city, had a population of 13,200 in 1921, and a bridge to connect it with vancouver proper was in course of construction in 1924. point grey, with a population of 22,000, is a growing residential suburb to the south and west of vancouver. the permanent buildings for the university of british columbia, which has absorbed the british columbia branch of mcgill university were begun at point grey in 1921. stanley park, an arca of virgin forest, is now the property of the dominion. vandervelde, emile (1866- ) belgian statesman, was born jan. 25 1866 at ixelles near brussels. he studied law at brussels university and in 1885 took his doctorate in social science. in the following year he joined the belgian labour party and soon became its acknowledged leader. he first en- tered parliament in 1894 as socialist member for the charleroi constituency, but after ro0o was returned continuously by brussels. in the chamber he achieved both influence and prestige and played a prominent part in the struggle to attain universal suffrage, a struggle which resulted in more than one national gen- eral strike. on the outbreak of the war, vandervelde devoted himself to the problems of national defence, the liberation of his vambery—variable transmission gears invaded country, and in aug. 1914 was summoned to join the government as minister of state, later becoming member of the cabinet. at the time of the peace negotiations and the signing of the versailles treaty, he used his influence to obtain the insertion of labour clauses, relating especially to the eight-hour day. as minister of justice in the “‘ liberal-catholic-socialist ”’ cabinet formed after the war, vandervelde eifected great humanitarian and scientific reforms in the prison system. after the important successes achieved by the labour party at the general elections of 1925 he entered the ‘“ socialist-catholic coalition ” cabinet as minister for foreign affairs and played an important part in negotiating the locarno pact in 1925, which he signed on behalf of belgium. | considered by some to be the most powerful socialist orator in the french language since the death of jaures, vandervelde has played a very conspicuous rele in all the international socialist congresses since the beginning of the present century. his speeches in the congresses of amsterdam (1904), stuttgart (1907), copenhagen (1910) and the berlin international con- ference of 1922 were epoch making. in 1922 he went to moscow to defend before the sovict tribunal the ‘“‘ revolutionary social- ists’? threatened with capital punishment by the bolshevik authorities, and managed to save their lives. he was for many years president of the international socialist bureau, and it was due to his interest in the education of the working classes that the “ artistic section ” of the brussels ‘‘ house of the people ” was formed in 1891. in 1924 vandervelde was made professor of political economy at the university of brussels. he wrote for this encyclopedia part of the article on belgium and was elected a member of the belgian academy of moral and political sciences. bibliography.—among his works are le socialisme en belgique, with j. destree (1898); le collectivisme et l’evolution industrielle (1900, trans. by r. p. farley 1907); essats sur la question agraire en belgique (1902); la belgique et le conge (1911); la greve generale en belgique, avril 1913 (1914); le socialisme contre i’ etat (1918); realisa- tions socialistes (1923); le parti ouvrier belge, 1885-1025 ae + lo) van dyke, henry (1785 2- ), american writer, was born at germantown, pa., nov. 10 1852. he studied at the brooklyn polytechnic institute, princeton university (mla., 1876), the princeton theological school and the university of berlin. in 1879 he entered the presbyterian ministry, being pastor at new- port, r. i., and from 1883 to 1900 pastor of the brick presby- terian church, new york city. in this capacity his preaching gave him a national reputation. from 1900 he was professor of english literature at princeton. during 1902-3 he was moder- ator of the presbyterian church in the united states. in 1908 he was appointed american lecturer at the sorbonne and in 1913 was appointed, by president wilson, minister to holland and luxembourg, but resigned in 1917 to enter the u.s. navy as lt.-commander, chaplains’ corps. his books, both prose and in verse, give him a high place in modern american literature. among his best works are his “ outdoor essays,” especially little rivers (1895) and fisherman’s luck (1899). his other publications include the reality of religion (1884); days off (1907); the house of rimmon (1908); out-of-doors in the holy land (1908); collected poems (1911); the unknown quantity (1912); fighting for peace (1917); the valley of vision (1919); camp-fires and guide-posts (1921); half-told tales (1923). van horne, sir william cornelius (1843-10915), canadian financier (sce 27.894), died at montreal sept. 11 1915. van’t hoff, jacobus hendricus (1852-1911), dutch chemist and physicist (sec 27.896), died at steglitz, near berlin, march 1 1ort. vaporisation: see heat. variable transmission gears.—this article is con- cerned only with variable transmission of power, and not merely variable speed transmission, otherwise it would have to include the consideration of a multitude of appliances such as speed indi- cators, integrators and many kinds of calculating machines. power transmission gears may be divided into two classes (1) that in which there is a step by step change and (2) that in which variable transmission gears 927 the change is gradually effected and an infinite range of velocity ratio is possible. step by step variable gears are familiar to all who have passed through an engineer’s workshop, in which lathes, drilling machines and other tools operated by stepped belt pulleys or trains of wheels of different size, these devices being employed to effect variable transmission. a familiar example on the roads is the motor-car, in which with very rare exceptions the change of speed is effected by means of a gear box containing a number of different sized pairs of tooth wheels, arranged so that a change of speed can be effected at will by operating a gear lever. on the other hand infinitely variable transmission is less familiar and until comparatively recent times had no particular application except for very light powers. the increasing employment of infinitely variable transmission is due to the fact that the electric motor, steam turbine, but most of all the internal-combustion engine (g.v.) have for many purposes superseded the steam engine. one feature of the greatest value in the steam engine js its flexi- bility, the inferiority in this direction of other kinds of motive power being very marked. hence there have been continual at- tempts to find either by frictional, electric, hydraulic or other means some form of infinitely variable transmission. a case where variable transmission would constitute a vital factor is the heavy locomotive, and here up to the present no effective variable transmission has been devised which can re- hm | = = nt iw : ii nn fic. 1.—diagram showing the step pulley system of frictional gear for an ordinary lathe. place the ficxibility of the steam locomotive, although there are enormous tracts of country where water is scarce, which the internal-combustion locomotive might open up to civilisation. probably more inventive effort has been employed in finding some form of infinitely variable transmission for motor-cars than for any other purpose, but step by step mechanism with all its obvious disadvantages still holds the field, being up to the pres- ent superior in the two vital points of weight and cost. before proceeding to describe various types of variable trans- mission it must be understood that it is quite possible to vary the speed of transmission by a slipping clutch and claims are often put forward for such a clutch as effecting infinitely variable transmission. this, however, is not correct, being only a case of variation in speed by a direct loss of energy and not true variable transmission. the true test of variable transmission is as to whether it fulfils, at any rate approximately, what is called the law of work that “‘ what is lost in force is gained in speed ” and vice versa. thus, for instance, apart from the fic, 2.—diagram showing an infinitely variable transmission by pulleys. unavoidable loss of efficiency due to friction either with a train of wheels or a combination of belt pulleys, these obey the law of work and overcome a greater resistance by a corresponding reduction of speed. on the other hand, if a lesser resistance is to be overcome the rate of operation can be proportionately in- creased by suitable alteration of the train of wheels employed. classification of methods.—in order to understand the different types of variable transmission they may be classified under the fic. 3.—modern form of variable transmission, as applied to motor-bicycles. 928 following heads: frictional, mechanical, pneumatic, electric and hydraulic. frictional.—tig. 1 shows the step pulley system for the ordi- nary lathe, the lower series operating in the headstock of the lathe, and the upper on the driving shaft. as the belt is shifted from left to right off and on the different pulleys the speed of the lathe can be increased; or when from right to left the opposite | | sat “ji ang oe ell ssk —— wn in ny am, ae = ati “ i ees t=\" fig. 4.—cross section of pair of expanding pulleys. effect is obtained. corresponding to step by step movement, fig. 2 shows an infinitely variable transmission by pulleys which forms part of the cotton spinning device of houldsworth invented just a century ago. here the variation of speed between the driv- ing and driven pulleys is obtained by shifting the belt to and fro along drums of nearly conical form. fig. 3 shows a modern form of variable transmission as applicd very successfully to motor bicycles. here the pulley is of a “‘ v” shape, shown in cross section, and having driving and driven pulleys both capable of expansion and contraction. this expansion or contraction is effected by shifting in one direction or another the conical mov- able side of the pulley. the closer the sides come together the greater will be the effective diamcter of the pulley. this device is given in greater detail in fig. 4 which figure shows the pair of pul- leys in cross section. in the latter diagram it will be scen that powerful springs ensure that the driving belt shall be gripped on both pulleys, an idle wheel running between two pulleys in order to accurately secure the correct location of the conical surface. a development is now taking place of this type of variable transmission for larger powers. fig. 5 shows how the variation of the distance apart of the conical sides of a driving and driven pulley is effected when much power has to be transmitted. the hand wheel (uw) operates a combination of worm and worm wheel (b) so as to turn to other worms (cic2) and thereby move levers, fic. 5.—diagram showing how the variation of the distance apart of the conical sides of a driving and driven pulley is effected when much power has to be transmitted. variable transmission gears which respectively operate the movable cheeks (d,d.) of the conical pulleys either inwards or outwards. probably the oldest type of infinitely variable gear is that shown in fig. 6, in which a friction roller is moved edgeways to and fro on the disk or cir- cular flat plate g. if, as in a certain type of motor-car, 7 is the fic. 6.—infinitely variable gear in which a friction roller is moved edgeways to and fro ona disk. ; driver, the speed of the driven disk » is least when r is nearest the edge, and the greatest driving force is then obtained. on the other hand, when r has approached towards the centre of the disk p the greatest speed is obtained with corresponding reduction of driving force. \"the power transmitted with this gear is how- ever comparatively small as it depends entirely on frictional con- tact of the surfaces and the wear of the frictional surfaces is considerable. a novel type of variable friction gear was exhibited at the london power exhibition in 188s, operating a sackhoist. this transmission, although not of much practical importance, has certain interesting features, and although not used for power pur- poses has been largely employed as a spherical integrator. the principle of operation is the movement of a sphere which in the above case was a steel ball. this sphere was supported ina frame by four friction rollers, the frame being pivoted on a ver- tical axis. in contact with the sphere, in a horizontal plane, were the driving and driven rollers, the axes of which were at right angles to each other. by swivelling the frame the relative velocities of driving and driven wheels could be altered between any required velocity ratios; a discussion of the theory being given in phil. trans. roy. soc., part ii. 1883. mechanical —variable transmission of mechanical kind can be said to be practically confined to the employment of trains of toothed wheels. ‘the most familiar case of all is that of a lathe headstock shown in fig. 7, in which by coupling up the largest fig. 7.—variable transmission as used in a lathe headstock. step pulley to the spur wheel to the spindle of the headstock a direct drive 1s obtained from the speed pulleys. if this coupling is released and by means of a handle shown in the figure the back axle is approached to the main spindle so as to bring into engagement the two pairs of tooth wheels, the driving now takes place through the pinion driven by the step cone pulley variable transmission gears through the axle shaft, and back to the lay spindle. increase of driving force is thus obtained which in the present example is nine times as great as by the direct drive. fig. 8 shows the clutch type of motor car change speed gear box, in which the engagement of different ratios of spur wheels and pinions is obtained by sliding the teeth sideways into mesh with each other. this is called the “‘ clash” engagement. in rws sse sebel uate egfr eae fic, 8.—clutch type of motor car change speed gear box. this diagram the whole transmission is shown out of gear. in another type of gear box no such sideway sliding takes place, the teeth of the respective wheels being always in mesh with each other. change of gear is effected by sliding the different jaw clutches into engagement with each other. tor many years inventors have aspired to produce an infinitely variable mechanical transmission on the principle of ratchet motion, in which by altering the positions of the fulcrum of a lever operating a driving pawl, relative variation of force and speed is obtained without any variation in the driving effort. one form of this gear of extremely interesting character is a french invention by de lavaud which operated quite effectively on a small motor car. more attention, however, has been at- tracted by the torque converter of constantinesco. the latter is a ratchet gear, but has practically no resemblance to the gear of de lavaud and appears to introduce an entirely new mechanical principle, in which as the velocity of the motor increases the inertia effect of a weight changes the position of the fulcrum of the lever and causes an impulsive effort to be given to what is called the secondary shaft by the movement of what is called the primary shaft. although this gear has been applied both toa motor car and to a locomotive, as well as to other purposes with remarkable results, it can scarcely be said at present to be beyond the experimental stage, the difficulty of the invention being to find an effective friction ratchet, or as the inventor calls it, a ““ mechanical valve,’’ which will satisfy all the conditions of prac- tice. pneumatic—many attempts have been made to introduce pneumatic variable transmission since 1900 by dunlop, lenz, lebach and others. the best account of these pneumatic de- vices as applied on a large scale to locomotives was given in la revue generale des chemins de fer, may 1923, by brille. up to the present these devices, though extremely interesting and ingenious, have not proved efficient in practice. electrical.—various inventors have suggested continuously variable electric change speed gear. as already explained a mere reduction of effort by interposed resistance is not a true variable transmission, and all the most important examples of real variable electric transmission as on electric railways, is of essentially a step by step nature. locomotive systems such as crochat’s mine locomotive, the loco-tractor of moyse, those of nn 929 westinghouse, of sultzer diesel and dewa are on the same fun- damental principle as the tilling-stevens electric transmission, namely, production of electrical current by a dynamo from an internal-combustion engine and the operation by an electric motor of some form of gearing to turn the wheels. in the electric railways there is no variable transmission apart from the driving motor itself, and as far as the writer is aware there are no cases on a large scale of an internal-combustion motor operating di- rectly through self-contained electrical variable gear. hydraulic—the last form of transmission, and in its de- velopment the most recent, is hydraulic transmission. a beauti- ful device by hastie was described as long ago as 1881 in the gth edition of the encyclopedia britannica, in an article on hydro- mechanics. by means of two springs the stroke of the crank can be altered, when the crank pin slides in a groove. when the effort is great the springs are compressed and the stroke is increased correspondingly, enabling a constant hydraulic pressure to overcome a variable resistance. this resistance may vary within wide limits, with the corresponding economy in the matter of the fluid employed. in the foregoing case the working fluid was water supplied by a high-pressure hydraulic main on the principle first introduced by armstrong. modern hydraulic variable gear in which oil is the working fluid has been brought into extensive operation chiefly to enable electrical power to overcome very variable resistance without unduly causing a rise in the amperes transmitted. the principle of operation is exactly identical with the generation of electrical current by a dynamo, which current is transformed into mechanical effort by an electri- cal motor. in the case of hydraulic transmission, however, a variable stroke pump is employed to produce a flow of incom- pressible fluid, generally oil, which operates in a hydraulic motor. thus it is only necessary to effect a change in the stroke of the pump when any required hydraulic pressure can be obtained without unduly increasing the driving effort on the pump. hydraulic variable gear has been employed for a great variety of purposes such as cranes, lifts, winches, tipping wagons, rail cars, gun mountings, planing and broaching machines, swing bridges, etc. the chief types of these gears may be distinguished as parallel and radial. fig. 9 shows a section of the janney- williams gear (cither pump or motor) which is an example of the former type, and it will be seen that the variation in effort is attained by varying the angle of what is known as the swash = fic. 9.—section of the janney-williams hydraulic variable gear. plate, with the result of altering the stroke of the pistons. as the stroke is reduced the working pressure can be increased; thus when the stroke is very small a very great pressure can be produced and consequently great resistance can be overcome. the necessary valve action is obtained by causing the opening at the inner end of the cylinders to pass alternately in succession supply and exhaust ports. a fig. to shows the operation of one radial type pump. the cylinder body revolves about a fixed axis which forms a cylindri- cal valve. as the cylindrical body revolves, the working iluid is alternately admitted from and exhausted into the two ports u and p respectively. ‘the fluid comes in under no pressure from the 930 port a and is driven out under any required pressure from the port ~ along the shaft. in order to obtain any required variation of pressure the frame to which the connecting rod ends are at- tached is made to pivot about the fixed centre ¢ by a rod attached fic. 10.—diagram showing the operation of one radial type pump. to the upper end of the frame. in the middle position there is obviously no stroke, and when the neutral position is passed the direction of flow is reversed. two views of another type of radial stroke pump are shown in fig. 11 (hele-shaw pump), the left-hand view being in half section, the lower portion of which shows ff a revolving case n ss sre fic. 11.—two views of the hele-shaw radial stroke pump. called the “‘ floating ring.” the sectional portion in the view shows that this floating ring is carried on ball races 7 and r, their object being to reduce the frictional wear of the rollers or slippers by allowing the floating ring to rotate ircely with the cylinder body, in which the pistons are carried. if the floating ring is moved bodily along guides on the line a-6b (shown in the other view) the stroke of the pistons and the fluid pressure can be varied as required, just as in the previous example. the action of the fixed cylindrical valve is exactly the same as in fig. 10, the employment of the four cylindrical passages in the shaft instead of two being simply a matter of design. variety theatre while the floating ring is found to give almost indefinite life to the pump there was a characteristic hum or noise which was after years of research traced to the same deformation that pro- duces the sound in a bell. the latest form of radial variable stroke radial pump, while employing the same type of cylindrical valve is found, when properly designed, to be almost silent. the floating ring is dispensed with, variation of stroke being obtained by allowing the pistons, which work inwardly instead of out- wardly, to be operated by the equivalent of a crank in the form of a live ring mounted with roller bearings on a compound eccen- tric. the compound eccentric consists of two parts of equal eccentricity, the stroke varying mechanism being arranged to effect rotation of these two parts in opposite directions with the result that the resultant eccentricity may be varied at will from zero to maximum in either direction. this gear was fully described in engineering, oct. 16 1928. biibliograpiy.—a. graham clarke, text book on motor car fen- gineering, vol. 1, construction (1911); e. butler, transmission gears (1917); f. d. jones, afechanisms and mechanical movements (1918); w. hi. berry, afedern aotor car practice (1921); g. constantincesco, variable transmission for automobiles (1924); g. h. c. hartman, les mecantsmes (1925). (h. s. h-s.)",
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