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    "source_title": "Encyclopaedia Britannica (1926)",
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    "title": "EXPLOSIVES, MILITARY",
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    "verified_text": "for military purposes, explosives are divided into two main classes: (a) propellants; (6) high explosives (disruptives). the difference in functioning of these two classesis obtained by regulating the rate of explosion, 7.e., the rapidity with which the explosive is converted into gases. in the case of a propellant, so as not to burst the gun, a relatively slow rate of transformation, corresponding to a prolonged push is required; whilst a very high rate (detonation) which is equiva- lent to a sudden blow, in order to cause fracture by shock, is ne- cessitated in explosives used for disruptive purposes. it follows, therefore, that the rapidity of the explosion or burning of a pro- pellant, must be strictly controlled and that of a disruptive enhanced as far as possible, so as to berate almost instanta- neously the whole of its potential energy, if the utmost violence is desired. propellants and their nature it was found that the temperature developed by the explosion of mark i. cordite (7.138) was unduly high, owing to its large content of nitro-glycerine. in order to [essen the wearing and scoring effect of the hot gases on the bore of a gun, known as erosion, it is essential to kcep the temperature of explosion of the charge as low as possible and a “ cooler ” cordite of modified composition, designated cordite m.d., was introduced, and re- mains to this day the standard british propellant for naval and military uses. the composition of cordite m.d. is:— nitro-glycerine, 30 °%. guncotton, bs 6: mineral jelly, as the so-called “ nitro-cellulose powders ”’ do not contain nitro- glycerine and find favour in france, germany and america on account of their lower temperature of explosion, thereby de- creasing the erosion of the gun barrel. they are made from solu- ble nitro-cellulose, which is gelatinised by a mixture of ether and alcohol and then dried in the form of sticks, cubes or flakes, as found most suitable for the particular ordnance intended, high explosives (disruptives)—ligh explosives for naval and military purposes are required for the bursting charges of shell, mortar and aerial bombs, grenades, mines (land ot sea), torpedoes and for general demolition purposes. for such uses, an extremely violent and disruptive effect (brisance) is neces- sary in their explosion, but at the same time a very high order of chemical stability and insensitiveness is called for, particularly in the fillings for shell, which are subjected to an extremely severe shock at the moment of discharge of the gun. to bring about the effective explosion of such stable compounds requires the employment of a powerful initiatory impulse. this may be effected by means of a small quantity of a more sensitive explosive, which then acts as a detonating agent or detonator. a high explosive is said to ‘ detonate ’ when its explosion is propagated at the maximum velocity possible for the particular substance in question. there is no definite line of demarcation between a “ high” and what may for the want of a better term be called a “ low ” explosive, such as gunpowder; the term is purely relative. no intensity of initial impulse can cause gun- powder to explode at a greater velocity than about 350 ft. per second, in the recognised high explosives the velocity of detona- tion is at least 2,c00 or 3,000 ft. per sec. and may reach to as much as 9,000 ft. per second. it is owing to this intensely sudden evolution of gases that the shattering and disruptive effect of high explosives is obtained. in order to appreciate the function of the detonator in initiat- ing explosion, it is necessary to consider that in an explosive substance each molecule in its decomposition gives out a surplus of energy, and so provides the initial impulse required to decom- pose the neighbouring molecules. when, however, a stable high explosive, for instance t.n.t., is merely ignited, the de- composition propagates itself slowly at first and may cease export credits altogether owing to external cooling; in any case, the velocity of decomposition increases but gradually or may even dic out, and it is only after a considerable quantity has decomposed that det- onation may ensue. by employing a detonating agent to initiate the explosion, decomposition of sufficient of the stable explosive in contact with the detonator is ensured, so that the explosion wave 1s propagated throughout the whole mass with certainty. the value of fulminate of mercury, discovered by howard in 1800 as a detonating agent for more stable high explosives, was not recognised till a considerably later date. this sensitive chem- ical compound may be readily caused to detonate “ ci musse ” by heat, friction or percussion, and the detonation is similarly imparted to any high explosive with which the fulminate is in contact. another detonating agent, for similar purposes, which is now increasing in favour is lead azide, for which better keeping qualities, particularly in hot climates, are claimed. by reason of their sensitiveness, the quantities of such detonating agents employed are strictly limited—in the case of percussion fuses, for shell up to about 10 grains. a greater amount of an explosive such as tetryl, which is intermediate in sensitiveness between the detonator proper and the bulk filling, may, if necessary, be used to increase the “‘ send off.” shell filling —for shell filling, very insensitive high explosives are required, on account of the severe shock to which they are subjected at the moment of discharge. in fact it was the fear of premature explosion in the gun which militated for many years against the introduction of h.e. shell. the high explosives most suitable for shell fillmg are nitro-benzene dcrivatives, derived from coal tar. the first to be employed for this purpose was picric acid, tri-nitro-phenol (see 10.82) discovered by woullfe in 1771, and used for many years as a dye without its explo- sive properties being recognised. it was first used in actual war- fare by the british army in the south african war of 1899- 1902, under the name of lyddite. picric acid is also the main or sole constituent of the french melinite, the japanese shimose powder and the austrian ekrasit. 11 has now largely been su- perseded by trinitrotoluene, which is known gencrally as t.n.t. it is very similar in its action and appearance to picric acid, over which it possesses certain advantages in ease of manufacture and facility of manipulation. in the world war economic considerations played a large part in determining the types of explosives used and their meth- ods of manufacture, and so led to the adoption of others hitherto untried and unproved for military purposes. for these reasons, the high explosive which was used in the greatest quantities by great britain was “ amatol,’’ the name given to various mix- tures of ammonium nitrate and t.n.t. the proportion most favoured was amatol 80/20, which contains 80° of ammonium nitrate and 20% of t.n.t., these being approximately the pro- portions necessary for the complete combustion of the t.n.t., through the agency of the extra oxygen required, which is sup- plied by the ammonium nitrate constituent. such an amatol can be compressed into the shell or other munition, in the form of a powder, or loaded in a plastic condition by heating it above the melting point of the. t.n.t. this ts a consideration of great importance, as it enables more rapid output to be ob- tained than in the melt process used with lyddite or t.n.t. the high explosive fillings described for shell are equally ap- plicable to the numerous other kinds of munitions, but in addi- tion wet guncotton has been used for the filling of torpedo war heads and floating mines. for hasty demolitions in the field wet guncotton slabs packed in watertight boxes are convenient, the detonator being inserted before use in a disk or primer of dry guncotton which fits into a correspondingly shaped cavity in the wet slab and thereby brings the whole to detonation. birpliograriy.—lieut.-col. h. w. l. hime, gunpowder and ammunition (1904); sir andrew noble, artillery and explostves (1906); e. m. weaver, military explosives (new york, 1906); i. brunswig, explosives, translated by c. e. munroe (1912); w. r. quinan, igh explosives (1912); a. marshall, a shori account of explosives (1917); s. i. levy, modern explosives (1920); a. marshall, dictionary of explosives (1920); r. c. farmer, the manufacture and uses of explosives (1921). (cod, c.) io95 export credits, a post-war scheme adopted by the british govt. to assist british export trade. the export credits scheme was initiated by mr. lloyd george’s coalition govt. in the autumn of r9r9 as a means of facilitating the finance of ex- port business and of promoting the restoration of british trade. importers in many markets found it difficult at the time to pay cash or do business on short-term bills; and it was felt that trade could be secured if facilities were provided for financing trans- actions which owing to the length of credit required, or for other reasons would not be acceptable to the banks. power was also taken to insure goods against risks of an ab- normal or exceptional nature, but this part of the scheme has remained inoperative. at first the application of the scheme was limited to business with certain european states, viz., finland the baltic states, poland, czechoslovakia, yugoslavia, rumania and others. this restriction was due in part to a concomitant policy of assisting in the economic reconstruction of countries which had been specially disorganised by the world war. in 1921 the sphere of operations was enlarged to include the whole world, subject to certain exceptions adopted in the actual administration of the scheme. the legislation governing the scheme is contained in the over- seas trade (credit and insurance) act 1920; in an amending act of 1921; and in the trade facilities acts 1921-4. it is admin- istered on behalf of the board of trade by the export credits dept. of the dept. of overseas trade, with the assistance of an advisory committee of business men. the aggregate amount out- standing in respect of credits at any given time may not exceed £26,000,000. credits may only be given for british goods. the period within which new credits may be given expires in sept. 1926, but icgislation was introduced in 1926 to extend it for another three years. under the original scheme the export credits dept. made ad- vances to the exporter in cash up to 80% of the cost of the goods, but early in 1921 an important modification was introduced, the object of which was to bring the method of operation into closer correspondence with the normal machinery of trade. instead of making advances the department was authorised to guarantee drafts drawn against the shipment of export goods. the advances system was in consequence discontinued, and the bulk of the busi- ness done under the scheme was effected under the guarantee plan. the modus operand: is that the exporter makes application to the department through his bank (an arrangement introduced so as to preclude overlapping with the banks), and if the proposed credit is sanctioned, the government’s guarantee is given usually in the form of a guarantee endorsed on the actual bills drawn by the ex- porter, but sometimes in the form of a letter of guarantee. an ap- propriate premium is charged. bills may be guaranteed up to 100 % of the invoice value if the length of the credit does not excced one year, and up to 85°, if the term is for more than one year. if ade- quate security is provided by the importer, usually in the form of a bank guarantee, there is no recourse on the exporter in event of loss; in other cases the department takes recourse up to 573 % if 100 % has been guaranteed, and up to 422% if 85> has been guaranteed. it will thus be seen that the exporter gets both assistance in his finance, since the guarantecd bills can be discounted or used as collateral security for a bank advance, and a measure of insurance against loss, since the department, in only taking partial recourse upon him, makes itself responsible for part of any loss which may arise. this is the broad outline of the scheme; there are variants in the actual facilities offered which need not be set out in detail. the total amount of business done up to dec. 1925 was about £1,750,000 under the advances scheme, and rather less than £6,000,c00 under the guarantee scheme. as the limit fixed by the original act was £26,000,000 it is clear that the scheme has not been taken advantage of to the extent expected. this may be due to various causes. the scheme probably has not been very widely known; business men have been perhaps sceptical about the handling of such matters by a government department; there may have been complications and rigidities of practice such as would discourage applicants. published figures, however, show that very much larger amounts of credits have been sanc- tioned than have actually been taken up—presumably because the applicant eventually failed to secure the business for which the credit was required, or perhaps was ultimately able to finance 1096 lye it without assistance from the department. on the whole, the | 7vades factiities and loans guarantee act 1922, session 2 (13 geo. 5 ch. 4); expiring laws continuance act 1923 (13 and 14 geo. § figures indicate that the scheme has been helpful to trade at a ci. s5a paode roclcecdct rosntrg ands ee soh a): quater: difficult juncture. statements of applications sanctioned and credits and guarantees bibliography.—overseas trade (credits and insurance) act 1920 | taken, published in the board of trade journal, london. sg (10 and if geo. § ch. 29), do. do. amendment act 1921 (141 and 12 (w. cl.) geo. 5 ch. 26)! yrade facilities act 1921 (11 and 12 geo. 5 ch. 65); eye: sce ophthalmology, end of twenty-ninth volume printed in u.s.a.",
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