^n^ \vvj ^. ^„^. ^.. \t: IMAGE EVALUATION TEST TARGET (MT-3) 7 // 1.0 ^BS "11^ ^11^ IS 18 11:25 i 1.4 1.6 d i_ Sciences Loiporation 23 WEST MAIN STREET WEBSTER, N.Y. 14580 (716) 87i2-4503 .^ 4» .^^m. S r o f.€^, M3 ^ ,. "wi" A CIHM/ICMH Microfiche Series. CIKIVI/ICiy/gH Collection de microfiches. Canadian Institute for Historical Microreproductions / institut Canadian de microreproductions historiques Technical and Bibliographic Notes/Notes tachniques at bibliographlquas The Instituta has attamptad to obtain tha bast original copy available for filming. Features of this copy which may be hibllographicaliy unique, which may alter any of the images in tha reproduction, or which may significantly change the usual method of filming, are checked below. □ D D D D n n n Coloured covers/ Couverture da couleur Covers damaged/ Couverture endommagie Covers restored and/or laminated/ Couverture restaurie et/nu pellicul^e Cover title missing/ La titre de couverture manque Coloured maps/ Cartes g^ographiques an couleur Coloured ink (i.e. other than blue or black!/ Encre de couleur (i.e. autre que bleue ou noire) Coloured plates and/or illustrations/ Planches et/ou illustrations en couleur Bound with other material/ RellA avac d'autres documents Tight binding may cause shadows or distortion along interior margin/ Lareliure serrde peut causer de I'ombra ou de !a distorsion le (org de la marge intdrieure Blank leaves added during restoration may appear within the text. Whenever possible, these have been omitted from filming/ II se peut que certainea pages blanches ajout«es lors dune rsstaurdtion -pparaissent dans le texte, mais, lorsque cela italt possiMe, ces pages nont pas 6ti film^es. Additional comments:/ Commentaires suppldmentaires: L'Institut a microfilm* le meilleur exemr'aire qu'll lui a 6ti possible de se procurer. Las details de cet exemplaire qui sont peut-dtre uniques du pomt de vue bibliographiqua, qui peuvent modifier une image reproduite, ou qui peuvent exigur une modification dans la m4thode normale de filmage sont indiqu^s ci-dessous. □ Coloured pages/ Pages de couleur □ Pages damaged/ Pages endommagias □ Pages restored and/or laminated/ Pages restaurd<)s et/ou pelliculdes |~1 Pages discoloured, stained or foxed/ i^^ Pages ddcolordes. tacheties ou piquees □ Pages detached/ Pages ditach^es FT] Showthrough/ [^ Transparence □ Quality of print varies/ Quality in^gale de I'impression □ Includes supplementary material/ Comorend du matAriml «iinnlAn.>.. Comprend du material supplementaire Only edition available/ Seule Mition disponible □ Pages wholly or partially obscured by errata slips, tissues, etc., have been refilmed to ensure the best possible image/ Lea pages totalement ou partiellement obscurcies par un feuillet d'errata, une pelure, etc., ont 6t6 film^es i nouveau de facon A obtenir la meilleure image possible. This item is filmed at the reduction ratio checked below/ Ce document est filmd au taux de reduction indiqui ci-dossous. 10X 14X 18X 22X 26X 30X y 12X 16X 20X 24X 28X ] 32X aire details ues du modifier g.v read /v " 140, " 6 " " for t" " t"K 46-I4.S 49-^52 J I*A.RT I. EXPLOSIVES, THEIR USE FOR MILITARY ENGINEERING LAND OPERATIONS. md to ' Aide rs," by I'^BT I. EXPLOSIVES. THEIR USE FOR MILITARY ENGINEERING LAND OrERATIONS. INTRODUCTION, 1. The purposes for which explosives are used in Milil.iiy Enp^inccring Land operations are numerous, and the effects to be produced vary considerably. The Military Enp^ineer must, there- fore, be acquainted not only with the methods of usinjjf the vari- ous explosives, but also (in order that he may be able to select the explosive best suited to each particular case) with their qualities and capabilities. This shows that the subject divides itself natu- rally into two distinct parts ; the first is the inquiry into the qua- lities and capabilities of the various explosives, and the second is concerned with the methods of preparinj:^ explosives for use and of i^niitinj; them, which includes the methods of testinj; adopted in order to minimize the risk of failure. To this must, however, be added a third consideration, namely, the storage of explosives. The study of the above will be greatly assisted by a knowledge of the manufacture of explosives, but more especially by an acquaintance with the chemical composition of the various explo- sives and with the changes produced by their ignition. These questions are, however, treated of in other Courses*, and will therefore not be considered here. *In the Artillery and Chemistry Courses respectively. 'I :\ CHAPTER I. lii QUALITIES AND CAPABILITIES OF EXPLOSIVES. 2. When owing to chemical decomposition and recombination a lar,-e volume of gas and a large increase of temperature are very rapidly produced, an explosion ensues. 3. 1 he explosive effect depends on the rate at which the che- mical action proceeds, that is, on the time taken to convert the explosive into gas ; and the shorter the time the greater and more violent tne effect. In the special case when the conversioTis almost instantaneous the explosion is called a detonation, and the effect produced is that due to an impulsive force or a blow. The rate of explosion is influenced by the following circum- (I.) By the physical condition -^the explosive. For instance to tirri'' "^''^' .more rapidly than pebble powder owing to tlie ditlerence in the size of the grams. .|f •) ^y tlie mode of ignition. Thus dry compressed guncotton will, if ht by a match, simply burn very rapidly but will not de- tonate, and gunpowder can h^ detonated under suitable conditions. There are two methods of igniting an explosive. (a) By the direct contact of a heated body. (h) By the previous explosion by heat, percussion or chemi- cal action* of a small quantity of a sensitive composition (such as the mercuric fulminate.) Detonation is practically obtained by the second method. 4. Fhe explosive eTect further depends on the degree of con- finement (or tamping) to which the explosive is exposed before Ignition. Thus gunpov/der produces but little useful effect unless strongly confined, but guncotton, dynamite and such like exolo sives, whon detonated, have a vjry considerable effect without any tamping ; their power is, however, increased by confinement. l-otasY"' "''*''"""' *'*' *''^'''" "'' ""'^''""" ''""* "" '•* ""^*"'-<^ "t' «»gar and chlorate of RATE OF EXPLOSION. 3 This effect of tamping' is in reality due to the rate of explosion, for in the case of detonation the transformation into gas is so rapid that the resistance of the air is sufficient tamping. The following extract illustrates this point : "With nitro-glycerine a volume of gas, nine hundred times that of the liquid used, is set free all but instantaneously. * * * It can readily be seen that the sudden development of this large volume of gas, which becomes at once a part of the atmosphere, would be equivalent to a blow by the atmosphere against the rock* ; or, what would be a more accurate representation of the phenomenon, since the air is the larger mass, and acts as the anvil, a blow by the rock against the air. It may seem very sin- gular that our atmosphere can act as an anvil, against which a rock can be split, and yet it is so, and, if the blow has velocity enough, the atmosphere presents as effective a resistance as would a granite ledge. The following consideration will, I think, convince you that this is the case : I have here a light wooden surface, say, one yard square ; the pressure of the air against the surface is equal, as I just stated, to about nine tons ; but the air presses equally on both sides, and the molecules have such great mobility that, when we move the surface slowly, they readily give way, and we encounter but little resistance. If, however, we push it rapidly forward, the resistance greatly increases, for the air- molecules must have time to change their position, and we en- counter them in their passage. If, now, we increase the velocity of the motion to the highest speed ever attained by a locomotive -—say, one and one-fifth mile per minute — v/e should encountei still more particles, and find a resistance which no human muscle could overcome. Increase that velocity ten times, to twelve miles a minute, the velocity of sound, and the air would oppose such a resistance that our wooden board w^ould be shivered into splinters. Multiply again the velocity ten times, and not even a plate of boiler-iron could withstand the resistance. Multiply the veloci-' ' once more by ten, and we shouldreach the velocity of the earth in its orbit, about 1,200 miles a minute, and, to a body moving with this velocity, the comparatively dense air at the surface of the earth would present an almost impenetrable barrier, against which the firmest rocks might be broken to fragments. Indeed, this effect has been several times seen, when meteoi ic masses' moving with these planetary velocities, penetrate our atmosphere. The explosions which have been witnessed are simply the effect of the concussion against the aeriform anvil at a point where the atmosphere is far less dense than it is here. So, in the case of the nitro-glycerine, the rock strikes the atmosphere with such a velocity that it has the effect of a solid mass, and the rock is shivered by the blow."t xi^. *'*i'i"idge of nitro-glycerine is supposed to have been placed on a rock. trhe New Chemistry, by Joaiah P. Cooke, Jr. ^■ f I I i f \ t! 4 QUALITIES AND CAPABILITIES OF EXPLOSIVES. DETONATION. 5. The effects produced by an explosive when detonated are so important to the Mihtary Engineer that detonation must be con- sidered a httle more in detail. 6. It has already been said that an explosive detonates when the transformation into gas is almost instantaneous. Certain substances in unstable chemical equilibrium are those most read- ily detonated, and once the equilibrium of any molecule is dis- turbed, the transformation is transmitted with enormous rapidity throughout the mass. Experiments have been made to ascer- tain the velocity with which detonation travels, and the following are some of the results : The velocity of detonation of dry gun- cotton varies from 17,500 to 20,000 feet per second; but the rate of detonation is greater in wet guncotton, thus the guncotton which, when dry, detonated at the rate of 17,500 feet per second, detonated at the rate of 20,000 feet per second when saturated with water. The rate of detonation of dynamite was found to range from 19,500 to 21,600 feet per second, but that of ni! o- glycerme is only 5,500 feet per second, which is probably due to the explosive being liquid and unconfined* As a comparison it may be stated that in a hose filled with gunpowder the explosion travels at the rate of from 10 to 20 feet per second. 7. The sensitiveness of explosives to detonation varies consid- erably. Some explosives, for instance the terchloride of nitrogen, t are so sensitive as to be, at present, practically useless. Others, such as the mercuric fulminate, although readily detonated by simple percussion or by the application of heat, can be safely used m small quantities. And lastly, guncotton, dynamite, etc., cannot be readily detonated by percussion or heat, but the more sensitive explosives are capable of inducing their detonation. Thus, in practice, fulminate of mercury is employed to start the detonation of guncotton and dynamite. 8. " The ma ner in which a detonation operates in determining the violent explosion of guncotton, nitro-glycerine, etc., has been the subject of careful investigation. It has been demonstrated experimentally that the result cannot be simply ascribed to the direct operation of the heat developed by the chemical changes of the charge of detonating material used as the exploding agent. *These experimeuts were carried out by Professor Abel by stretching insulated wires across a row of guncotton discs at intervals of six feet. The rupture of these wires, by the detonation, gave spark records on the cylinder of a N^oble's chronoacope from which the velocity was i;alculated. The experimeuts with dynamite and nitro- glycerine were made in a similar manner. tSee § 121 Bloxam's Chemistry, 4th Edition. t )NATION. 5 An experimental comparis'. the mechanical force exerted by different explosive compou , and by the same compound ex- ploded in different ways, has shown that the remarkable power possessed by the explosion of small quantities of certain bodies (the mercuric and silver fulminates) to accomplish the detonation of guncotton, while comparatively very large quantities of other highly explosive agents are incapable of producing that result, is generally accounted for satisfactorily by the difference in the amount of force brought to bear suddenly upon some poi tion of the mass operated upon. Most generally, therefore, the degree of facility with which the detonation of a substance will develop similar change in a neighbouring explosive substance may be re- garded as proportionate to the amount of force developed within the shortest period of time by that detonation, the latter being, in fact, analogous in its operation to that of a blow from a ham- mer, or of the impact of a projectile. "Several remarkable results of an exceptional character have, however, been observed by the author, which indicate that the development of explosive force, under the circumstances referred to, is not always ascribable to the sudden operation of mechani- cal force. These were especially observed in the course of'a com- parison of the conditions essential to the detonation of guncotton and nitro-glycerine by means of particular explosive agents (such as the chloride of nitrogen), as well as in an examination into the" effects produced upon each other by the detonation of those two substances, nitro-glycerine being very susceptible of explosion by guncotton, while the detonation of the latter can only be accom- plished by comparatively large quantities of nitro-glycerine. The explanation offered of these exceptional results is to the effect that the vibrations attendant upon a particular explosion, if syn- chronous, with those which would result from the explosion of a neighbouring substance in a high state of chemical tension, will, by their tendency to develop those vibrations, either determine the explosion of that substance, or, at any rate, greatly aid the disturbing effect of mechanical force suddenly applied ; while, in the instance of another explosion, which develops vibratory im- pulses of different character, the mechanical force applied through its agency has to operate with little or no aid ; greater force, or a more powerful detonation, being, therefore, required in the lat- ter instance to accomplish the same result."* 9. The following facts, established by experiment, exemplify the above remarks, and also illustrate some points on the trans- mission of detonation. t *The Recent History of Explosive Ageuts by Prof. Abel, F.ll.S., Professional Papers, Eoyal Engineers, 1874. tExtractud from "Notes ua Gunpowder and Guncotton," by Major Wardell, R,A. , ;i ! 6 QUALITIES AND CAPABILITIES OF EXPLOSIVES. (I.) A fiue containing over i oz. gunpowder, strongly confined exploded in contactvviti, compressed guncotton, only inflames k although the explosion of the fuze is apparently a sluirp on^ n}nl ^7*y-fiy^ e^^'-'^i'^s oi mercuric fulminate, exploded unconfined . on the surface of compressed guncotton, only inflames or disperses (3.) A fuze containing 9 grains of mercuric fulminate, strondv confined, exploded tn contact with compressed guncotton, detonates It with certainty. (4.) An equal quantity of fulminate, similarly confined does not detonate nncompresscd guncotton, in which it is en'bedded but merely disperses and inflames it. ' (5.) One hundred and fifty grains of compressed guncotton de- tonated in proximity to dynamite, detonates the latter. (6.) Three ounces of dynamite, and much larger quantities detonated in contact with compressed guncotton only disperses it.' (7.) Detonation being established at one extremity of a con- tinuous row of distinct masses of compressed guncotton or dyna- mite, travels along the whole length thereof. . . :. Jll ,A/«V^?"''^"°" ^'f^' ^° ^^^^ ^°"&' o'S inch apart, can all be detonated from one end. (9.) Discs of guncotton, weighing about 8 ounces each, placed six inches apart in the open, are blown away or broken up by the detonation of the central disc. ^ ^ (10.) A disc of guncotton, 2 ounces in weight, inserted into a wrought iron tube 5 feet long, and detonated, transmits the deto- nation to a similar disc at the other extremity of the tube. EXPLOSIVES USUALLY EMPLOYED FOR MILITARY ENGINEERING PURPOSES, AND THEIR APPLICATION. 10. The explosives principally used in the British Service are Gunpowder, Guncotton, and the Mercuric Fulminate. In some foreign countries dynamite is used instead of guncotton but in France dynamite has lately been replaced by guncotton. Besides these there are numerous explosives in the market, such for in- stance as Nitro-glycerine, Lithofracteur, Dualine, Cotton-powder Glyoxiline, and Horsley's powder, the effect of the explosion of which IS generally similar to that of guncotton, but they do not appear to be as suitable for Military purposes as the Ser .ce article. I -' ^'-" <-* use in the field and at stafnrfi^f ^^? ^^^" constructed for safely and expeditious^. ''^''"'' ^"^ ^^^^^'"^^ ^"t this operation an^^ d!;^;5S- X;h!^^^^^r ^^^t ^^^^^ - ^ ^«^^er are placed at a distance of abiut 6 fee fL ^^""""^ ^° ^^ "sed, connected together by mean, of ! • ?-^ ''^^^^ ^^her, and are length. The boiler is ^of?opperLd"i.'''^^^'^" '""^^ ^^ th^ of felt and sheet zinc. It is heated bt P'^'^^'M with a covering IS furnished with a .iJZt^lt^,' I ''''''-' ^^-coal fire, anf a short exit pipe, on to which ss]inn'^?;''°^K ^^^ ^^^^^^^ and rubber tube, wliich connects k w-fth t^^f d^M vulcanized india- chamber. It is also providedv i J n I "K^^."^°"°"^ed dryin/:r "u^e closed at the to^p with a diaphra^^^^^^ ^'^'^^ «" a ted with a damper, which is onlv to KpT / ''''^^ ^^^"^^' ^nd fit- the apparatus is in use. To the stand of tf f !"^" °P^" ^^hile fire ^xratin^, is attached a trav in wHrt '^ '^°''^^"' ^^"^^th the he fire is burning. The dryi^ clamber coV' 1° ^'.^^P^' ^^'^^^^ steam chest, made of stout sheet t^nnni S?f'T^'.°^ '^" °I^'on^ pipes at the two ends, one for ?iie entrance of . '"''^ '^^'° ^'^"^^ indiarubbertube, the other forte escane of ^f '*''"" ?"'°"^^^ ^he water The sides of the steam chestTre'r " "«"^^"«-^ towards the top, on to which fi^s a hood of b, I 1?",^' constricted handles, and provided with a shorf .n ""^ ^\''^^}^ sheet tin, with tube havin,. ^number o erfoiat onTl' STr '^'''^'- ^ -^de the top of the steam ches and fVm^=' ^^'''^ ^^^"^' ^^'^ centre of o^;er the different parts of the boUomof'tb'T'^ t' distributing/ which enters this channel by openr^s°^thro '^^"^^^' '^'^ ^°'^ ^' the chamber ; thus, when tlL apSus .'^ ^^'^ *^° ^"^^ of and rapid current of air enters and sdTtHbr?'"^' ^ ^""^^^"t lower put of the oven, while the lot .t^"?^ "'^^^" ^^^^ entire expelle . from the heated ^ncoUon wM^b"^'"/^'^'^^ "^^^^t^^'e, steam chest, escapes throu.^h tre^raft'of ?heL^oI?'"G"Pt"^ ''^ •Extracted fro. ..Instructiou in MiHt.r, Engin^oHn,,' Vol. I, .^^ " OSIVES. inite periods of ' cJry /L^uncotton ns of drying it, on the subject:* of drying gun- until it ceases inng dry wea- or wind. WitJi osure to open y, steam heat instructed, for this operation irts: a boiler t to be used, ther, and are tube of that th a covering coal fire, and •rfiiling, and ■ni^ed india- omed drying ' fixed on a J^e, and fit- open while beneath the ,kept, while an oblong two short trough the condensed constricted ■t tin, with • A wide e centre of stributing, 'le cold air o ends of constant :he entire moisture, upon the rating or fclV. DRYING GUNCOTTON. 9 shelves of coarse copper wire gauze are placed upon the top of ^ the steam chest, on either side of the ventilating channel, and the ^ guncotton to be dried rests upon these ; in this way the dry air ■ which enters the chamber is made to circulate on all sides of the discs or slabs of guncotton. The steam chest is enveloped in stout felt and enclosed in a wooden case, which is raised from the ground upon legs. The hood of the chamber is fitted with a pro- tected thermometer, which reaches down to the top of the steam chest, and projects sufficiently outside the hood to be read while in position. (4.) The mode of using the apparatus in the field would be as follows :— First, fill up the boiler with water, through the stop- cock, until the water rises to the top of the glass gauge. Next, ascertain that the exit pipe is free, and blow through the vulcan- I ized tube to make sure that the steam can pass freely into the ? hood chamber. Then light a charcoal fire in the boiler, filling the cylindrical fire-place not more than one third with charcoal, J and after the steam has been issuing freely from the exit pipe of this drying tray for five minutes note the reading of the thermo- meter, which should not be allowed at any time to exceed 150° Fahr. Then put the discs or slabs on the steam chest, cover them with the hood, and let them be exposed to heat for a period varying with the size of the disc. For 1^ inch disc 4 hours. Other sizes below 3-inch discs 6 to 8 " 3-incli discs and parts of slabs 12 " Whole slabs (according to thickness) 18 to 24 " (5.) The water in the boiler must be replenished from, time to time, so as to keep it within the glass tube of the gauge. In re- plenishing the charcoal fire it should not be m.ade up higher than about one-third up the cavity. (6.) A rough but good indication as to whether the discs or slabs of gun-cotton are dry is aftorded by holding a small piece of cold clean plate glass up before the face, and placing against it, for a moment, one of the pieces of guncotton in the warm condi- tion, as taken from the oven. If moisture is still being given off, a 'Aim of dew will be at once seen upon the glass, round the surface of the gun-cotton. A piece of plate glass mounted in a frame with handle is supplied for this purpose. (7.) In drying the large primers used for submarine mines, or for any special operations in which the 9-ounce or other large primers are used, it is important to ascertain that complete dessiccation of the gun-cotton is effected. For this purpose, after the drying has been continued for nearly the prescribed period, one or two of the discs or slabs are removed from the oven, left exposed to the air 10 QUALITIES AND CAPABILITIES OF EXPLOSIVES. V :l i ' ■f • I 16. Guncotton is a suitable explosive to use : (I.) When a shattering local effect or a blow is required. (2.) When tamping cannot be resorted to. Submarine mines. NITRO-GLYCERINE. , 17. Nitro-glycerine is an oily liquid ; its effect when detonated i^ similar but more powerful than that of guncotton." Prof Abe says of this explosive : "The economy in time and "abour eftected ^rea4"Tin3er 'ff "^ if *"""^"^"^ "^ ^^^"^ ^^^^ is undoubSl ^ gieatei undei favorable circumstances, than with truncotton or any of the most powerful substitutes for guncotton. Moreover tlS iq Id form, high specific gravity, and insolubility in water if nitio-glycenne are peculiarly valuable properties under some cir cumstances ; thus, blasting in wet holes, o? actually underrate ' aiwsrechK^^^^^^^ "fo glycerine expeditiously^nd wrthout au} special appl ances. * * The poisonous nature of nitro-o-jv cerrne, which injuriously affects the health of those hanS "uid usmg It, IS one of its defects. It is stated, howeveT 'hat ' le human system may become accustomed to its influence * * The comparatively high temperature at which nitro-gK-cerine fiee.es and the slowness with which it thaws, even at nomal atmospheric temperatures, constitutes a source of inconvenience and, in some respects, of danger. * * It has been eslab lished beyond all doubt that the material is much less susreptible of detonation m the frozen than in the liquid condition £ i . T lie accidents which occurred with frozen iiitro-glyc;rine appear Ohemi^tify'lKdilir""'"*"""^' nitro-glycerine will be found at § 412 Bloxam's l I'ES. J, (the weight the weight is 1 to the heat !, the diying DYNAMITE. II luired. for Military ition of iron down trees, ng in gates. stance, de- !, etc. letonated is Prof. Abel )ur eftected idoubtedly n cotton or reover, the 1 water of " some cir- der water, d without nitro-gly. idling and , that the * * -glycerine at normal venience, stablished ^ptible of * *- -* le appear 2 Bloxam's to have arisen from a recklessly rough usage of the material ; and, so far as the apparently great safety or inertness of the hozcw substance will lead to recklessness, it does constitute a source of danger. The necessity for thawing the nitro-glycerine (and its preparations) for use, unless exploding arrangements ot a special character be provided, has also proved to be a source of accident. * * * "The principal defect of nitro-glycerine, when employed in its pure state as a blasting material, arises, however, from its liquid nature, and its consequent tendency to leak out of receptacles in which it is transported, stored, or used. In blasting operations the nitro-glycerine with which a hole is charged will flow into fis- sures in the rock, and may thus be conveyed to parts where its existence cannot be suspected, and where it may be afterwards accidentally exploded during the boring of other holes."* DYNAMITE. 18. J. Dynamite is a nitro-glycerine preparation formed by the ab- sorption of that liquid by a silicious infusorial earth called " Kie- selguhr." By this means several of the defects of nitro-glycerine are removed, and the explosive thus produced is still very power- ful. The effect of nitro-glycerine on the human system is not, however, entirely prevented, and there appears to be a chance of the oil exuding which might possibly cause a premature explo- sion, and there is the same inconvenience and possible danger owing to the high freezing point. Kieselguhr will absorb 3 times its own weight of nitro-glycerine, and this forms the strongest dynamite, called Nobel's No. i dynamite. There are several other similar preparations in the market in which the Kieselguhr is replaced by other inert porous solids, or in some cases partial- ly or entirely by porous explosive solids, such, for instance, as : Horsley's blasting powder, consisting of chlorate of potash and nut-gall powder with 20 p.c. of nitro-glycerine ; and Dualine, which is Schult2e's sawdust powder impregnated with the oil. Dynamite is made up in small cylindrical parchment cartridges, this form being very suitable for blasting rock. 19. Treatment of frozen dynamite. — If dynamite cartridges are frozen and are thereby stuck together, they should on no account be separated. Frozen dynamite can be thawed by placing it either in a warm room or in a vessel surrounded by hot water, but in no other way should heat be applied. The detonation of 2 lbs. of dry guncotton or of 2lbs. of thawed dynamite will induce the detonation of frozen dynamite. * "The recent history of explosive agents," by Prof. Abel, F.E.S., Professional Papers of the Corps of Rcyal Engineers, Vol. XXII, 1874. I 'A I:, 12 QUALITIKS AND CAPAniLITIER OF EXPLOSIVES. Application. 20. Dynamite can be use.l f„, .he same purposes as Runcottcn. MERCURIC FULMINATE. st.ac\ or when I c, &o™ 36o»'f°" 1^], ^''''l ''"''T" ^^''^ small chan-esof from /tn ,„ ,r ■ • "'i"''^ "P f"'' "so in nite of po?ash ami Vh. ,*-•,' ""■""' ""'' "ifate and ehlo- nators."*"^ ' P''"='^'' '" f"^'^^ "'■ <=^'P=i called " deto- COMPARISON OF GUNPOWDER, CUNCOTTON AND nvKAMITE. t..an,l;o«r,'^r:'^yrt;ti^;;"C';i;irc'°''\'!'"^\^'™«- absolute sense because tho nS r ^ ^^""""^ ^'^ ^'^^^n in an enfireT."'" "''''°"™ '' '°'"'"'''' "" <=™P-ison evidently fails lent'r„7tsTctir?hlf n: V"d"^ ^'T^^-^^ ^^^^ -*"- ™°« vlo. some experiments earde^ont »f ?-1*T,"'' ''= ™= exemplified by .>oweverrobtai„:rf:ri;i-e^^ri:i'rcts^^^ pre^"d'°s™eo"t?ri°'tha1'lrf r*^ ^" *''™"'="''« -" "m- gently pr-essedhUo IneqthtLs." "'""'' """"■' "' "^ >"='■'« % \ RIVF.S. 5 as p:iincotton. ti.'il detonation 'iolence when up for use in trate and chlo- :allcd '*deto. DYNAMITE. imes strong-er be taken in an totally differ- Jver, be made ^mped. But ct is to move vidently fails er more vio- emplified by y result was, mz in 1873. 3 over com- of its beiny- 1!; ■4 m CHAPTER II, 'es see P. 101 IJ.ai., Vol A. IV. CONSTRUCTION OF MAGAZINES FOR THE STORAGE OF EXPLOSIVES. 23. It is clearly of the p^reatest importance that the site and desi^m of the buildmgs m which exnlo«;ivc-s are stored, termed magazines, should be such as to reduce to a minimum the risk of an explosion. 24. As regards the ^itc the following are the principal points to be considered. To reduce to a minimum the chance of an acci- dental explosion the magazine should be as isolated as possible and especially kept at at distance from towns or villages factor- ies, wooden buildings, etc. In order that an accidental explosion may produce the least possible damage the magazine should also be isolated.especially from important buildings, and a hollow site is an advantage in this respect, but it may interfere with the drainage. Lastly, the site should be chosen with a view to easv communication. ^ As for the dcdpi it will depend to a considerable extent on the kmd of explosive to be stored. The design of magazines to contain gunpowder and guncotton will now be considered. GUNPOWDER MAGAZINES. Tnf;^''^^''" '" Y^' gunpowder is stored in barrels containing 100 lt)s. of powder each, or else in metal-lined wooden cases if the magazine is damp. These barrels are placed on racks made of wood the object of which is to be able to obtain sevemUiers of barrels without pressing on those in the lower tiers A con venient arrangement of the racks is to have them on either side of the magazine, leaving a central passage. There should also be side passages but if this is not possible a space at least 6 inches wide should be left between the wall and the barrels for ventila- tion. A small crane is usually provided to shift the barrels • it is placed on an overhead traveller running on the top strino-eVs of 14 CONSTRUCTION OF MAGAZINES. IS' < i Jj: The Willis of the ma^'a^fine should he of masoniv l)iickwork or It l)c less t a 1 9 inches (2 ring's). To prevent damp, which is the pnnc.pal chfhcnity to he contended with, hollow vv i Is u mid e used, hat ,s, a tinn hnck wall should be built, leaviuL^ lllo w space between .t and the main wall ; and further, the trch of the pcifoia ed bricks placed in the centre of the arch rinir and care should be taken that the perforations are throuKdi tl i len^f the bricks, and that the bricks are so laid in the work that there wH be a continuous air space from one end of the arch to t e otl^r A more effective arran^^ement, however, is to build an inner arch' in con inuat.on of the brick lining of the walls, leavi g hoTow KikST:" W ""'t," """V"-^''- ^^'^^ '^^'^ arches^shouid nc" be bonded together. 1 he ventilation is another point to be carc- fu ly attended to, and for this purpose air-inlets and outlets en- able of bein,^ opened or shut to any desired extent, mus b pro- vided. These inlets and outlets should be closed with wire rauze o prevent the chance of sparks entering the magazine hr'ough r;i, ^"f ^^^°"'^">' ^^"'.^o^vs can be placed in magazines, but this, though convenient, is not necessary ; in any case however arrangeinents should be made for lighting wkLut havinrto bnng lights into the body of the magazine. Til is X^^^ do^.e by ineans of 'Might passages, ^ as%hown in Figs, f and ^ 1 i^. Vll, CD., Text-book of fortification. A shifting room or ante-chamber should be provided at the entrance to the body of the n:agazine. For security all the metal fittings in a magazine are made of gun me al. To prevent grit, and also damp, the floors Tre con- s ructed in the fo lowing special manner. Battens are secured to an asphalte floor laid on concrete, and to these, and placed across them, small battens are screwed down at close intervals The spaces between the battens are filled with cork shavings. " Magazines may either be exposed to Artillery lire or not in either case the internal arrangements are the same as gi'ven above. The exterior will, however be different. MAGAZINES EXl'OSED TO AKTILLEKY FIRE, 26. These magazines must have earth or concrete protection thick enough to leave a large margin beyond the penetration of the heavx's^ t-dnance at all likely to fire against the magazine. I he bee; V : ngp^ent in this respect is to sink the magazine in the ground, rts siiown at e, Fig. i, PL. XV. P. F, Text-book of *In large inaga/.incs it wiU probably be accessary to use two or mora arches. I r.UNCOTTON MAr,A;;iNKS. 15 y, brickwork or s dcpciHliiif^' on no case .should ip, whicli is tlie ^alls siioiild be iviug a hollow the arch of the ^-e one rin^' of rin^', and care le lenj^th of the that there will to the other, an inner arch, ving a hollow les should not nt to be carc- id outlets cap- must be pro- ith wire gauze izine through agazines, but ase, however, ut having to 5 is generally igs. I and 2, )vided at the I'ortification ; but if this cannot be done the walls should i)e pro- tectcd by masses of concrete or earth, and the roof covered in the same way ; see I'igs. i and 2, I'l., VII, (M)., Text-l)ook of I-'ortilication. In any case the magazine should be hidden from view as much as possible. MAOAZINKH Nur RXPOflKT) TO AIITII.CRRY FJRK. 27. If the magazine is not exposed to Artillery fire the dimen- sions of the walls and roof will be determined by the stability of the structure, and it must be remembered that tli()S(> walls acting as tlie abutments to the arch of the roof will re(juire to be thick enough to resist the thrust. Further, to localize, as far ..s pos- sible, the effect of an accidental explosion in the magazine, the V Uls should be made thick, so that the force of the explosion may be deflected upwards. This is the arrangement adopted for large store magazines, and m order to minimize the effect of an accidental explosion it is best, instead of building one large magazine, to build several small magazines to contain from 1,000 to 2,000 barrels each, separating them from each other by earthen or concrete traverses! The roofs of such magazines should offer as much resistance as possible to falling splinters, and for this reason they are made in the form of parabolic arches ; tliese arches should be covered by an ordinary roof to throw off the rain. Fig. * shows a suitable arrangement on plan for a large store magazine and Fig. * gives a section of one of the magazines. are made of oors are con- ire secured to placed across ervals. The ings. e or \iot, in me as given e protection netration of e magazine, iiagazine in ext-book of lor^ arches. GUNCOTTON MAGAZINES. 28. Guncotton was formerly stored in tanks containing i ton each, and in which it could be kept in a wet condition. Special wooden packing cases containing 50 lbs. (nominallv) have now been sealed for use. These boxes are made of deal payed on the inside with marine glue, and to make them air- tight the lid is screwed down over a layer of flannel. They are provided with an air-hole in the lid closed by an ordinary screw, and also with a hole for draining off water, placed low down in one of the ends of the box, and fitted with a brass socket and screw plug. The guncotton in these boxes can be re-wetted, when neces- sary, by attaching a flexible tube by means of a screw-junction to the plug hole, opening the air hole and forcing in water. As soon as the water has risen to the level of the air hole it is let out again, and the holes are closed. *Drawn by the Cadets. ■I I ■f I t6 CONSTRUCTION OF MAGAZINiiS. ground, arL'p'iirdllnVw^ ^^°^t ^^^^ -"^^ -der , leaving a central passaL Thn 1 '''.k""."? "^"^ '^^^ ^^ ^he store, the .oof, shoXr;;vli7,lr;-S4te'^„''.°^S ""^" "'"' for the purpose ori"„7 s^daf '^rsir/nr' '" ^"P'^"^'* P™"''^'' EXAMPLES. the'-brr?4*'bi;f i'r?aj;;1=r ,r"*"" ?"° ^"^'= <" P-*''- depth. Ca,eu,a.faC']re1ca^„.,?4rorth^':«it^^^^^^ "> feet high. Find the thickness nf fL u ""• '''''"^ ^^"^ ^ supposWone oaheJ;''expt3?: I-i^^re^^^-rdX^^^^^^^^^^^ 5. The thickness of coverintr of the rnnf ^^f ^ .v.^ boxes'''"'' "" '"""="^'°"^ "f '^ S-ncotton magazine to contain 200 ilf sunk under de of the store, zine should be ces the maga- e boxes being- :om flush with ton. ecautions are •proof. s in hermeti- ards provided s of powder, d 2 ro\\'s in y timbers. i,ooo barrels ired. 5,000 barrels fed over by a walls are 6 ng the arch le other not. ine, exposed oportion of contain 200 CHAPTER III, USE OF EXPLOSIVES. INTRODUCTORY. 30. When explosives are used for any purpose a certain quan- tity of the explosive, called the charge, the weight of which is de- termined by calculation or by estimation, has to be placed in a position depending on the object in view. Occasionally, instead of using one large charge, two or more charges are fired simul- taneously. The selection and the work to be done in reaching and preparir^, the receptacle or chamber to contain the charge are questions treated of in other parts of the Course of Military Engineering, so that it will only be necessary here to consider : 1. The preparation of the charge. 2. The preparation of the firing arrangements. The manner of making up the charge will depend principally on the object for which the explosion is required and on the ex- plosive to be used, but the method in which the charge is to be fired will affect the first steps of the preparation. The firing of the charge can be effected by heat, by electricity or by percussion. When firing by means of heat a "leader'' containing gunpowder is used. These leaders burn either slow- ly or practically instantaneously; Slow-match and Bickford's fuze belong to the former class ; Quick-match, Powder hose, Ord's hose and Bickford's instantaneous fuze to the latter.* When electricity is the means adopted a current of electricity is con- ducted to the charge by wires ; and when by means of percussion a blow has to be given to the charge. Electricity should, if possible, always be employed when the charge is covered by tamping, when it is required to fire at a given instant of time, or when several charges arc to be fired simultaneously ; but when not available can, in these cases be replaced by instantaneous leader. Bickford's fuze is most suitable for a single charge, which can be reached up to the last moment before firing, *Thesc various leaders are deacribetl in §§ 71-75. tii 11 i8 USE OF EXPLOSIVES. either mealed powder or merci.Hrf,*- *''''P'??'T''' ff^erally prodticedbyeitlieroftl?JthS. .1 1""'"=""' '*''"<='' 'ff"i'ion is arrangemeiilfc^lled a "fu.e!"*'"''''"'"' ■"""»™'' ='''°™- This FUZES. con'st.So'i'deU'idT-l"" °'-^^^^'-'" <'"f--* X-ds. and their 1. On the explosive. 2. On the method of firing. poId"rXltt^::t'fir™«"''' ^J"^"""'" ^°"'-" ™-l^d fulmina'te, and ar" Slled ^"d^tSors.''?''"""''' """"" "'«"'™ meTh:d':'fl'ingr''™^"°"°f*^'"^^ "^P-'ds. however, on the FUZES POR USE W,T„ SLOW OR INSTANTANEOUS LEADER. ■ entf;sTlfhS;7a°ndtni'es ir '' "'>'"^^^' ^ ">^ '-''- thif br?s's artt ste':'f°!;rai^ouiif ■ ~t'^*^ °f - -p'y attached a tapering th, tube aTnel?" ' I "^ ""'"=' '°"?' '° ^^'"^i is g.ains of me?curic fulminatr Over tl-' ~"'^'I""K ='''°"' ^° small plug of wood is n lareH tl..^ i "'^^^rcuric fulminate a matcht is passS to comtvthn fl f f 1 " '^'""'^ "^ l^'^k To use this'^detonat™. bS, d>s ftel ^n "" .'^i'^'^fo';'''^ f-e. tube, whieh is then slightly tn^.o^Saln SfBl^Cl*"^ ""P'^ or'*' rreytr:!:; Z!:t",'=.frPins '7' '""''^'1 "^^' - ^^'°-t- use them Bickford's fuze is'pressed i^I'?/'" ^"''"'"='''^' ^-"^ '° tube. It should be noticed tlfft ""o^'be empty part of the "sed with dynan,ite buf hat a S i!'Z"°'-/''='°"?*°''* '=™ ^'^ ful enough to detonate guncotton*^ ^ "'''" '^'' '" ™* P°"«- gul;V,:,f:!;fbfrL"\,'tlfft;T:,'^,:'f^"';;f'*'o-'^^ .n.te caps, side by side,,, 'Si:!:i,t::^Z^:f^:;\^,- If <;yna- +See Ji "72. failed to do so wii.^n u!ic.,.n!iiie- ..er described under tL h^a'd of •'oi4"^"'f'l;f "' •"'^ ,"="!,- f?insi:l::d^~ctTe^-^^^^^^^ should, if possible, be used with |uncotton in rJj ^"^Tfu'^'' *See §§ 117, 118 and 127. +Seo §g 87 and 89. ^Provided a quantity dynamo, which will fire wire fn^oc ;<, n.f -i ^^ §Official Treatise on Ammunition, 1878. ' * ^'^''''''^'• IjThis applies only to Laud operations. ij i' tion of the 1 wire fuzes ed.* Tliis e}' are less pparatiis is er of these t appears, e ready for w tension' 3r testing, 3 blasting, :h tension frictional ilways the s forms of from the PREPARATION OF PRIMERS. PREPARATION OF PRIMERS. 23 ze ready, ntaneous ict them the man- nsulated s circuit me is an re firing, tonators of them Jcted to through onator ; head of le. GUNPOWDER. ,.o!!'i'^lf ^T"^""' '^ generally* placed in a bag, and for this our pose leather bags are issued from store, but if these irp nnf ^ curable a sand-bag can be used ; this sLk! b.!^g si ould be ta e^d" f there is any likelihood of damp, and always'if the charters to remain unhred for any length of time. cnai^e is to When the bag is about one-third full, cither tliP ^^A ^f fi should be taken to prevent the leader ^^i^s ^^^^SZZ the tefr, and tins can be done by n.aking a knot o,*; flULZ or tJUNCOTTOX, 48. A small disc or slab of dry -uncotton ;« f-^l-o,. 1 .1 shanks of the detonators are gendv- pS into Vlt ,' f ""^ *^'' vided for the purpose. If the holL'a^e tTo sn all tl ey shodd' 1?^ enlarged by means of a tool called a ''rectifiei '' o>if o" some paper should be wrapped round the s nnk A w*°°-f ^? . pnjvided with each box^ detonatoit^^li^:;^ L "^^i, .^n^';^^^!^:-^::;^^^^ ^^ ^-^^ i-o the "This cylinder of detonators contains a 'Rectifier ' detonator shonld en't^l^td'u;;,;";.!.;'^';;:"!! by'twi^n,:'!:^'' '"^ scribed for preparing a ffunpou ler p in S "S'u «;'';r' '''■^ '^^■ precautions. To insure thaf no n,oisture Hnd its 'fv f '''T wet^Runcotton into the primer the montkolZV^'lZr'f: ^Blasting rock and (luarrying are oxcoptions. tThese india-rubber baga are articles of store. l/"' 24 USE OF EXPLOSIVES. I > DYNAMITE. 50. To prepare a dynamite cartridge as a primer it is on^nnrl and the liead of the cap or detonator is ^en^ly pressed iZ ho Tt'T: '' 'V''' ^^^^°P---tion it is wdl to imfke a m" ho e r o!l ; '"]°f \P^^ce of wood. The cartridge should then be closed and the fu^e secured by string or wire. PREPARATION OF CHARGES FOR VARIOUS PURPOSES Jtl'fp^, :l Z -e?" tU'^^- n,f« charge for tlie various cases in wliich explosive, are ',sedn edsSirt"'""-' ''^"' """■'^'""^ "■"' '''"^f°^^ "'^^-'^d- cas^e^^caTifelt:S^a1r t,^: £ j';-^lVrI;°''^ LAND MINES. 52. Gunpowder is the explosive best suited for land mines and he charge is placed ma chamber prepared at the end o the ^al Icry m the manner explained in sS 47, F.t The ch ,r°f> 1 T ." made up in bags, and this is also tite'most co ?e ie f a "^^.^J^ ment for brmgmg the powder to the head of the gallery T^rmj sand-bags can be used for this purpose, but in^dia-n bber ba's v IcamzedHS contaniing each loolhs. of powder, shou d be usJd I the mme is wet. The mouths of these"^ india-rubber ba's are The bags are packed as tightly as possible in the chamber the nmmg bag ben.g placed n the centre. If the mine is to be hred by powder hose, the hose IS laid in a wooden trough Jaced for the purpose along the bottom left-hand corner of the gallery!^ *Prepared by dissolving imlia-nibber in nai.Jitliji JIt sliouM lie remembered tbat thi- following oidv deals uifl, fi,„ . ^ 1 up and placing of the .'barge. " ^ '*'' *'"^ '^'•*"'^' making gTheso india-rubber bags are articles of store. llSee § 49. d consistiiip^ meared with is closed by )Oves should 5o that they t is opened ied into the small hole Id then be OSES. LAND MINES AND FOUGASSES. 25 p^e depends : up of the re used in e consider- 2 followiner I. mines, and of the ^al- S'e is best t arran^'-e- . Tarred bbor baj^'-s d be used baf,'-s are ' for gun- inber, the o be fired laced for gall er}- tbougli that t tlie leader on of linng luvl making But if instantaneous hose or electricity is to be employed for firing, the eader or the wires are secured to the upper left-hand corner of the gallery.* They should not extend much beyond the end of the tampnig, and with electricity the final connections are niade just before firing. The best method of firing land mines is by electricity, more especially so if two or more mines are to be fired simultaneously. The charge being placed, the next operation is the tamping which is best done by means of sand-bags about one-haTClt The amount of tamping should not be less than iftimes the L. L. K.t for a two-lmed crater, or twice for a three-lined crater. on^h^ ^"^^"tity of Gunpowder to be used in a land mine depends on the depth of the charge below the surface (the L.L.R.), and ti"ateVo^7il - 'on,.,..,-. Hick. . elect, icity. The two l;,st ,„etl,o,ls .„c prefen.blc. .nite°s i,rih '■;",!;:-;t:;:;^'f "' ''>■ "-.•>.- "f.„„c„„„„ „,„,.,„. clKirres are best,,!?.. "' '"'■ '''"""'>^>''"^^- to ^^^ fonned, and this can be hand. ^ '"' explosives used as in blasting rock, or by a llnlrmim' '^'''"'^'' '' ^'''^'^^'"'^ '' '' ^^^^'^^ "^ ^ ^""^^^^ ^^y to In this case if there is no difficulty about tamping, gunpowder o guncotton are ec,ually effective; but it siiould be cMnembered hat guncotton requires a smaller chamber than gunpowder If :b^;:;uit:':^-:^S^il:---^' ^^-^ ^^^ --^p^-^ ^i^-ott:^ i^ ^^ The charge can be fired either by Bickford's fu2e or by electri- ARCHES. 67. In demolishing arches a chamber has to be prepared in the pi!"xLI v! F.F * ^''''^°"' '^^'''" '" ^ '"^ ^- ^'"^ ^^1^0 I' 4 4, If an arch has to be hastily demolished a charge of guncotton o dynam. e can be placed over, or better still, mider tl e c^own of the arch and can be fired most simply by Bickford's W unless the bridge is to be '< prepared "^ir deinolit^n,%v^er; the charge should, if possible, be fired by electricity. used.*^''"''' explosives are not available, then gunpowder must be HASTY DEMOLITIONS. 68. Hasty demolitions differ from deliber *Text-book of rortiticatiou. ate demolitions in that 30 USE OF EXPLOSIVES. !, (f , very little time is available. Tamping? must therefore horp^n.^^ 1= SL 1irs.;-n,,rs,t'i.s;r.r£ PREPARATION OF FIRING ARRANGEMENTS. nf?hl^^ ^'f' already been pointed outt that the immediate cause of the explosion of the charge is the explosion of the fu7e and 1^ tt:'::X'. ^"" '".' '"" ^'^ three^distinct ^el^rof dl^^ 1. By slow or instantaneous leader. 2. By electricity. 3- By percussion. _ Firing by percussion is so rarely used that it will not be rnn detS ' ''''' '"°'"' method's will now be hlqu^red intoTn PREPARATION OF FIRING ARRANGEMENTS WHEN USING SLOW OR INSTANTANEOUS LEADER. 70. The object of the leader is to enable the person firinp- thp charge to beat a safe distance when the explosion takes p^^^^^^^^^ This can be done by attaching the fuze to a short length of slow Xr^a/iirc^oSd?^ ^"^""^^^^^- ^-^-' ^^^ot^!^ in ?hTseL?cf '"rhf for '"^ instantaneous leader are employed h\^A X TA following IS a description of the more usual kinds and of the manner of using them. bickford's fuze. 71. bickford's fu^et is made by placing a train of mealed powder between two coatings of jute-thread twisted in oppoTiL diSus A peculiar waterproof composition is then laid on, and thrwhole L?:r\'croniin^/?o%f *^P^;.^Vta-percha, thread^ o, evintf Intended ^^ particular use for which the fuze is Seratli:y:"R:grwinb\'lourd"ai*r^^^^ - 1858, by Lieut. seriptionSlarge'bTa L w tl gfut.wdefAUea'.f:'rr^ Kngineering/ A do- the same work at pa-es 532 and 5 Fnr f*f'>^^^^^^"'J at Seaford will be found in Singapore see p. d, Cl. ll^Sttbnaf JipTrrRE 5 "' """^ '^'^^^^"^ "^^^^^-^ -* tSee footnote. *p. 1 S. 2 reduced an rarely and that ipowder. ig" up the INSTANTANEOUS LEADERS. 31 Lte cause s, and it of doing be con- into in i SLOW ing the place. 3f slow I kinds ployed 3 usual •owder ctions. whole ven of uze is f Lieut. A do- ound in tions at 1607, The rate of burning is from 2.4 to 4 feet per minute, and the more the fuze is handled the quicker it burns. If it is required to be able to ignite the fuze at any given instant of time, the exposed end should be prepared by splitting the fuze, inserting a few strands of quick match*, and securing with twine or tape ; but the usual way, although the fuze will not ignite so readily, is to cut It diagonally with a sharp knife. The other end of the fuze is either inserted into gunpowder or into a detonator, as already explained at page 18. QUICK MATCH, 72. Quick match k made by boiling cotton- wick in a solution ot mealed powder and gum. When unenclosed, quick match burns at the rate of about 0-2^ feet per second, but if enclosed in a tube, the heated gases rushing through the tube increase the rate of burning to about 15 to so teet per second, and it then forms an instantaneous leader, such as Ord s hose or Bickford's instantaneous fuze. Quick match, when unenclosed, is used for priming.t POWDER HOSE. 73. Powder hose consists of a linen casing filled with gunpow- der. The case is an article of store, but it can also be made as tollows : Strong Imen is cut into strips 4 inches wide, the ed^es are turned over outwards, then the double parts are brought to- gether and " serged" by passing the needle through the four thicknesses of stuff and back again over all. This makes a tube one inch in diameter, which is now filled with fine powder by means of a copper or a paper funnel. Twenty feet is the lontrest length of hose that can be filled easily at one time, and this tilling can be done from the upper window of a house. Powder hose is used with charges of gunpowder, and then only when the other descriptions of instantaneous leader are not to be bad. No preparation is required at the exposed end, and the other end is secured to the charge, as explained in § 47. This hose is very liable to injury, and must, therefore, if pos- sible, be surrounded by a casing of wood. The angles are the points most liable to injury. The method of placing the hose in a gallery is explained in § 52. Powder hose burns at the rate of from 10 to 20 feet per second. OltD's HOSE. 74. This hose consists of several strands of quick match en- closed 111 a casing of coarse painted canvas, cemented together *See § 72. +See § a. 11 32 USE OF EXPLOSIVES. M Bs Joint secured with ZMr.T:^^^^": f-"""^ ---fed only affects this liose temporSly Dampness, however, "'^'"'"■'"■S I^■S•^AMTA1IK0US FUZE and ^nSts'Satl'd" ''::ir;!^fell""" "?,"">■ "'*™''"-'<. of gutta-percha and wlrpi^ftrpf IndT'cl'^r "'""f ^''y"' water. ^ '^'^P'^' ^"'^ ^^ can be used under .na4h!t\,';:tin;,"?,14:i^!f^[ht^^;Kr;fr.^''°'' '™f"'%°f "- "'-^ the joint. If tie ioint iffn ?i , j"PP?S a shp of paper round damp, it should bfwate proofed^ hv't'"''"' f™'^'^' ^ '^ "^'W'^ t° in tl>e same way as™, etorical ioh^"" Vf- '"*=^--*ber tape, R. ,. J, . operator to g-et away, /. J:^^°'-^ ^ ."stantaneous fuze burns at the rate of 3^fat per FIRING THE CHARGE. ■eadt,TaT-.,:st'''"e:atfred';: tZV" ''7 "I ■-'-■taneous wl.en prepared as described ,ho, T""' 'i'"' °' ""^ '^ader, answer tlie purpose l^ut velvS^ V '^" "/''"'^^'y '"="<=!, will and port iirrca'^n .So'bl usVd 'wiTh Sv^lu^asfr""^- ''^'"^^ ■"■■"^'■ PREPARATION OK K.KtNG ARRANCKMENTS W„EK US.N,; ELECTRICITY. dct"„,;^r,ed"bf,^;r,:;'ra"r'of'^;reSri^'"," "' •='='^'™»' f-- - inK to the nature o t he fu,e eith,^ , I "'nentnhich, accord- wire or decomposes a se, sftivn " '' """ ""''° platinum ■owm, are, the';.efo?e! ;:^m::,^,trhr?:;'j;;al;S-^ I. A source of electricity, or firing machine. fA'^''!'t::^'uL:n;'^!:ii:^-,^^^^' ---t tin.„,h ti. crcuif witi, the source^? Jlectrieity. "" '"-' P'^"^"' " "' *See§100. SOURCES OF ELECTRICITY. 33 III SOURCES OF ELECTRICITY. 78. On the nature of the fuze depends the qttality of the elec- trical current to be employed. Wire fuzes require the heating effects of electricity, and for this a current of large quantity is necessary,* but the potential need not be high. Tension fuzes t require the chemical effects of electricity, that is, the current must have a high potential*, but its duration need only last long enough to start the decomposition. A high potential is also necessary in this case on account of the high resistance of these fuzes. SOURCES OF ELECTRICITY WHEN USING WIRE FUZES. 79. Two descriptions of apparatus are used to fire wire fuzes, namely : 1. Battery of Voltaic cells. 2. Magneto electric machines. Battery of Voltaic cells. 80. Since a powerful current is required, each cell in the bat- tery must have a high electro-motive force and a low internal resistance. Such are Grove's, Bunsen's and Poggendorff's cells.:]; The cells in use in the Service are the " Military Grove" and a special pattern Leclanchd. Military Grove cell. 81. The cells are flat, and the outer cell is made of ebonite. The zinc plates are curved round the porous cell to expose as much surface as possible, and the surface immersed is twice ^"X2^". The platinum plates are made of sheet platinum, and have ^"X2j" immersed. Twelve of these cells are placed in a box, and the terminals are connected to two binding screws placed outside the box. The box has a compartment for I2 spare porous cells. 82. "To prepare the battery for use, all metallic contacts should first be brightened with emery paper. " Sufficient sulphuric acid to charge the battery is then diluted with water in the proportion of i acid to lo water in a wooden or earthenware vessel, the zincs having been first amalgamated. § *See §§ 819 and 784 (ianot's Physics, 9th Edition. IHigh tension fuzes are usually called tension fuzes. tKor an evplanation of tlio, thnni-y of Voltaic cells, reference is made to Book X. chap. 1, Gaaot's Physics, 9th Edition. §See "Sundry Recipes." 34 USE OF EXPLOSIVES. "The plates and porous rplleTc^!'S:„«^'-rj;;~J%,|2'>^ t...„,, pieces, and "Amalgamate the .i„o plate and clean the hindi,,, scew, Jnrtr.,cti„„ ,„ Military E„gi,„„ri„g, p„t j y U«k.., SOURCES OF ELECTRICITY. 35 Make up a saturated solution of sal ammoniac in one of tlie ebonite cells, letting the level of the liquid be ii inches from the top. Mu, iij a^jnc amalgamated plate, and try if the current from It in conjunction with each graphite in turn will redden the standard wire. A fuze does very well for this. "Put aside those graphite plates which do not redden. "Coat the binding screws of the remainder with earth or puttv and submerge the head to a depth of i inch below the surface of a molten mixture of equal parts of china clay, and pitch. When this lias hardened remove the clay and brighten the screw. it Zhlu}'^fu^^^''^^^ P^^^' V}"^ ^^^^ °^ °*^^^^ P°^o"s cell, and pack It tightly with equal parts of binoxide of manganese and graphite nn't inVn''"'^' ^^^^^ '^'^ '''^ °^ ^ P^^ '^ P^^^^ble, but in\J case put in nne powder. "Choke the felt cell at the top, taking care that it does not reach above the bottom of the pitch mixture. paraffin.^^^ ^'^^"^ ""^^^'^ ^^°"'*^ ""^^^ ^"^ ^^^ "^^P^^ °^ ' ^"^^^ "^ "^o^ten whX''^ f" }}'^ !'"'' P^-''^ ^'^\ ^"'^, *^^" ^^^ graphite, and close the whole of the top with crude black paraffin, leaving one hole through which to pour the liquid and another for the escape of air, these holes being kept open by pieces of quill or glass. "The lead head should stand at least i inch clear above the top oi the ebonite cell. ^ "Pour in the saturated solution of sal ammoniac until it reaches to within I inch of the top of the cell."* 85. These Leclanche cells are not as powerful as Grove's cells as they have a smaller electro-motive force and a larger liquid re- sistance, consequently a greater number must be used to produce the same effect ; but they are far more convenient to use. Makeshift battery. 86. A makeshift battery, capable of firing wire fuzes, can be made as follows :t " Cut from sheet copper a number of plates each 6 inche-, by » inches. The exact thickness is immaterial, but No. 2^ B W G IS very suitable. ' ' * " To each copper plate solder a strip of thin copper. " Cut a number of zinc plates of the same size, and if possible one tenth of an inch thick; if this thickness is not at hand ordinarv roohng zinc will answer very well. *In8truction in Military Engiueeriiic, Part IV. i-A represeutation of this makeshift battery is "given in Fi.'s 1 to''. Plof«l in § .inuouscirct:.%,rjte:,,?S;,,:,^ so f„.es at once in eon 1 iie rnacliine is nl-ipo^ f !i ^'iou])s of two in divided cirr.„>' 4 dynan^o, but can b'e S"^,,'^, L^fr" ^'"f °,' ''°' as ll.e "S;,! of a key, by tl,e bo.v bei ;>p' itfed WarV" 'f!'"' l'^ "'« ' ''-nee sion panited on it. t'a'ntea black, and by tlie word " Ten- '.andle sC!y::^,;f J^^ll^^^^^';^ is as follows: Turn the ofthe automatic tin,,,,';..,,'?;' , '°'"','°"' ""''1 tlie sharp dick turn wires and turn tle'-iandfei ml '';,""-'" ?"' "" ""= hue a'^d 're^ or fu2es."t "'"'P'J "''•'^'^"■nes, this fires the fu2e wires, vvhich 'mus?L'i,'sT/hVd ?"^"^t^d ^^^'ough the fu^es bv wire, called the " line wfre '' 1..^%^'"^''^"^ ^^^-^^ ^^v leaka4 On? firing apparatus to the W. ^ ^'°"^ ^'^^ P^^^tive po1f ^f^ ' wire," from the fu^es back tn'., ""^ ^"^^^^^^'' ^^lled the "ret, n ment is known as the 'drcu^ '^'^ ^]'^^'^'' P^le. Th s'armX" erally called the -cable"' ^^'''^ ^nsuhted wires lie get ^o^^S:^'Z'^lii;^;S T"^^^^ ^^-^ ^^- wires have a most important requisite^ '"'^°" ^"^^^ -"-^^ insulation is the .o B:W.5..ttteVst'l?s'o'f*?;^°i'f-*-- ---sts of seven No tnen with platted string. ^^' ^ith a layer^of felt and fict*nt'^'uttitr'tl"'pal't ':r/, -^^i'^"^ - can„ot be had in s, f *.See§97. ^^""^^ of the circuit under tinstruction in Militarv v. ■ **See §, 784 and SSSf!"'^^- .^-t IV. ' '^""^ ^ I'hysics, yth Edition. ARRANGEMENT OF THE CIRCUIT. 39 Srround Muist be made of insulated wire, and if not obtainable ready rn=.de, some bare wire can be insulated in the fol oW man! ner: Thm strips of linen or cloth well saturated with tar are ;p-apped spn-ally round the wire to be insulated, and then paved iJZ-Tt ^ 7f' -^^ ^:''^ ''^"^ '''' "^^^^^' P"t °" l^«t- Marine tfflue if obtainable is a better material than tar or pitch. 95. When using tension fu^es the return wire may be dispensed with, usms- r earths" instead, that is the return wire from the fu^e, which in this case may be bare, is attached to a metal plate placed in the earth, and the negative pole of the firing apparah s is connected in the same way to another earth plate. The Fa h then becomes as it were the return wire. Earth plates should have an area of about 30 square inches, and if the inrndTs not damp It should be made so. These earth-plates increase Te resistance of the circuit, and should therefoie on no account be introduced into a circuit connected to wire fuzes.* 98. Short lengths of wire are frequently required in making up he circuit. For this purpose, either portions of Service cabfe or insulated copper wire not less than No. 16 B.W.G. can be used. ARKAXOEMKNT OF THE CIRCUIT. 97. When only one fuze has to be fired the circuit is called a simple circuit ; the arrangement is represented in Fig. i. j't* When several fu/es have to be fired swinltancously the circuit may be arranged so that the current fiows tluoughil the Ss in succession without being divided ; the fuzes are then s "id to b ^ connected in " continuous circuit," as shown in Fig " Or the curren can be divided, before passing through the fu.;s, kto as many branches as there are fuzes; this arnmgement is called *Thia aijplies to Land operations only. If u 40 usii OF nxPLosivrcs. s&i;^----,-^^^^^ Fiffs. 4 and 5 °' ""="= ""■•-"'Keir.ents are shown in Selection of the arrangement of the circuit respect, therefore,!^d iis^on of t - -^-'"'^''^ ^^ "'""'''' "' °"' battery power re niired I^^.t n ^J ^["^ ends to increase the tlie circuit dimiS es the eWH^^^ ^^ the current.* Ttapplls the efore'll'''''""? ^^'^i'^'^ ''""^'^^'^^ to ascertain the n^ost econom^^^^^^^ ''' matter for calculation regards battery power. ''''"°"^'"^^ airangement of the circuit as continuous ci -cuit a d iscotSn"VfT'^ " "^T^V"^.^ ''^''^^^^^^ '^"^^ only cause the failure tfTe" fui^o"; }!;resr ttt taS'^^^' ''"'' will ignite first, tl?us cauing a dist^nrcl^'^^n^th^^^^^^ '"'^^f the remaining fuzes. '"'^^'on, and tlie failure of On the whole it appears, therefore, that one should • *See § 145, 1^'. J. Or pted when JOINTINC, 41 and con- shown in lit is em- ch must 1; in one sase the 'ision of ticreases culation ircuit as e failure -led and les, will id they >r fuzes ilure of (c.) " If the wire fuzes have been tested (resistance measured) and can be relied on, it is better and simpler to use a single fuze ni each charj^'e, otherwise they should be in pairs divided." .lOINTINO. 99. Electrical joints have to be made when the circuit wires are too short, when usin;,' divided circuits, and, as already seen, wlien preparnif,' electrical hues for use. The joints are the weak points of a circuit ; they must there- fore be very carefully made. When hrinj,' with wire fu/es it is essential that a perfect metallic contact be obtained, for otherwise the resistance in the circuit will be increased, and possibly to such an extent as to cause a failure. It is most important therefore that the ends of the wires to be connected should be perfectly bri^dit and clean, and, shice the metal soon bej^ins to oxidise,which increases the resistance, the joint must be soldered, unless the charj^e is to be fired shortly after makinjL,' the joint. Further, to prevent leaka^^e, the joint should be insulated more or less effec- tively according to the liability to leakaf,'e. When firing with high tension fuzes the same precautions should be taken, but in this case the insulation of the joint is the most important point to attend to. The method of making electrical joints in circuit wires is as follows : Tvo-ifoi/ joints. 100. jfointing the xcircs to>rcthcr.~'' For jointing insulated wires, strip the msulating material for two inches from the ends of the wires, avoiding using a knife when pliers are at hand, but in any case takmg particular care not to nick the wire itself when cutting the insulating covering ; next clean the bared wires until they are (juite bright, then cross the wires, as shown in Fig. 6, and I inuous livided Flij. (1. Fhj. n 2 are a num- Fhj. S. Fi'j. 9. bend them round each other as siiown in Fig. 7, then with each end take two full turns, Fig. 8, and bend the projecting ends so If I 42 VSE OK KXPLOSIVES. t;./^^^e'rz:t^;-;:i-;;:i .- 't;;'^ T- f''-^ ^^" /V'vi|| rise or- swc^ "'',,^?'' '^ -^"ently hot, the jo.nt, fornnn« a perfect metallic union'! ''" ""= '''''' "' "'^ as it is*c:„'rd"ta^%oT- whe!uhr'-''T '"'"" ""' ""• ^'i"*^. or bolt, whilst hot, ,mt°l t is ,ite Id,? "" ?"=• *!'? ""= ""' of the or.nc, and rnh iton .f:.s^"r^^Sty-'tltirta?l;^^^:;i'= insnS'tirtii;";;!!^,^^^^^^^^^^^^ f:::'""' '!- J-"' ^■■o"M be tape whilst «Tap,,,fr it an7nrrHn,J' I'"!""' "' ■■^'■otcl.inf; the ceding one by cSl h^lfitrbreadth f^ig?^.) ''" "'"''''^ ""^ P"-- Failin be -r<;Lr:d':bS'Xnid''L:,';;st,i:^''i^, ^^■■' ,-- --^ -c not the joint can be inspected un to /h, I ''^' '''■' '"''• =""'' "here is not necessary to Fnsula eTvi h r bb r 'taTe"'r tr '"'■^ """«• " -.ffiee to frap then, ronnd with someToos?"ope Virns.'"" '■' "'" »..t »;!i,iut:t;';;tts;t'':i,f it :^z!{iz 'r '""""" '""•• -« ■'-. '"■' " i. imitact iiisural. ' ' I'™"'l»'l "»« tlio wire is well clciiiu,! and g,,,,,' II /'V//. l.t. ;* this lilorido 2d with Ider, is wires ot, the of the CONNFXTING FUZES TO THK CIRCUIT WIRKS. 43 used with the india-rnl)her tape, smeared over the successive layers of tape as wound round. "*t micccssivc r.U^^r ^°i?* '' "'^ -'^"l^iored, no india-rubber sohition should be placed on tlu. wires, for It nii^dit ^'ot between the wires, which would increase the resistance in the circuit. In this case there- re, a layer of india-rubber tape should be put on before apply- ing any of the solution. ^^ ^ ^l.ll-^-f'ff '^",'''''''•"•1°'"*' '" iron wires are made as above desci bed, but sal ammoniac is the best flux for solderiiiL' If the If iron wires are too stiff to bend, it is best to make a - Brit- tan ina joint. This joint is made by slightly bending the ends of the wuxvs to be connected, placing them side by side, with an overlap of about one inch, and then frapping them toge her with fine iron or copper wire. See Fig. ii. 21iree-wai/ joints. J^l' ?''^^-y:Y i°^"ts ^re "sed for connecting up divided cir- cuits. Two of the wires are twisted together as for a two-way in 1 ; ^"? ^}'^'' ^^'f ^A^i °^ ^^'^ ^^''^ ^^'^^ '^ ^I'PPe^i l^etween them and twisted round. (Fig. 12.) The remainder of the joint is made m a similar manner to a two-way joint. Connecting electrical fuzes to the circuit wires. • ^®f-T,he joints connecting the fuzes to the short! lengths of insulated w-ire must be made with all the precautions mentioned ^reT;d '^" f}^-«/^Provided with short leads§ the connections are made by a simple two-way joint, or by a three-way joint if two fuzes in divided circuit are to be placed in each charge. But if the fuze has eyelet holes the short lengths of insulated wire are connected direct to the fuze, and in doing this great care must be akento prevent the possibility of the current passing between he wires instead of througli the fuze. The manner Sf making the joint in this case is as follows : i'^Kuig 'Instruction in Military Engineering, Part IV. tWhen the joint is to be placed under water it can he fuiiher insula*-^,] on,l strengthened by slipping over it. after the tape has been laid on nhoTlem H?' of nKha-rubbertubnig, the ends of which are then «rmly secured with twine *^Th« india-rubber tubing should, of course, be slipped on to one of the ZwL W jnen^ahe joint. This india-rubb;r tubil^ i. Z^::^J':^i:'^;:,:::S:^ tSee § 45. §Scc § 37. 44 USH OF EXPLOSIVES. i ' ■ ;■( If ■■ with a:,;^;i'nan o > :r otic' "f '^ ^^"'^^'"^^ ^-d out contact. The ^vires sl^ then 1 1 u'^r ^^ T"'' ?^ '' ^^"^^^ '"^^alhc inches and inserted tln-ould t e ube ol ^, " ^'''f' «^ '^^«"t if opposite directions, tlie ends nrot.f' ^ , *^"""^^'' ^^^^^ tube) in an inciu Tliese ends sho Id theZ T"^ J''^""* three-quarters of ;ng part just below t e n u at o • t^^^^ '"""^ *^^^^°^^" «tand- brou^dit together, and the head of Vlf"^'''''"^ ''^^"^^ then be wires should be frapned round ,n ^^^^^^ther with the Hne ape^ commencing fi^Jm tl e lo we t 'art of tr^"'"' /"dia-rubber tendm- to at least an incli abovn fh f ^'f ^"^^ '^^ad and ex- If the india-rubber tlp^is not 't t" '^^ ^^'^^ ^"""^^•* tar will answer. See Fi^^ i^. ^ ^'^"^ "°"^"^«" tape dipped in in h^tl^ dloS'll::,^^!f ;t^-^ of the ^- i" this way serted nUo the charge, ^r^ i:tS:^J^Z;i:l^?^^^y "^ LAYING THE CIRCUIT WIRES. 103 Tl open a,.e m! 2^ tt ""o.S^.T/'.l.r^^^L "''"'' T"' '° '» ■•" *'- ben,,r placed abom a yard apart b?,tVf h,'' "V^"'"'«'. "'« «ires are attached to poles, L me.itiin^d in § 94 ""'"' ""' ^"''^ ""=>■ rolldi-alU^^HtTe^d'to'l:;:!!-^^^^^^^^^^^^ - '- it any"st'';l"™m!"yrth^^^ '? =* "«'" "- '<> P-ent of t.,e ,anery fra;;es to pr^^r ilj^^^rn'.'^^^^f -' ^ «'<= '°P wiS' t":rsysi,?st;,i',ts™ ■ ^-'''^ *° ■■' ™" °f --^ted rent! capable of firing the fuS ^ '''' '''" "^^'"^'-'^^ ^"i- FIRING THE CHARGE. 104 Fir ing Keys. is p.ovidVn.i^i;T£i;?rkeyrri°;r^t \t' !^"= ^^""'V'^- «>■"-"" an automatic firing arrMKcmem: ^''""°" %"a"'o l.as wj^tadm^'t'rSnii^iL^^ebdn?™"^^^^^^ ^'?"^'= ''atteries, allows of a fir„, contact beinf^o^SnedTV-''^^*-''™'! ""'='"' ''"d of a brass spring, to winch is solWH ^'"J ".■■"'- ^oy consists wh,d> ,s connected to a b.di '^ .^^'^r^xLtS iStZ," FIRING. 45 mcnt IS made of ebonite, and a second platinum point is attached to this base and is connected to a second bindin^^ screw. When tliese phitiniim points come into contact the two bindint^ screws are clcdncally connected, and to prevent this occurrin- prema- turely an ebonite catch is provided which, when in position, pre- vents the brass spring being- depressed.* FIRING. 106. If the charge is to be exploded by means of a Voltaic bat- tery, the return wire is connected to the negative pole of the bat- tery on the order to prepare to fire. The line wire is then connected to one binding screw of the key, the safety catch being in posi- tion, after which the other binding screw is connected bv a short lead to the positive pole of the battery. Oiithewoixi "A>," being given, the safety catch is released and the key sharply and hrmly depressed. 107. When the charge is to be fired by a dynamo the circuit wires are connected to the binding screws, on the order to prebare /oyzrc, and the handle is placed in position. c cS" *'f '"''^'"'^ "^'■''^" the machine is worked as explained in '_<> oo or S) go. EXAMPLES. 1. A makeshift wire fuze has an iridio-platinum wire brid-e .00144 diameter, and .3" long. Calculate the resistance of the bridge at the fusing point. 2. The L.L.R. of a common mine is 18 feet. Calculate the charge. Within what radius from the charge would a gallery belonging to the enemy be destroyed, supposing it to be on the same level as the charge ? 3. The L.L.R. of a mine is 15 feet. It is required to produce a radius of rupture of 50 feet. Find the necessary charge. 4. Find the dimensions of the chamber for a common mine to contain 500 lt)s. of powder. The ground sill of the gallery at the point where the charge is placed, is 17 feet below the surface iMnd the position of the chamber relatively to the gallery. 5. It is wished to form a lodgment 50 yards long and about 10 yards wide. 1 he gallery, in which the mines are to be placed is 15 feet under ground. Ascertain the number of charges recuiired and calculate their weight. ^ ,. *A repreHciitation of this Firing Key is given in Figs. 3 and 4, PI IX In<,trii<- tum in Military Engineering, Part IV. ' ' ^"^^""- n 46 USE OF EXPLOSIVES. At what distance fro„/^l e comteM„". "" 'f ' 'olojv the smface. lodfjerl, so as iust to destr™, ,,,"""=' -'""le should the chaive be being x6 feetVelovv the suTfa« J™"''""""'"' "'= ^'«='^'< K-'nery vw>ich\lSat':dtiij:ftL''s:t tet' n't^^ "- -^f-^. fence gallery. Calculate the ch^r"^ f,' ""^ '^° f'='=' from a de^ may just be ruptured. What clfLwi'f.'" '''=f*'"" gallery defence gallery were 24 feet below Ji;: ^"il'','^ "''^''"'''i '' thl slopini^etri-^'rl'tntelr^'it^t-crt^erl ^ ''^y^ ™»'er- means of gunpowder, Arra. ™ th^c nf °''"'.^5 feet high, by 10. A rolled beam 12" deen anri h^ • n mean thickness, is to be ci^ tinouti 3 '"^^''^" ^^^^ ^"^ -9' the amount of the charge ^e.ui^ ^^^ ih^^rll^.^S J centre, . -"wu^u iiie lower Hi of ?:oi!:tdt':;,';t1r™'°ITes'8?:;-H^°'"Pr''' =•* "- «: boom is connected to tl e web bv =^ Y""-" '""^ <=acb i" thick. The culate the charge re,u'il.'e7t?c!;f th tgK: ^^^ X i". Ca,! of bnuKi: t;t; xr e^ota^^? ^^^' =•'-■ '^ ^°-d amtytc^^farf - ^^ ^ ^--^^^e^^::z:t^ -'''x-«?''^c'Safet^?e°i,:rS':;' "''th that of placing Pla J'rSr'"- *■• "' »-" « "■•">«« »U. be to,.„.. ., „.. P,.„„„i„. ^„„„, EXAMPLES. 47 Kr^^' A l'°<=''^='<^ constructed of timbers 12" square is to h» Calclt tL^n""' °f S™^""™- The breachisto'be 8 feetwide tl,P I n^,H « ■"*^'' lequired, and show liow to arran/re it™th the Land Service sizes of guncotton. * " r,;f ; ^{""i "'" j'""'?'' "f f?"ncotton required to breacli an iron rail stocl series of guncotton changes S wei"h- I' i"thkk and' 8te't°r"- f ','' ''""''' ** f'' "■'d'^ '■" ••' brick wall plaJ{j^in-fthV»^^^^^^^^^ fr{n|,p±s3tL^°rer ?r?'k '°6°'';hkk 'V"'5 ^r' "'T "■'"''""• The wails are of Mick, lb thick. Find the number and weights of the charires of gunpowder required to demolish the house" cliaiges riv^-^'nf i'Tp °'f 'i" 'l'''o"'°'' ■' J"'"!'"' "' h.ncstone so as to give .111 L.L.R of 3 feet. How many inches of gunpowder slioiiW be^put^m to blast the rock? What would be ^tegmicoUo;! 'ii^n2:^!::i^i::^.:^^J^!''^l,J^'^^ -hlar a„d the der required to'^lemolish each p er Vh^c h nb'er'md ?f «""P°™" leading .0 it are to be formed bTbialiy itriui otto" 'fill mate the weight of guncotton required for this pmpo'e? 23. A brick arch 3 feet thick carries a road 20 feet wide ind i. 11 , il .1! !l arranged, and calculate its amount 'f 48 USE OF EXPLOSIVES. 26. A segmental brick arch 24 feet span carries a road 30 feet wide. The angle of the segment is I2o^ and the thickness of the arcn ring is 26 . Charges of gunpowder are to be placed behind one of the haunches in order to demolish the arch. Arrange these charges and calculate their weight. ,..27- A common mine wl)ose L. L. R. is 16 feet is to be fired by Bickford s instantaneous fuze, to which is connected a short length of Bickford's fuze. The length of the gallery from the bot- tom of the shaft to the charge is 80 feet, and the height of the shaft 12 feet. Find the length required of both kinds of fuze. 28. Three wire fuzes are to be fired simultaneously. Would it be a good arrangement to connect two of the fuzes in divided cir- cult, and the other in continuous circuit with them ? JO feet of the )ehind these l!» PH ed by sliort e bot- •f the :e. uld it d cir- CH AFTER IV. TESTING. 108. In order to guard as far as possible against failure, the whole of the stores and apparatus employed when using explosives should be examined and tested as carefully as time and circum- stances will permit. This can only be carried out to a very limited extent when the charge is fired by means of slow or instantaneous leader ; but when firing by electricity, and using wire fuzes, the condition of the circuit can be ascertained at any time up to the moment of firing. 109. The various electrical tests required for Land operations can be performed with the apparatus contained in the " Field Service Jointing and Testing box," namely : I Box of resistance coils with thermo-galvanometer attached. § 167. I Three-coil galvanometer. § 170. I Test cell. § 163. I Reel of iridio-platii um wire .0014" diam. A description of these instruments will be found in the § stated against them. The examination and testing required under various circum- stances will now be considered. EXPLOSIVES. GUNPOWDER. 110. There does not appear to be any ready method of deter- mining the strength of gunpowder, unless provided with the proper apparatus, such as used at Waltham Abbey,* but the fol- lowing tests can be applied in the Field to ascertain — (\) " // the powder have a proper colour, a proper amount of glaze, a sufficiently hard and crisp texture, and if it be free front dust. — These points can be judged by the hand and eye alone; and re- *These methods of testing guupowtler ure considered in the Artillery course. 50 TESTING. f \k i' [I quire a certain amount of experience in the examiner. The clean- frn?n °^ *^^^ P°^ where C is current, /' the electro-motive force, r the external re- sistance in the circuit, /> the liquid resistance in each cell, and n the number of cells, so that if C, P, r and /> are known, n can be found. If there is only one fuze in circuit, C is taken as the current required to fire the fuze. But if two or more fuzes are in circuit, the current must be sufficiently powerful to fuse the iridio-platinum wire of each fuze,t and the value to be taken for C, in each case, will depend on the fuzes employed and on the arrangement of the circuit. Thus if the fuzes are in continuous circuit, C will be the current required to fuse one platinum wire, but if the arrange- ment is "continuous circuit with divided fuzes" the current must be capable of fusing two platinum wires, and so on. The current required bj' the Service fuzes has been found by experiment, and is given in Table I, but when using unknown or makeshift fuzes, the strength of the fusing current must be determined as explain- ed in § § 242 and 243. The electro-motive force P, of the Service cells, is given in Table I, but when working with unknown cells it can be found as explained in §§ 234 and 235. The liquid resistance of the cells in use in the Service is also given in Table I, but if not known it can be found as explained in § 221. r can be estimated as follows : First, supposing the fuzes to be in continuous circuit, then the resistance will be made up of the resistance of the fuzes (at the point of fusion) together with that of the line wire, of the connections between the fuzes and of t\ie^,:^/rr' return wire. Thus, if / is the number of/fuzes, r^ the resist- ance of each fuze (at the point of fusion), / the total length of line wire, connections, and return wire (say in yards,) and r^ the resist- ance of this wire per yard, then But ifthe arrangement is "continuous circuit with divided fuzes,'' *See § 145. tThis has been ascertained by experiment. V ^^ TESTINr,. the resistance of the fu/cs will bcluilved hnf fl..f r *i remains tlie sa.ne. Hence, in this case '/C''^' "/j';" ^""^"'V meet a,, cases^,,. , ^^^^l^^, ^':^:r^:Si3t^'''' '° to make up for defective joims.le'aks.eic ""'"' '° "''' =" P'^- under the followinj,^ conditions : ^^^^tery poucr rccjuirod Circuit wires of Service cable. Line wire, len^'th, 200 yards. Connections, " 40 " Return wire, " 180 " Fuzes : No. 14 Low tension Land Service Cells : Leclanche. It will be found on reference to Table I that • '''''oClZ'y.76:'' ™"' ''=°-'5 "'-. ^ =-6 ol.ms, r, =„.oo6 Hence — A A u Z^^ "" ^'^'^ (200+40 + 180) 0.006=7.72 ohms I And by Ohm's Law— 0.8=— H1!L__ '' 7.72 + O.I5W X «=4-7- The number of cells used would therefore be 7 divides, X60 ytrds,°;nd"he saTe d fl^ce fro'.rthr o^ t^'l""^ current re-unites back to the bitferu T i m r P"" "* '''^'^''"^ >:vv*» A :ircuit, m be ed to tance , the ' p.c. ; are lirod V A D06 /I X ed nt re in id e- !4 Hence Thus TESTING DYNAMOS. 2.84 , • — ^=1.42 ohms. r=i.42 + 2X 160x0.006=3.34 ohms. i-45« 55 2 X0.08: 2 X 0,8= 3.34 + o.i5» Or ^=4.4. And 7 cells would be used. 3rd Example. Same arranf,'ement as in the second example, but usin^' Military Grove cells instead of Lccianche cells. The resistance of the circuit wires will not 1h> altered, but dif- ferent values nuist be j^iven to P and to />, namely : P=i.956 volts, />=o.o8 ohm. Therefore, 1.956 n 3.34 + 0.8 n And »=2.g. 4 cells would therefore be used. QrANlUTY KYNAMO. 122. This machine is tested by fusion of wire in the same way as a battery. TENSION DYNAMO. 123. This instrument is tested by measurinp^ the lenp^th of th>» spark it produces, and, as already mentioned, it is provided with a spark measurer. The test is carried out as follows : The micrometer screw is set to zero, and the two platinum points are then made to touch by screwing,'' up the lower adjust- ment screw. The two points should only just touch, and this can be ascertained by passinj; a current from :i test cell through the measurer, with a {,'alvanometer in circuit, the needle will be de- flected the instant the points touch. The micrometer head is then unscrewed to -gp^/ and the spark is passed by working,' the machine,* the interval between the points is then gradually in- creased until the spark only just passes. If the instrument is in good order a spark /ffVir" lo"?? should be ootained. CIRCUIT. 124. The circuit wires should be tested both before and after connecting up, in the following manner : TESTING CIRCITIT WIRES. 125. The circuit wires should always be tested for continuity and sometimes also for insulation. These tests are performed as ex- plained in §§ 247, 210 and 212. If it is found that there is a disconnection in the wn-e. the next operation is to localize the fault, and this can be done as explain- ed in § 248. *See § 90, 56 TESTING. |i I I h inspect!.,. ,„. ot..„v:Jj^tSnt.i:rL^-Jre '^^Ltt 10.x. «1„,„, ,u, wil 1 e ;,^n L ^^"'« "<'K.''-'<='"1 'f 't is not Ic-ss thun sistuncu of a circurt be o ^„| ,T ^'l rX'.'i'rh;""!'' ° ' '■''""' '=• looo ohms, and, for simnhVi. J '"siilation resistance leak ; then fC it °c^'tZS,tT''"'^'«''' ''""'" '" ""ly ""-^ ductor beyond the leak wilTbe™ ' P'"' """'"« "'""K »'■<-■ eon- iooo + io^ = -99C so that the loss can be neglected. mc!lt i,;:;Zi;;f £- ^;^- -l-'l^^tlon test is the across the iiiterva (if o^'too . re.t 'f ^ '"''' 'J/" '^''''^ ^^•'" i"'"P wire where there . -wHc ^ !^ between the two ends of tlie the hi,h resist" e'of'\"drcuiT*r' IoST^'^'^"^ '-^^^^^^^ '' considerable, if tlie i, sni-it on u T 7 l"')^''^'' ^^""^^l ''e appear by the fo Wi "ex^^^^^^^^ de ective as will cuit be 2o,ooo ohms (abon to f^ • i "" resistance of the cir- insulation resist^ ce tl be 1, onn f '''" ^"f"'^' 'f^ "'^"PP"'^^ the current passin, thrould, fees if"" ' ^'"" ^^"' I'^^-^^" "^ ^'-^ 40,000 40,000 + 20,000^"^^ wnere C ,s the total current, so that one-third of the current is TK.STIN(} COiMPLETE (■tKCUIT. .J^^f ^''"^ complete circuit can only be tested in th^ p;»u 1 wne fuzes are used bpnnc» ih ^ ., , '. "^ tlie f^ield when to measure the 1 ?'h ^S^^"^""' ^''''^'^^? ^? '^^' ^"^^^i^^^t are employed. The n ethr^^^ 'f . .^"'^ ''''^" ^"^''^ tension fuzes explained in § 199 ^^'""^ °^ ^^'^'''S the complete circuit is ma^e^arrn^aTSi^c-rcuitl^'lri f,''^ T^"^ ^'-"^^ ^e where K^illeries havL fn hi . ^T^,'^^^^"^' '''"^ "^ those cases interval^ wh Is t lie tanioin' T'^''^ '^'^ ^''^ ^*'°">^ ^^ '»^^^e at any injury to tlie cirSl^es. '"""'' °"' '° ""' '^ ^^ °"-^ ^^^-^ thi'c^cuit'aTesTF'^^'nT- ''""f !?W'T'-"^ ^'^ --^^ance of (See § 247 ) ' continuity should, if possible, be made. *See " theory of divided circuits," § I47. HXAMI'LliS. EXAMPLES. 57 1. Find the number of Lechinchd cells required to fire one N0.14 fiue, the length of Service cable in circuit beinjj; 500 yards. 2. Find the number of Leclanche cells required to fire two No. 14 fuzes, arranged in divided circuit, through 500 yards of Service cable. 3. Find the number of Leclanclul cell^ required to fire two No. 12 detonators, arranged in continuous circuit, through 500 yards of Service cable. 4. Find the number of Grove cells required to fire two No. 12 detonators, arranged in continuous circuit, through 500 yards of Service cable. 5. How many cells, whose electro-motive force is i volt, and whose liquid resistance is i ohm, would be required to fire one No. 14 fuze through 500 yards of Service cable ? If the Hquid re- sistance were increased to j.i ohms, how many cells would be necessary ? 6. Show that 36 commercial pattern* Leclanchd cells can be so arranged as to fire two No. 13 detonators, connected in divided circuit, through 200 yards of Service cable. (Electro-motive force 1.45 volt, liquid resistance 2 ohms.) 7. Three gunpowder mines are to be fired simultaneously. The primers are prepared with divided fuzes, and the length of circuit wires is as follows : Line wire 200 yards; from No. i mine to No. 2 mine 30 yards; from No. 2 mine to No. 3 mine 40 yards; return wire 190 yards. The line wire consists of Service cable, the connections between the mines of No. 16, B.W.G. insulated copper wire, and the return wire of No. 13, B.W.G. bare iron wire. Find the battery power required, using Leclanchd firing cells. 8. Ten guncotton charges are to be fired, simultaneously. Each charge contains two No. 13 detonators, connected in divided cir- cuit, and the charges themselves are arranged in two sets of five each, each set being connected in continuous circuit. The line and return wire are together equal to 300 yards, and each branch is 10 yards long; the former consists of Service cable and the lat- ter of No. 16, B.W.G. insulated copper wire. Find the battery power required, using Grove cells. 9. The fusing current of some makeshift wire fuzes was found by .experiment to be 4.5 webers and their resistance, at the point of fusion, to be 0.17 ohm. Two gunpowder mines are to be fired by means of these fuzes ; each mine contains two fuzes in divided *See § 803, Ganot's Physics, yth Edition. 58 TESTING. ?i rn?f Tif r ";'"? themselves are arranged in continuous cir- cult The hne and return wires each consist of 20 yards of No. 18 .'?' i^oPPer wire, insulated with tarred tape, to which is con- nected 180 yards of No. 9, B.W.G. bare iron wire. The con- No ?? R W r" *^^ "'^"'' ^'- ^5 y^^ds long, and is made with ll \ .: ^•^•^- ^°PPef wire, insulated with tarred tape. Find the battery power required, using Leclanchd firing cells. firfn°; K ^^J'*^ *^f ^^r""* ?^ connecting a Leclanchd test cell to a firing battery of 10 Leclanchd cells. nJ\^ii ""'"^^ ^""^ connected in combined continuous and divided rlTi' f J?'"^' ^^- "^ P'^^^^ ^" ^^^h continuous circuit. The resistance of the circuit is measured, and is found to be 4 ohms * dard .0014 iridio-platmum wire through 14 ohms (not including renuirpH t'"f 1'^' ^^''^ ''''^^'^ • ^^^^"^^*^ ^he number of cell! TelUo be 15'ohm. ''''"''' ^"PP°^^"8^the liquid resistance of each 12. Two guncotton charges are to be fired by electricity • thev are connected m continuous circuit with divided fu^es ' The ZZ resistance is found to be 3 ohms. How many Grove cells would be required to fire the charges.* f ^"^A ^u'^ffS^'y^ ^i^''^^^^ ''^^^^ '^ ^^*d i" position, and it is then found that the insulation resistance is only 100 ohr^s. A bare re" turn wire of No, 14, B.W.G. iron wire laid on the ground has celk fn'fir' ^'"m *^" ^^"''y ^"^"^^^^' "^i"^ Lecknch/firLng cells, to fire one No. 13 detonator, the length of line wire and of n^irpH r^'fi ^T5 '^'^ T y^'^'- ^^"^ ^^^° the battery power re- quired to fire 6 No. 13 detonators in continuous circuit, nl^lect- mg the connections between the detonators. How many Sis i""!ated r^""' " '°'^ '^^ ^'^^^ ^^^^^ ^^the return ^h'e^ere 14. Given that a certain quantity dynamo can fuze the standard .0014' iridio-platinum wire through 54 ohms and that the internal resistance of the machine is 13 4 ohms, find the num ber of No. 12 detonators, and also of No. 14 fuzes the machine could be rehed on to fire through 300 yards of Service cSe 15. With the same data as in example 14, find the number of th';'/ t'TT', ^"^^°- '^ f»-es, arranged in pa s d^Tded cable '^ ^^ '^' '""'^^"^ through 300 yar^ds of Service is oniv^^''"''loV/"'Hn ^' '''''\r^ ^t is found that the spark IS only „^^^ long. How many No. 5 detonators, connected in continuous circuit, would this machine probably fi^e ? " .Up '^"^ *,^i' ?*'^' V^^^^^^l^^ the cells are perfect, it is not „ecp,«ar^' to -.-U 50- n o t. tne calculatcu number ot oeil« sim-p th^ n,t„„i «n„,„4. ■ ■^. . " ''^ P*'- to resistance thus measured SuS,2nftI.rf '^^"'*'r'"^ in circuit is known. The increased at the poinrof f "sion "'"' '^'''" "''^^' «"•' t*^'« resistance is to a PRECA UTIONS, TO BE TAKEN WHEN USING EXPLOSIVES.* EXPLOSIVES. GUNPOWDER. 1. Examine the gunpowder, and if its quality and strength is unknown, test it as far as time and means will admit. ^ no. 2. Powder barrels are to be opened with a mallet and a copper drift, or else a wooden drift, but never with a hammer or an iron drift. 3. Only one barrel is to be open at the same time. 4. All tools (scoops, scales, etc.,) should be made of copper. 5. The charges should be r/iade up outside the magazine, and if possible on a skin or mat. 6. The men working should take off their boots. 7. No naked lights to be allowed near the powder. 8. Care should be taken not to spill any powder. 9. Sentries to be posted to prevent any one approaching with a naked light or smoking. GUNCOTTON, 1. Dry guncotton should be stored in a spark-proof air-tight box labelled " dry." § 29. 2. Wet guncotton should be inspected periodically to see that it contains the proper percentage of water. § 15. 3. Ascertain that the guncotton used for primers is perfectly dry. § 15, (6,) 4. When carrying guncotton made up in charges, it should be protected with a spark-proof covering. *Thesc precautions are principally abstracted from the preceding part of the book, Keference has been made to the correspouding paragraphs. 6o ' i ■' PRECAUTIONS. separated DYNAMITE. until JhalS"ti^ '""" ''' '"^""''^^^ "^"^^ "«^ ^^^ ca^i.'^l'ig!''''"'"^ °^ ^'°''" ^^"''^"^^^^ "^"^^ ^^ d«"^ ^vith great snLiS bnfottat"'^ ^" ^°'' ^^^^^^^^ ^^-^ ^^ - ^ --Ptacle coCr^^^u^twdfr.'^ '^"'^^' ^^^^^^ ^^- — - -^^y^-n- ^ 5. ^If the quality of the dynamite is unknown, test it, if possible. PREPARATION OF FUZES. General. I. See that the proper fuzes are selected. For gunpowder the fuze contains mealed powder. § 31. For guncotton, detonators must be used • fhp^7 Iia,L o" 1 . pamted red containing mercuric fulmfnate '^31 Table I ""^ fuzTsllTctVabirr '"^" ""^ P'-^^"*^^ -'^^'' ^'-- «^ tension 2. E.xamine the fuzes and test them, if possible. §§ii7andii8. When testing electrical fuzes use a test cell whnc;p \\n..\A ,- • ' tance is not less than 12 oJuns. § 163. ^'""^ '^^'^^ Electrical detonators must be placed in a stront^ box whil.f bemg tested, m case of accidental explosion. ^ ' 3. Secure the fuze to the leader, or to the circuit wires in such a manner as to prevent any chance of separation. ' The thin brass tube of the Bickford detonators for ^runcotton Z.Tr' '' ""^ "°' '° '''^'"•'' ■" --y "-y the mercuric ftl! ELECTRICAL FUZES. I. See that the joints between the fuze and the circuit wires are good 1 e., wires and eyelets scraped and brightened a firm con tact obtained and the insulation good.) Wl^^n ustg w rT fSzes look especially to the contact ; when using high tcnsioiT fu 4s L the insulation. <> 102. iLUbion uucs to PREPARATION OF CHARGES. 6i 2. See that the fuzes are perfectly dry outside. 3. If the fuze has eyelets, wrap the head with india-rubber tape to more effectually insulate the leading wires from each other. This must always be done when working in water — in damp soil, and when the charge is likely to remain any length of time un- hred— otherwise it is not necessary, but it is better to do so if time permits. § 102. 4. It is better to place two fuzes, connected in divided rcuit, in each charge. This should always be done, if possible, hen using guncotton. § 45. PREPARATION OF CHARGES. General. 1. The charge should not be made up sooner than necessary. 2. Take great care that there is no chance of the fuze or fuzes being withdrawn. 3. Never apply solder to any wires near the charge, unless the charge is well cove'-ed with some fire proof substance. GUNPOWDER CHARGES. Primer. 1. Use a tarred sand-bag if the ground is damp, or an india- rubber bag, hermetically closed, if the ground is wet. § 47. 2. Place the fuze or fuzes at about the centre of the primer. 3. Secure the leader, or the circuit wires, in such a manner as to prevent them from being withdrawn from the primer. § 47. Charge. 4. Place the charge in tarred sand-bags, if the ground is damp, or, It the charge is to remain unfired any length of time • but if the ground is wet, use india-rubber bags or other water-proof re- ceptacle.* § § 52—68. GUNCOTTON CHARGES, Primer. 1. To prepare the primer, insert the "rectifier" into the detona- tor holes to Its full extent, and then withdraw it by twisting, § 48. 2. Insert the shank of the detonator without twisting or other " force," and see that it fits tightly into the guncotton. § 48, 3. Detonators should be secured to the primer by means of string or wire, •Blasting rock is an exception. ■'I [ill 62 PRECAUTIONS. Primer for wet guncotton. „cJA ^""^ ^^^^ ^^^"i ^"^"^ ^^- ^^ ^^/^^^6' dry guncotton should be used as a primer for wet guncotton. §§ 14 and 49. 5. Having inserted the detonators, seal the primer up in a water-proof bag, the mouth of which is closed hermetically by means of a clip and india-rubber solution. § 49. Charge. 6. Insure as close a contact as possible between the primer and the charge,- absolute contact is imperative when wet guncotton is employed. ^ 7. See that the continuity is good. ^^8. If the charge is to be placed under water use a water-proof DYNAMITE CHARGES. Primer. 1. Insert the detonator, (the leader or circuit wires being at- tached) into a hole made in the explosive by means of a smooth piece of wood. § 50. 2. Close the cartridge and frap with wire or string to prevent the detonator being withdrawn. f^cvcui Charge. 3. Take the same precautions as for guncotton charges. PREPARATION OF FIRING ARRANGEMENTS. FIRING BY MEANS OF LEADERS. 1. Examine the leader and ascertain its rate of burning. § 114. 2. When using slow leader use a sufficient length to enable the operator to ^et to a place of safety, 3- Instantaneous leader should generally be fitted with a short length of slow leader. • t When the leader is to be ignited, without fail, at any given instant, it should be fitted with quick match. § 71. 5. Carefully secure the leader to the primer to prevent any pos- sibility ot separation. ^ ^ 6. Great care must be taken not to injure the leader during- the process of loadmg or tamping. § 52. PREPARATION OF FIRING ARRANGEMENTS. 63 FIRING. I. Use a vesuvian or a port-fire. § 76. FIRING BY MEANS OF ELECTRICITY. SOURCES OF ELECTRICITY. General. 1. See that the connections are bright and clean. 2. Test the firing-apparatus (battery or dynamo) shortly before firing. §§ iig, 122, 123, 244, 245 and 246. Batteries. 3. Calculate the number of cells required. § 120. 4. Make up the battery with amalgamated zincs. § 82. 5. When using Grove cells do not pour the acids into the cells sooner than necessary. § 82. 7. See that the cells are kept as dry as possible, rejecting leaky ones. CIRCUIT. 1. Examine and test the cable. §§ 125, 210, 212 and 247. 2. See that all the joints are good. §§ 99 — loi. Solder the joints, if the charge is not to be fired for some time. Insulate any joint that will be pk.ced under ground, using in- dia rubber solution if there is any wet or damp. If the joint is not soldered do not place any india-rubber solution on the wires. 3. Serve exposed portions of the dielectric with spun yarn. 4. See that no stress can come on the cable during the process of laying, and also afterwards, and that it is not injured during the process of loading or tamping. § 103. 5. When using wire fuzes test the complete circuit for resis- tance. §§ 126 and 199. cess 6. Test for resistance, or continuity, at intervals during the pro- ss of tamping. §§ 199 and 247. In both the above tests a test cell whose liquid resistance is not less than 12 ohms must be used. § 163. 7. When a number of charges are to be fired in succession by means of the same cable, the cable should be tested for continuity after each discharge. § 247. 8. To be safe from the effects of atmospheric induction, a roll of insulated wire should not be connected to a charge except just before firing. § 103. I i-. I 64 PRECAUTIONS. CAUTION.— The man who attaches wires to a charge is re- sponsible for his own safety. He must see both ends of the wires he connects. riRINO. 1. The firing apparatus is not to be brought up sooner than necessary. 2. With voltaic batteries— Use a firing key, and see that the safety catch is in place. § 105. 3. With dynamos— Remove the handle. The person in charge of the operations must keep the handle in his own possession. 4. No connections at the firing point are to be made until the order to " Prepare to fire " is given. This order must only be given after all is reported clear. §§ 106, 107. h -c ' '^ ; r Ci,^ ^^" :harge is le- , of the wires sooner than see that the son in charge )Ssession. .de until the lust only be TABLE I. EXPLOSIVES. GUNPOWDER. Size of grain of various gunpowders — R.F.G'^. will pass througli a mesh 12 to the inch, but not through a mesh 20 to the inch. R.L,G- " " 4 " " " 8 " RLQ.^ " " 3 " " " 6 " P ; cubes §" side. P2. ; cubes li" side. Volume — I It), of gunpowder occupies about 30 cubic inches. Price per tb., (average) 6 pence not including carriage. GUNCOTTON. Dimensions and weights of the principal forms of compressed guncotton in the Britisli Service. NATURE. Dimensions in Inches. I WEIOHT WHEN DRY. ? I I 2 ozs. ii ii NO. OF Perforations. I I I PURPOSE. Submarine mining. -Blasting. ) 6^ X 6i X i^ 6ix6ixii^ 6^x6^x1^ , 23- lbs. 4* li" I f Torpedo and ( Submarine mining, 3t J! Demolition of A.f 'j stockades,walls,&c In store,' guncotton shbuld contain 20 p.c. of water ; 15 p.c. will prevent it from burning. Time required to dry various discs and slabs : — i^ inch discs 4 hours. Other sizes below 3-inch discs 6 to 8 " 3-inch discs and parts of slabs 12 '' Whole slabs (according to thickness)... 18 to 24 " *Only a certain iiunibor of these discs or slabs are perforated (25 discs per ton), and these are intended fur primers. tA proportion of 2ftis. slabs cut in three and of l^tb. slabs cut in two are supplied with the E.E. Field company e(|uipnieut. i,l: ¥: »r- Pi u I 66 TABLE I. Price per ft. (average) 2*', not incliuling transport. DYNAMITE. Dimensions and weights of various cartridges : — Length, 6| inches. Diameter, i to i|^ inches. Weight, 3 to 5 ozs. CONSTRUCTION OF MAGAZINES. GUXPU WDER MAGAZINES. Gunpowder in bulk is stored in barrels or metal-lined cases. Mdal-lincd wood cases for ammunition. WeiijhL I E.rfcrnal Dlmi-nifiiius over Cutte. Remarks, Whole common Empty Filled . OZB.' Half common ) 1 Empty ^ Filled . «-"»(», common lbs. 45 varies. I'J'J 138 70 27 varies. 57 8 16 8 varies. 79 l«-75x l()-75x 20-(J2 13 44x 13-44 X 16-75 10-25 X 10-25 X I \ .„-„,. ) i3-88 " \ With tilled cannon cartridfj:e.s. With 330 fuzes, mortar larye. With 1,000 fuzes, mortar, small v\ ith 20tt)S of (juiek match. With tilled cannon cartridges. With 1,200 fuzes, hand grenade. With tilled cannon cartridges. With 125 fuzes, time, Armstrong. Barrels used for ammnnitiou. Weight. Dtriwnmotis. Diameter. End. Middle. ' lbs. ozs. ^ Whole.. 31 Ammunition, 1 Gun. J fHalf.... 19 Budge, Mark! ; 10 Cartridge, small-arm. Powder . . i^ Half.... I / Quarter 1 Whole . . ) Half ... J Quarter ( Eighth 12 7 12 30 18 11 S 5 lOi ni. 15-3 11-!) 10-2 ll-!» 10-2 15- 11-9 10-2 7-1 in. 17-3 13-4 11 6 13-4 1 1 -6 17- 13 4 11-6 8-5 Hehjht BemarkK. in. 21- lG-9 14-2 16-9 14-2 21- I6-!) 14-2 11-5 For filled cannon cartridges I ill ilrji muiidziiii'-t or for ' occasional transport. tI)o,, seldom tiscd. For loose powder for mortars. ( To contain re.s])ectively ' 2,240 and 1,300 cart- I ridges, blank, Ro.xer, for ' Snidor rifles. -577" hoi'e, I Mark II. i To contain resj)ectively 100 50, and 25 Itis. of loose r powder, H.I,.(I. or L.G. Kor conveyance of small ijuantitie.-i of loose powder. es. rt ridges or for iiortars. I'ctivt'ly cart- xer, for i"' bore, ■ely KM) >f loose )r L.G. mnall lowder. CONSTRUCTION OF MAGAZINES. 67 Racks. — The iliinensioiis of tlie timbers used in the racks depend oil tlie arran^'einent of the barrels, and are to be obtained by cal- culation. Barrels should not be piled in more than 5 "iers with- out support, and can be stacked to within 2 feet of the roof. /^f.v.s7;.i,'-c',9.— Louf^ntudinal passages 4 feet wide ; side passages 2 feet wide ; in any case 6" must be left between wall and ends of barrels. In large magazines transverse passages, about 4 feet wide, are useful. WALLS. Height — From 6 to 10 feet. Thickness— Walls acting as abutments to the arch of the roof must be designed to resist the thrust. Minimum thickness 2 feet. Walls exposed to Artillery Hre ; thickness of protection (earth, concrete or masonry alone or combined) li times the depth* to which the heaviest shell the wall is likely to be exposed to will take effect, when tired with delayed action percussion fuzes. Walls of store magazines, 5 feet thick, to minimize the effect of an accidental explosion. Brick lining. 12 14 IG 18 20 22 r4" I'o" I'd" l'7i" n»" I'loi" 1 8 13 14 15 IG 1 7 24 o'O" 19 ^ -42 " thick when attached to outer wall, q" when detached. Damp-proof course of asphalte on floor level, V thick. ROOFS. Magazines exposed to Artillery fire. Thickness of arch ring when carrying earth and concrete covering: Span, in feet [ 10 Thickness for \ Segmental 120'. 112' block-stone arches ( semi-circular . . ) 1 1 J Brick arches are made as nearlv as possible of the same thick- ness as the above. Rubble stone arches should be made about one-fourth thicker. t Protection against damp.—U an inner arch is built there should be a clear space of 3 between the two arches, and they should not be bonded together. The inner arch should be capable of standmg unloaded. Roof cover ing.—Cox^cxeie over arch, i foot thick, nearlvfillin-the spandnls and forming a valley for drainage. Asphalte over "con- crete !f thick. Earth covering, i^ times the depth* to which the heaviest shell to which the roof is likely to be exposed will take *See Table A, Guide to the Course of Military Engineering. tExtracted from a table given in "Instruction in Construction," by Col. Wray, l\. E. 68 TABLE 1. m effect when fired with delayed action percussion (yize?>. If this thickness cannot be obtained, then for every foot of earth in defect add 3" to the concrete over arch or to the arcii rinj,' itself. The concrete is supposed to be made with Portland cement ; i cement to 7 shinj,de and sand. Ma^fazinea rw! t;.\i^uscd 10 Artillery fire. The arch must be designed to support the foUowinj^ loads : (i) wei<,dit of arch itself, distributed, (2) wei<,dit of ordinarv roof over arch, of snow lying on this roof, and the wind pressure', con- centrated at the points of contact. Roof covering. — Slates, tiles, or metal. FLOORS. Thickness of concrete 9" to I'o". Thickness of asphalte f". (Joined to the damp-proof course in the walls so as to form a continuous damp-proof surface.) Oak plates 4" x i". Battens 3" x i^", 'i" apart, and secured to the oak plates with brass screws. VENTILA-'ION. Outlet shafts. — The total effective area should allow i sq. inch to 100 cubic feet of interior space, if the magazine is above ground ; and I sq. inch to 60 cubic feet, if the magazine is under ground. In large magazines the air can be taken at two or three points, and then led to one up-take shaft. Inlets, about 9" X 3" in section, should be placed about 10 feet apart, and their total effective area should be at least equal to that of the outlets. In magazines above ground the inlets estab- lish communication with the outside air, and in underground magazines with a passage surrounding the magazine. Inlets and outlets should be provided with shutters or louvres, so as to allow of their being entirely closed or opened to anv de- sired extent in any weather. The air-space in hollow walls should alpo be ventilated. LIGHTNING CONDUCTORS.* Lightning conductors should be placed on the highest part of the building, and 50 feet apart, so that no portion of the building is more than 25 feet from a conductor. Height of rod above the magazine, about 5 feet. *Tiie following data have been taken from a ijaper by Captain Bucknill H E Vol. XXV, R.U.S.I. 1 .^ f ., .J CONSTRUCTION OF \fAGA;jlNRS. 6g The roil is best made of iron (f,'alvani;ie(l) vveif^diiiif^' (> !l»s. per yard, and it is advisable to terniinatt it witli several points. The coudiictny attached to the rod is best made (A j^Mlvanized iron wire rope, round or flat, weij^diin},' 8tbs. per yard if one con- ductor is connected to each rod, and 4lbs. per yard if two are used. Etif'fh'i. — A conductor should always be led both to a shallow and to a deep earth, the former to provid j for wet weather and till' latter for dry weather. Shallow earths aie best formed by layin/^^ the conductor in a shallow trench, i foot deep for clay, and 2 feet deep for sand, i^ravel or shin.s^le ; 25 feet lonj,' in ordinary ^oil, and 50 feet lonj^- in very porous soils. This trench is filled with coke and ashes. Deep earth connections can be made by sinkinj^ a well about j feet in diameter to a depth of 10 feet below the subsoil water level in tin; driest season. No mortar 01 cement to be used for the lininf( of the bottom 10 feet of the well. The "earth" itself consists of a galvanized cast-iron pipe offering 24 sq. feet of outside sur- face, and it should be surrounded with lumps of coke. All joints should have an area 6 times the area of the conduc- tors they connect, and should be run in with molten zinc or lead. All metal surfaces, except those entirely within the magazine, should be connected to the lightning conductors. This can be done by means of a horizontal band. EARTHEN TRAVERSES BETWEEN STORE MAGAZINES. Crest, 5 feet above top of roofs. Thickness at top, about 20 feet. Top slopes, I. Side slopes, -f. Distance of foot of slope from wall of magazine, about 20 fee.. GU2iC0TT0N MAGAZINES. Dimensions of guncotton 50 lbs. boxes : Length, 2 4"; height, I i"; wif' h, i' 9". Internal dimensions of magazine. Width, 10 feet ; length, depends on the number of box* • to be Stored. These boxes can be piled in 5 tiers and in two row ;, one on either side of the central passage ; 6 feet in excess should be allowed for shifting the boxes at inspections. Height, 7 feet. Floor ; made of concrete, 12" thick, sloped to the centre of the passage to a drain covered over with perforated tiles, or with ;ui iron grating. 7') 'I h TAHI.j; I. 'JW/v ; the l.uil.lin- is half-si.iik. and earthen traverses about AW/; ordinary. A Unk, holding lo gallons per ton of guncotton, shoul.l l,e pro- can be .it .id., ,1 ; the I,ott.„n of the tank to be Hush with the roof A small shiftn.-.nx.ni should be provided. FUZES. Fuzes for use with slow or mstantancons leader:* fv^aZ"^^? ^'f /"'^'"'—Enipty brass tube 6" Ion-, diameter, .25": taperni^ tin tul,e 2 lonf,s smallest diameter .Tt". CharL^ 20 grau.s of mercunc fulminate. Paiuted red all over. ^ ' Official name-Detonator No. 8, for Bickford's fu;?e, Mark II. Gmcottcm dctomitor for instantaneous /«.~..-Similar to detonator Sn dk fnr'v ''•';. ^^'^- '^' "li^^ ^■•^^1"""^'^^ ^«'- "^^- ^"tended pnncii)aliy for Naval Service. Painted red all over. ^Official name-Detonator No. 15, for instantaneous fu^e, Dv;m;m7. r.yi.-Made of copper, length i", diameter i". Charge about 5 grams of mercuric fulminate. ^ ,aJ I /- ^ •Biokf-rd's fu/.o is innhulcd under the head of proparatioi, of fi nag arrangements. ELECTRICAL FUZES. 71 h O J w o > w (/5 5 J;^ I £ £ '^ S '*' i «9 X ?- * :, a ~~ -5 B = * ■ i a (I) u ^ N i ti. ; j^ CI o o o J , < I o ' I— « q: ! ^ .5 < ;,i •O.tll ■''>'// ii/Ji xs 1 ao ^ S ^ « c4 ^ •^ ii -(J a> >. W «2 2 ?2 3 ci i ■c »•• H H 5 IS 6C -2 <« So »- • « d so 'm 9 < < f4 «! '■a £ a ^ 1 60 Q *«5 .>l.>,t,)<^< a » ' . ^ — OJ • (0 . s • »3 • « 1 ri ^ 1-^ f^S f ^? 1 1 -i-TS ' « ' Heat Shoiil Tube Heac Shou Tube 1^ CI 1-^ 2 d X^ C9 'l(.t)))Vii/.ii .11)/ ■KlllfOpfx^ a > 1— 1 .1— < -ir*"^ »— ^ >« r-l — ' ■:i! \n —1 -•.J:^ . t- . ^ * &» =« C '■' '^JJ i^:?; >*■- «^ •aaaMojKnf) ^5 OJ xg s «5 l-H J---^ ^1 ;^,s •.Nroxj.oo.s:nf) o a 9 o u .3 «o J3 01 S iffi ci a u > J3 - iP 'C _ S ^j D a ™ NO,, aj a I"? o Q 3 3 o 8 t. V o « 1 39 f-A I i fr, ' 72 TABLE I. PREPARATION OF CHARGES. PRIMERS. Bags. India-rubber bags for gunpowder primers containing loolbs. of powder. Body; length 12", width 15" (when en:pty). Neck; length 5^", width g". Leather bags for gunpowder contain 90, 50 or 28 lbs. of powder. India-rubber bags for dry guncotton primers (for use with wet guncotton); length 14", width 5:^" (when empty.) FOBAfULjE AND DATA For calcidating the weight of the charge required for various purposes, arranged in alphabetical order. ARCHES. In the f©ll«vving C is the charge in lbs., L the line of least re- sistance, T the thickness ©f arch-ring and B its breadth, in feet. Gunpowder, Charges placed behind the haunch of the arch at two-lined intervals — For moderate demolition C=^LK For violent demolition These charges to be placed at a distance of 2L from side walls and not less than 3L from top of road. L should not be less than i| feet and not greater than 5 feet. Tamping at least 3Z. Single charge, or a line of charges, placed in a trench cut over crown of arch to the keystones and covered with the excavated rubbish — Guncotton or dynamite. Charge placed on crown of arch, uncovered — placed in a trench and covered- C='iT'B BLASTING ROCK. Gunpowder. c — aL^ where c is the charge in 02s., and a is a constant, depending on the nature of the rock. For rock of ordinary tenacity a = L Ex- WEIGHT OF CHARGES. 73 periments should be made to determine the value of a if the blasting operations are to be of any extent. L is the line of least resistance in feet. Tamping, not less than i\L. Guncotton or dynamite. Charge placed in a bore hole and the hole filled with sand or powdered brick. c=hLK For soft slaty rock h=\ ; for hard granite 6=i. A 9-0Z. disc of dry guncotton, placed on a hard boulder 3 feet in diameter, will generally shiver it. BLOWING DOWN HOUSES. Gunpowder. Charges placed in angles of building, or against walls at two- lined intervals, and tamped. where C is charge in lbs. and T thickness in feet. See also demolition of walls below. BRIDGES AND COLUMNS OF WOOD OR IRON. The charges of guncotton required for the demolition of iron and wooden bridges and columns can be found from the formula given under the head of cutting through iron and wood. CUTTING THROUGH IRON PLATES AND THROUGH WOOD. The charge of guncotton or dynamite required can be calcu- lated from the following equations, in which c— charge in 02s. i=thickness in inches. 6=breadth in inches. Wrought iron — Charge placed in contact with the iron. For solid plates C — 2.lt^b. For laminated plates (rivetted), each lamina h" thick. (Not applicable if /2L the mine is said to be overcharged. *These are empirical formnliP. and ^lo nr.t appear to be very reliable for ov^r char,M m.n.s. F.nuula (a) g,ves possibly t.o ilL accurate w4lxtS the charge tThis formula was obta'ned from some exueriments imrlp nf ri,ofi,n.^ i WEIGHT OF CHARGES. 75 I* The weight of the charge can also be obtained from the follow- ing formula : -UtI (a) C=^U- i^l +^ Camouflets.~The maximum charge for a camouflet can be obtamed from formula (a) by putting Z)=o, hence Increase in charge to allow for deficient tamping. Increase in charge. i Diminution in tamping. _2 3 No tamping. Charges for day and rock — For clay, 1.55 C. For rock or masonry, 2.25C. above^'"^ ^^^ corresponding charge for ordinary soil, as given Tamping.— Length of gallery to be tamped— For common mines, i^L. For an overcharged mine giving a 3-hned crater, zL. PALISADES. ^ A palisade of triangular timbers, 8" x 8" X 10" in section, with 4" m the clear between the timbers, can be cut through by L charge of guncotton i^ lbs. per foot run. ^ ^"drge PIERS OF STONE OR BRICK. In the following, O is the charge in lbs., L the line of least resistance, B the breadth, and r the thickness of the p"er, l feet! Gunpowder, Compact solid mass of masonry or brickwork, with circular polygonal, or square base. Charge placed in centre of mass ' For moderate demolition For violent demolition C — 3 /• 3 High bridge pier, 20 or more feet high. Charges nlar^n a+ f, lined intervals, in the centre of the ma^sonry. ^ ^ ^"'"^ ^^ *'^°- 76 TABLE I. \l For moderate demolition For violent demolition High bridge pier founded on earth, with an equal pressure of earth on both sides. Charges placed under the centre of the foundations at two-lined intervals. For moderate demolition ^ . . C=^LK For violent demolition C — era Short thick bridge pier. Charges placed in the centre of the masonry at two-lined intervals. For moderate demolition For violent demolition Short thick bridge pier founded on earth, with an equal pres- sure of earth on both sides. Charges placed under the centre of tne toundations at two-lined intervals. For moderate demolition For violent demolition Tampmg— Not less than i^L for moderate demolitions, and 2L tor violent demolitions. Guncotton or dynamite. Small piers. Charge placed in contact, untamped. Charge placed in a groove ^T deep and tamped. Large piers. |ths of the coiresponding gunpowder charge. Taniping=L. RAILWAY RAILS. 8 ozs. of guncotton will blow away i foot of a rail weighing 70 lbs. to the yard. One-third of a 2 lb. slab (lof ozs.) will cut through the hea* iCst rail. REVETMENTS. In the following, 6' is the charge in tt)s., and L the line of least resistance in feet. sure of of the of the pres- tre of , and ig 70 I cut least WEIGHT OF CHARGES. 77 Gunpowder. Revetment without counterforts. Charges placed at the back of the revetment at two-lined inter- vals. For moderate demolition For violent demolition If the charges are not placed at two-lined intervals they should be altered in the proportion ^^ "^ ^" ^^^^' . The interval should not exceed 4Z. Revetment with counterforts. Charges placed in the centre of each counterfort, at two-lined intervals. For moderate demolition For violent demolition Charges of -i^L^, at cwo-lined intervals, demolish the inter- mediate parts, and shatter the counter-forts without bringing them down. Charges of ^Z^ at i^-lined intervals produce violent demolition. If the distance between the counterforts be unusually great, in proportion to the thickness of the revetment, one or two mines should be placed between the counterl'orts at the back of the wall. The L.L.R. should be about equal to the thickness of the wall at the base. Counier-archcd revetments. Charge-! placed in the centre of each vaulted chamber L is measured from centre of chamber to face of revetment. STOCKADES. Gunpowder. Single stockade of 10" timbers : One charge of ^o lbs. if cov- ered with sanri= .ITS, or one charge of 60 lbs. if uncovered. Single stockaile of 14' timbers : One charge of 100 lbs. uncov- ered. Double stockade of 14" timbers, with an in^^erval not exceedinjr 3' 6" between the rows : One charge of 200 lbs. uncovered. If the interval between the rows exceeds 3' 6", two distinct explo- sions must bo made. r : ) 78 TABLE I. Guncotton or dynamite, C=o.023^8 tte'^to'ikL'e in'^iSs!" '"" ^'' "'°' ™"' ^"^ ' t""^ "-"ness of ,' ^'^K^I' Stockade of 14" timbers, with an interval not exceeding 80 lbs '""" '■ ^5 *'■ P^' f""' '""• °'- °"<= charge of inirchth:r:'^1tk1er tTr/" "==• "-^^ '-'' •'"'-<' 'o-''- TREES. -^ Charge of guncotton or dynamite placed as a necklace Charge of guncotton or dynaraite placed c=o.o8G2 where G is the girth of the tree in feet and c is the char>: m auger holes". ge in 02s. WALLS. Gunpowder. By blasting -The, following table* gives the charges placed at two-hned intervals in various holes required ioT^conMdehil moderate demolitions. The L. L. R. is mLsured in feet ad s S^JLrntVs?;?^"-^ '' ''' -^"- -"'^ charges^fhould' b^ Description of hole. Diajneter of hole in inches. Single . . , Single . . , Single . . . V. X 2L ... 1.33 i/ . L Depth to which each hole is to be bored in feet. L ... 0.67 L 1.5L 1.75L 2.2 L Charge in lbs. 0.33 L\ OAL^ \ 0.5 L^ ) Remarks. 1.4L I0.33Z/3 2L 0.33Z8 This is the best size of hole. Single holes should be bored alter- nately from opposite yides of the wall. Half the charge, or0.17/v«, tobcineach hole forming the V; the holes should overlap slightly. Half the charge, or 0. 17/.9, to be in each oj the two holes forming the X. fi^'m '"^^ ^^\ ^'Owever well built, can be breached by charres of tfe^han'Tte-fS'tal-f ^^"'-''^^^' '"^ ^"-«" '^^ * Abstracted from Instruction in Military Engineering, Part IV. tAide-memoire to the Military Sciences. I f FIRING ARRANGEMENTS. GuncoUon. 79 Charges placed uncovered in contact with the wall, and when breach >2^. C=o,oo35/2 where is the charge in lbs. per foot run and t is the thickness of the wall in inches. Charges placed in holes about ^t deep, 3/ apart, and covered with sand or earth. c=o.o3^' where c is the charge in each hole in ozs. and t the thickness of the wall in inches.* FIRING ARRANGEMENTS. SLOW AND INSTANTANEOUS LEADERS. Rate of burning, — Bickford's fuze 2.4 to 4 feet per minute. Powder hose 10 to 20 feet per second. Ord's hose 15 " <« Bickford's instantaneous fuze... 30 " ** ELECTRICAL CIRCUIT VV^IRES. Resistance of various Service Cables. Resistance per yard in ohms, ^ ^ ,.r ^ . , ateO'-Fahr. No, 10 B.W.G., insulated with gutta-percha, to 8 B.W.G.... 0.008 No. 22 B.W.G., 7-strand, insulated with Hooper's india- rubber to 0.23 inch, and served with hemp braiding. External diameter 0.32 inch, (C Troop cable) 0.006 No. 20 B.W.G., 4-strand, insulated with Hooper's india- rubber to 0.24 inch, and covered with two servings of tarred hemp. External diameter 0.44 inch 0.007 There are numerous other cables in the Service, but they are intended solely for Submarine mining, or for the electric light. Resistance of copper and iron wire. If, for any wire, ^2 = resistance per yard in ohms at 6o°F. rf— diameter in thousandths of an inch. ,V= number of yards per ft. * This formula has been deduced from experiments made by the author at ixibraltar. ■■' ^ \»v^1>?^*■ a*'«-».jaKte-CiLS. ?.«»r_p y' !'.:"*■ jfi. n ^0 TABLE I. then, — For copper wire whose conductivity is 88.6 p.c. that of oure copper wire — ^ For iron wire — * 05 ■* 8 .* g 2 S B » B 8 9 10 II 12 13 14 15 16 17 18 19 20 21 22 23 24 165 148 134 120 109 95 83 72 65 58 49 42 35 32 28 25 22 No. of yards per lb. 4 5 6 8 9 12 16 21 26 2>5 46 62 89 107 140 175 226 — 34 r 2=0.0002,1 N. '^'~d» _2I0 ^2= 0.00166 iV ; telegraph wire. ^2= 0.0019 AT ; ordinary wire. COPPKR. Jieslstance at 60" F. Comiuctiviti) S8.6 p.c. of pure copper. 0.0012 0.0015 0.0019 0.0024 0.0029 0.0038 0.0049 0.0066 0.0080 O.OIOI 0.0142 O.OI9J 0.0277 0.0332 0.0434 0.0544 0.0702 Conductivity 72.2 p.c. of pure copper. 0.0015 0.0019 0.0023 0.0028 0.0034 0.0045 0.0059 0.0079 0.0096 O.OI2I 0.0170 0.0231 0-0333 0.0398 0.0520 0.0653 0.0843 Iron. No. of yards per lb. 5 6 7 9 II 14 18 24 30 37 52 71 .T03 123 161 202 260 Hemtance at 60^ F. (Telegraph ivire.J 0.0077 0.0096 O.OII7 0.0146 0.0177 0.0233 0.0305 0.0405 0.0497 0.0624 0.0875 O.II90 O.I714 0.2051 0.2678 0.3360 0-4339 Corrcctwn for tcmpcminrc.-The resistance of copper increases o 21 p c. for a rise of one degree Fahrenheit and that of iron about the same. The electrical conductivity of copper is much affected by im- purities as will be seen by the following table:t ^ *bovItr?£""' ^^'fT •''' ''^°""''^' *^*^ diameter of several Nos. of B. W. ■ the above are from careful measurements of Mr. Holtzapliel. ' tExtracted from apaperby Capt. BuoknilL R.E., in Vol. XXV., Journal R.U.S.I. / SOURCES OF ELECTRICITVi 8l Pure copper loo. Lake Superior g8.8 Commercial 92.6 Burra Burra (Australian) 88.7 Best selected 81 .3 Bright wire 72.2 Tough 71.0 Demidoff 59.3 Rio Tinto 14.2 Temperature about 60° Fahr. The resistance of wrought jron is more constant than that of copper. SOURCES OF ELECTRICITY. Grove*s cell. E. M. F., 1.956 volt. Liquid resistance : 0.08 ohm. Din:ensions of Military pattern cell : outer cell (ebonite), 3" X 2" X 4f " deep ; inner porous cell, 2|" x ^" x 4" deep. Dimensions of box containing 10 cells with 12 spare porous cells : 22^" X 4^" x 8^" deep. Charge for one cell : sulphuric acid, dilute (i acid to 10 water), II oz. fluid ; nitric acid, strong if oz. fluid. Zinc plates : purest rolled zinc, No. 10, B. W. G., U shaped, with twice 4" x 2^" immersed. Platinum plates : sheet platinum, 4" x 2^" immersed. Mean distance between zinc and platinum plates, ^". LeclancM firing cell. E. M. F., 1.45 Volt. Liquid resistance : 0.15 ohm. Dimensions of Military pattern cell: height, 14^^''; diameter, 4^". Dimensions of box containing 10 cells : i' 11" x 9" x 10^" depth (inside). Charge for one cell : sal ammoniac, 3 oz., dissolved in f pint of water; weight of binoxide of manganese and graphite mix- ture, 2^ lbs. Quantity dynamo. The data in connection with the qiantity dynamo at the R. M. College of Canada are as follows : E. M. F., 50 volts, (approx). Internal resistance, 13.33 ohms. BSmH ff« 82 TABLE r. Resistance of revolving coil (Siemens' armature). 8.03 ohms Resistance of electro magnet coils, (both together) 5.3 ohms Extra resistance cut out on depressing firing key, 24.64 ohms. ^Z\l '"^*i!""^ r" ^""" ^^^ "•°°3" standard iridio-platinuin wire through 15 ohms., and the 0.0014" wire through 54 ohms Tension dynamo. The machine should be able to produce a spark ^-2°.' io„„ 5000 ^' ■i ! SUNDRY RECIPES. Amalgamating zinc plates. 1st Method. First dip the plate into dilute sulphuric acid irrfll^'^ ''^'""^ until a. strong action commence' then pour a little mercury on each side of the plate and spread bv means of cotton waste attached to a stick. If the plate is groasv first wash It well with a strong solution of carbonate of sodl! ^' 2nd Method. First clean the plate by means of dilute sul- phuric acid as in the first method ; then place for one minute n a bath composed of one part by weight of Hg NO3 (proto-nitrate of mercury) and 20 parts of water. The plate m'ust afterwards be well washed in water. -^iwctiub Cleaning copper plates. If the plate be very dirty, place it in dilute sulphuric acid, and then rub with sand and water. Should this failf heat the plate red-hot before applying the acid. ^ ^^ Moderately dirty plates can be cleaned with sand and water Cement for Leclanche firing cell. "^IFu""^^ ^^ weight of French resin, 48 of bitumen, 3 of oil of naphtha, and one of india-rubber. ' J "" 01 Crude black paraffin may be used as a substitute for the above ana tor repairs. ' l^M SUNDUY RECIPES. gq Solder for jointing circuit . irea. I Tin and i Lead, fusing point 320° Fahr. Fluxes for soldering. Metal to be united . Fluxet. Copper. Sal ammoniac, Chloride of i^inc, or rosin. ^^?"' Borax, or Sal ammoiiiac. 2'\nc. Chloride of zinc. Soldering fluid is a concentrated solution of chloride of zinc. Marine Glue. •' One part of indiarubber is dissolved under t-cntle heat in 12 parts < f mineral naphtha or coal tar. When melted, 20 parts of powdered shellac are added, and the mixture is poured out on metal plates to cool. It is applied by a brush in a melted state, and is specially suitable ^or all work e^ josed to wet or moisture '' ^> IMAGE EVALUATION TEST TARGET (MT-3) // /Aft #^/. 7/ 1.0 I.I 11.25 2,0 UUu. 18 JA ill 1.6 V <^ /; ^a V] PhotogTdphic Sciences Corporation 23 WEST MAIN STREET WEBSTER, N.Y. 14580 (716) 8712-4503 rN>^ ^. '^i^ >V> ^i,^^- '% .V M,, i/.A m MttdKiiMiMHMri P^RT II. ELECTRICAL MEASUREMENTS. » il ::i :• I n ;■ List of symbols used. resistance,"total in a circuit. of wires, unplugged in a box of coils, etc. liquid of a cell. of a galvanometer. of a shunt. of a shunted galvanometer. R: r, n, etc. />: S- s: Sg ' P : highest potential in a circuit^-'alsorelectro-motive force. Pa-- potential at the point ^ in a circuit. C : current, total in a circuit. c, c„ etc., currents in the various branches of a divided circuit. K: capacity. Q : quantity of electricity. ^ : deflection of a galvanometer, in degrees. d: deflection of a reflecting galvanometer, in divisions of scale. -; of^^Sl^r^ ---' ^"^ --cL for rek ro» c^, etc. : unknown resistance, unknown current/etc. i \ r*A.RT II. ELECTRICAL MEASUREMENTS. CHAPTER I. )rce. ircuit. scale. (( resist- INTRODUCTORY. ^}:?:'^' '^^ purposes for which current electricity is used bv the Mihtary Engineer are very numerous, nd, in most cases, various measurements, more or less accurate, have to be made The followmg pages contain the more usual methods of making thp ■ •> tit will be observed that this is not a strictly correct term, for it is a nuestion ot work and not of force which is at issue. f A JThe comparison of this phenomenon to the flow of a Huid is convenient; the probability is, however, that a transfer of vibrations takes place. iJ ig two con- place. t on a body s a certain le quantity oint. The )n of elec- equired to om infinity ired) of the tion not to ■ electrical the differ- f potential ference of iure bears vo points, imount of e point to : tends to press the ference of r instance ential be- :ential be ictor, for o-electric ong the vintained. b is mea- ty which )f time.J 2r to the don, B.A., a question jnient; the UNITS. 89 186. Capacity. — The capacity of an insulated conductor is the quantity of electricity required to raise, by one unit, the potential of the conductor. Hence it K is the capacity, P the potential, and Q the quantity contained by the conductor : Q=KP 137. Polarization. — This is the name given to the deposition of hydrogen, on the negative plate of a voltaic cell, which occurs when the ce!! is placed in circuit, and which causes a reduction in the E. M. F. of the cell.* The amount of this polarization depends in the first place on the nature of the cell ; those cells which polarize rapidly are called inconstant, and those which polarize slowly are called constant. Daniell's and Grove's cells are amongst the most constant. The amount of polarization also depends on the resistance in circuit, and the higher the resistance the less the polarization. As will be seen in the sequel polarization renders inaccurate several of the methods of measurement. UNITS. 138. The measurement of all physical quantities can be made to depend on the fundamental units of space, mass, and time,t and the units of nieasurement thus derived are called absolute units in contra-distinction to arbitrary units. For scientific purposes the centimetre, the gramme and the second have been chosen as the fundamental units, and the units derived from them are said to belong to the C. G. S. system. The various measurements made in connection with current electricity are expressed in terms of units based on the magnetic properties of an electrical current,! and are called electro-mag- netic units; the absolute electrical units, thus obtained, belonging to the C. G. S. system, are, however, of inconvenient magnitudes, and they are therefore multiplied or divided by some power of 10. The following are the principal units required. 139. Current. — It is found by experiment that the attraction exerted by a current on a magnetic pole is ^^ where m is the a intensity of the magnetism of the pole, / the length of the current acting on the pole, C the strength of the current, and a the dis- tance of the pole from the current. The absolute unit of current on the C. G. S. system is clearly therefore : The current each centimetre of which attracts a unit magnetic pole, placed at a distance of one centimetre from it, with a force of one dyne. *See (5 796, Ganot's Physics, 9th Edition. +See § 932, Ganot's Physics, 9th Edition. JSee § 810, Ganot's Physics, 9th Edition. 90 UNITS. forcI'Tvhich 'Vn'iT'V^^''''' 5'^!" ^' ""' ^-'y'''^^' -"d -the loice wnicli c.in, m one second, increase the velocity of opp gramme by a velocity of one centimetre per second )^ The practical unit is called a Weber and is=C. G. S. unit x iq-i . 140. Qmntity.~The absolute unit of quantity is the ouantitv of electricity flow n"- in nnr^ <^nnr^,.A I 'my is cue quantity V.ILJ, iiuyviii^, in one second, past any cross-sect on r>f a conductor carrying a unit current. ^ ^ The practical unit is called a Weber*,and is=C.G.S.unit x iq-i . nnlf^'/if''''"'' ''//'«f''«^'«^ «'' £/^c/;-o-;,w/n^./orc..-The absolute (The erg is the unit of work of the C. G. S. system and ,•<. tb. amomit of work done by one dyne working tSg"?; one '^e^^ The practical unit is called the Volt, and is=C. G. S. unit x io«. .n!:f;/^''"u'''u~J'''^ absolute unit of resistance is the resist- ance through which one absolute unit of difference of poten ial can cause one absolute unit of current to flow. Potential The practical unit is called the Ohm, and is=C. G. S. unit X io». Occasionally very high resistances have to be expressed and for this purpose the megohm=-i,ooo,ooo ohms, is -iseT ^. The^ohm IS sometimes called the B. A. unit (British Associa- 143. Capacity -The absolute unit of capacity is the capacity of Lh ^Stir'"' ''" '°^' °"^ ""^^ °^^"^"^^^y of electSy'al The practical unit is the Microfarad, and is=C.G.S. unit X iq-^^ The Farad is 1,000,000 microfarads. fZ\L7jdX^l '''' "^^^"^^"'^ "^ ^^^^- "-^^ -" be obtained The electro-motive force of a Daniell's cell is about 1.12 volt The resistance of a pure copper wire, 48.5 metres lontr one millimetre in diameter, at 0° Centigrade, it one ohm ^' The current produced by one Daniell's cell in a circuit, whose lesistance is one ohm,^ is approximately one Weber. *€alled "Faiiicl" by some authorities. +The value of the ohm was determined »v a ('nminJffon ^f +1 b x- 1. » ;l;Iiu;ludiug the liijuid resistances of tiie cell. ■« '4 I; and is the ity of one nit X iQ-i. 2 quantity ction of a nit X iQ-i. 3 absolute sntial be- o be done repulsion, nd is the me centi- nitxio«. le resist- potential nitXio*. ssed and Associa- pacity of tricity at UNITS. 91 "The capacity of most submarine cables is about one-third of a microfarad per knot."* 144 A. The follovvinj^ are the decisions of the Electncal Con- gress, held in Paris in the autumn of 1881, on the subject of the Nomenclature of Electrical Units : I.— On adoptera pour les mesures electriques les unitt^s fonda- mentales : centimetre, gramme-masse, seconde (c.g.s/ 2.— Les united pratiques, I'Ohm et le Volt, conserveront leurs dehnitions actuelles. 3.— L'unitd de resistance (Ohm) sera reprt^sentde par une colonne de mercure d'un millimetre carrd de section, a la temperature de zdro degrd centigrade : 4.— Une commission Internationale sera chargtie de determiner par de nouvelles experiences, pour la pratique, la longueur de la colonne de mercure d'un millimetre carre de section, a la tem- perature de z6to degre centigrade, qui reprdsentera la valeur de 1 Ohm. A ces quatre premieres rdsolutions ont dt4 ajoutdes les trois propositions suivantes : 5-— Qu' on appelle Ampere le courant produit par un Volt dans un Ohm. ^ 6.— Qu'on appelle Coulomb la quantity d'eiectricite definie par la condition qu'un Ampere donne un Coulomb par seconde ; 7.— Qu'on appelle Farad la capacite' definie par la condition qu un Coulomb dans un Farad donne un Volt. tXio-^ obtained [2 volt, ng, one , whose Associa- tances of *Rough Notes of Lectures on Electricity, S. M. E., Chatham. CHAPTER II. Vh M ELECTRICAL LAWS ON WHICH MEASUREMENTS ARE BASED. I 1 OHM S LAW. 146. This law asserts that r where C is the current, p-p^ the difference of potential between two points in a circuit, and r the resistance between the same two points ; and by properly choosing the units this law can be written. c=tzh r Such units are the absolute units on the C. G. S. system, and likewise the practical units derived from them; so that a difference of potential of one volt will produce a current of one weber in a circuit whose resistance is one ohm.* This law is of the greatest importance for electrical measure- ments. Graphic representation of Ohm's Law. 146. Ohm's law can be represented graphically as follows : Let r be the resistance between two points, A and B, in a circuit carrying a current C, and let pa, pb be the potentials at A and B respectively. Measure ab=r, to some convenient scale, along an initial straight line representing zero potential, then a will represent the point A and b the point B. At a and b raise per- pendiculars aa and bb' to the initial line, equal to pa and /j, to the •From Ohm's Law : (absoluteC.G.S. unit of current) 10"^ - (^oI^»teaa S^unitof diff^ (abaolute C.G.S. unit of resistance) 1(F That is, lOhm. TIIKORY OF DIVIDED CIRCUITS. 93 MENTS il between I the same aw can be 5. system, so that a nt of one measure- ows : i B, in a ials at A •.a.\e,a.\ong en a will raise per- Pb> to the )otential)10 same sctie on which ah was measured ; then if d is the angle of inclination of a'b' to the initial line. tan d='''''~J'L=P^~-P'>=.C ab r The straight line a'h' represents, therefore, the current flowing in the conductor, tiiat is its strength is measured by the tangent of thf' angle of inclination of this straight line to the initial line. Further, if the resistance between, say, A and any third point -Y on the conductor is known, the potential at A' can be found graphically by n^^asuring ax in the proper direction along the initial line and raising a perpendicular xx' to meet a'b'. The intercept xx' measures the potential at A'. THEORY OF DIVIDED CIRCUITS. 147. Suppose that a circuit, carrying a current 6', divides into two branches at the point A and re-unites again at the point B Let the resistance in the two branches be ^j , and r^, and let c c ' be the currents in the same branches respectively. Let pa be"the potential at A and pb that at B, and r the total resistance in cir- cuit between the points A and B. Then by Ohm's law : C Pa—pb And clearly Hence it will be found that _ pa—pb C=Ci +Cj ^2 = Pa—pb ^1 _2j C^ = .C .C A similar method of investigation can be followed in the case of a current dividing into three or more branches. EFFECT OF AN ELECTRICAL CURRENT ON A MAGNETIC NEEDLE. 148. If a wire, carrying a current of electricity, be placed in the neighbourhood of a magnetic needle, and not at right angles to it, the needle will be deflected in a direction depending on the direction of the current, and on the position of the wire accord- ing as it is above or below the needle.* Ampere has given the *See § 810, Ganot's Physics, Oth Editiou. 94 ELECTRICAL LAWS ON WHICH MEASUREMENTS ARE BASED. followins: meinoria tcchnica from which the direction of deflection can in all cases be obtained. " Imagine an observer placed in the wire facing the needle in such a manner that the positive current enters at his feet and issues at his head, then in all cases the north-pointing j)ole will be deflected towards the left of the observer." When a horiirontal magnetic needle is deflected by an electrical current it is under the action of two couples, namely, the couple produced by the current and the couple due to the magnetic attraction of the Earth. As the needle deflects, the values of the moments of these two couples vary : that due to the current diminishing, and that due to the Earth increasing, until the posi- tion of equilibrium is attained, when both must have the same value. If the needle is swung on a horizontal axis and is counter- weighted, the counter-weight will produce a couple, whose action corresponds to that due to the Earth's attraction in the previous case. The moment of the couple produced by the electrical current is found by experiment to vary, amongst other things, directly as the strength of the current. 149. An Astatic system is a combination of two horizontal needles having the same magnetic intensity, rigidly connected to the same vertical axis and placed parallel to each other, with the S pole of one needle facing the N. pole of the other. The Earth has therefore, no directing effect on an astatic system ; but it can be shown that the deflecting couple produced by a current is in- creased. In the astatic systems in practical use, however, one needle is slightly stronger than the other, so that there is a small directive couple. For both reasons, therefore, an astatic system is much more sensitive to an electrical current than a single needle. ^ 150. The construction of galvanometers depends on the above properties of magnetic needles. CONDENSATION OF ELECTRICITY. 151. If an insulated conductor, in connection with a source of electricity, be separated from another conductor by a dielectric (air, glass, gutta-percha), electricity will be condensed on the con- ductors, and the amount of condensation will depend on the dielectric and on the proximity of the conductors.* The poten- tial on the condenser connected with the source will clearly be the same as that of the source, so that condensation of electricity See § 755, Ganot's Physics, 9th Edition, ' 'i BASED. deflection placed in e positive 1 all cases eft of the electrical he couple magnetic uesof the e current the posi- the same counter- Dse action : previous 1 current irectly as RKI.ATION HETWKEN CURRENT AND \V(JKK. 95 means: increase of electrical density without increase of potential. An instnnnent capablf of accumulating electricity in this manner is called a "conder ." RELATION HETWEKN CURRENT AND WORK. 152. To show that the work done is proportional to the square of the current flowing through a conductor. Let C be the current flowing through a conductor. Select two pomts, A and B, on this conductor, and let r be the resistance, and pa~h the diff"erence of potential between them. Then the potential energy transformed in time / is ^Pa-pt>) Ct, But r Hence C rt =work. And since r and t are constants, C^QCwork. d needles the same i S. pole arth has, it can be :nt is in- ;ver, one s a small c system a single le above ource of lielectric the con- on the 2 poten- learly be ectricity ! i CHAPTER III, Of III t: -■ V INSTRUMENTS AND APPLIANCES USED IN MAKING ELECTRICAL MEASUREMENTS. CIRCUIT WIRES. 153. Circuit wires, or leads, generally consist of ccpper wire ;rerr^bry^htfo"J;;e^"^^^^^'^/"^^"^ ^' .utta-perchr^or ^1^! 154. Binding screws.— ^^ch terminal of an electrical mea<.nrmcr apparatus IS provided with a brass screw and nu L ord^ Xt ttf^ °^i^' connecting wire may be firmly secured. Separate double-bmdmg screws are used to make ten.porary joints in wTres! .h^^^U f ""f "^^'''"T u''-^^ ^'"the conducting wires should bo bared and brightened with a piece of emerv cloth with the back of a knife, so as to remove all the o^fde and thu. obtain a good metallic contact. When this is do^^e the ends are bent with a right-handed hook, round the binding screws whfch are then firmly screwed down. ^ ^^^t-vvs, wnich CONTACT KEYS. 166. A contact key is an instrument by means of which a circuit can be closed or broken at pleasure, and it is especiallv usefu when currents of very short duration n,e require?. There are several different patterns of these instruments, but general v speaking the construction is as follows : ui ^eneiaiij The base of the instru.nent is made of ebonite, or hard wood A small brass stud, to which a platinum point is sol lered is secured o the base and connected by a wire to a binding sci^w A brass lever ,s also attached to the base, and coimecJed o a second binding screw ; this le.er carries a second platimnn pohit p aced immediately over the first one, but kept separated from t' by means of a spring acting on the If.ver. J STANDARD AND TEST-CELLS. 97 MAKING ^pper wire la or silk, measuring 3rder that Separate s in wires. ing wires cloth, c and thus ends are vs, which a circuit ly useful here are generally "d wood, iered, is ^ screw, ted to a m point, I from it To use the instrument the circuit wires are connected to the binding screws, and when it is desired to close the circuit the le\';er is depressed until the platinum points touch ; the circuit is broken the instant the lever is released. Currents of very short duration can therefore be obtained by making rapid contacts, an operation which requires some little practice. KEVKKSINd KEYS. 157. A reversi' ,^ key consists of two keys similar to the above, each luuinga double set of points so arranged, that, on depressing one key continuously a current may be sent in one direction, but directly the first key is released and the second depressed the current will be re\erscd in direction. TKST CELLS, 158. It is a matter of considerable importance to select suitable cells for generating the currents required for testing ; and, when making co;npanitivc measurements, "standard" cells should be employed, 159. The standard cells for use when the method of testing requires a current of electricity, even if it be only of short dura- tion, should have the following qualities : 1, If made up with pure materials and with the same strength of solutions, they should give the same result, 2, They should polarize very slowly, that is, they should be what are called "constant cells."* Grovet cells and Daniellt cells are those which, at present, best fulfil these conditions, but they are "two fluid" cells, and the electro-motive force is diminished by the gradual mixing of the fluids. This will generally be the case with two fluid cells, so that a single fluid standard cell is a desideratum. -MDUiriEI) DANIELL's STANDARn CKLL. 160. Tile following is a description of a standard cell of almost unlimited endurance, proposed by Major Armstrong, R.E, It is a modification of the Uaniell's cell : " In two porcelain or earthenware chambers, separated from each other by a »o;^porous partition, are placed the :inc and copper elements respectively. A semi-saturated solution of sulphate of xinc is poured into the compartment containing the zinc element, and a saturated solution of sulphate of copper is poured into the other. *,See § 707, Ganot's Physics, 9th Editioii. +See g 71)1)! and g 7i)8, Oanot's Physics, Dth Edition. 98 INSTRUMENTS AND APPLIANCES. 3i i The cell may remain in this condition apparently for years without undcr^^oin<,^ any chan^^e : it is merely necessary to add distil ed water to the solutions from time to time, as their level tails thron<,di evaporation, and to prevent the sulphate of copper h-om creeping up the sides of the cell. ^ ''When it is desired to use the cell as a standard, a string of IwT """ Vr"'' '' '^'^'^''''\ ''\ ''"''^'''' ^^"^' "s^^' to connect the liquids, which are tlius connected electrically, but do not nv'.x for hours! At the conclusion of the experiment, the string is removed and thrown away and a fresh one used next day. "After employing some of these cells for a couple of years no trace of copper could be detected in the zinc cell. " The electromotive force varies under conditions, which I have not determined, from i 083 to 1.065 ohms ; that is onlv o.8j per cent from the mean value of 1.074. It is unaffected by raising the temperature to boihng point ; and I have not observed that action of light produces any alteration. "A number of these cells, when first made up, were within an extreme range of 1.069 to 1.077 ohms. '' It is probably desirable to employ commercially pure metals and salts, although I have not found that cells prepaid with the above precautions differ in any way from those made up with ordinary commercial materials. ^ " As I have already stated, this is not, in its present condition, an accurate standard, but it is a very convenient one for work no^ requiring greater accuracy than can be obtained bx' its means^ ^^ ^l^r^-l^^Tf'"''^"/ at starting by soni recogniz d standaid. * * The results obtained with a condenser are not yery accurate, as even the current required to till a capacity ot about ^ microfarad appears to lower the E. M. F of the cell at the moment about . p.c, ;^ ^^ * but when used with an electrometer gives results within one per cent, of the truth * .1 • 1 I'?"^ experiments made during the past two months I think that the value of 1.07 volts will not differ more than o s P c from the correct value. The previous experiments were made with a zinc wire soldered to the zinc element and a copper wire to the copper element, and the variations may have been caused to some extent by damp at the junction of th'e copper lead with the permanent zinc wire ; at any rate, since a copper wire was .soW^•m/ to the ziiic element the E. M. F. has only varied from 1.065 to 1.075 volts. The mean value of which "is which should be taken as the actual value. "* 1.07 volts. '"'A periiifiueiit standard ooll. papers, Vol. IV. I'y Afajor Anii^itn.iig, i!J.;., U.K.I., ,)^.e:,si sioiial STANDARD A\T^ TEST-CELLS. 99 for years r}' to add heir level of copper string"- of he liquids, for hours, loved and years no ich I have ' o.iSj per )y raising rved that vithin an •e metals with the up with ondition, work not s means, cognized mser are capacity the ceil I with an nth, * lonths I 10.5 p.c. re made per wire 1 caused ;ad with I'ire was ed from )7 volts, Ouc:isi(iHivl -,# I.VTIMKR CLF.l: -TANDARI) CF.U,. 161. When measurin.c,' the l" . F. of a cell or battery, and the method employed requires onl\ instantaneous current, it is not important that the cell should" )iarize slowly, but on die other hand, it is of p:reat advantage (i) if the E. M. F. of the test cell remains constant when set up (but not at work) ; (2) if the cell does not emit acid vapours, so as not to injure delicate instru- ments, and (3) if it requires little looking after. Mr. Latimer Clerk's standard cell fulfills these conditions. The following is a description of it. "At the bottom of a small glass cell is placed some pure mer- cury, which is then heated to expel moisture. On the surface of the mercury is poured a paste made of : zinc sulphate, dissolved m boilmg distilled water to such a point that on cooling a con- siderable quantity of crystals of the salt are deposited, and a per- fectly saturated solution obtained ; this solution is mixed with protosulphate of mercury so as to form a thick paste, which is tlien boiled to drive off air, and is poured hot on the surface of the mercury. A piece of pure zinc is then supported in the paste by means of a cork ; the whole is sealed up with paraffine wax Contact with the mercury is made by means of a platinum wire dipping below its surface in a little glass tube, or neck rcachint propor- e only be they are is swung;' sensitive, bbins on needle is tions on :x. The at ri<,dit t of the , whose leans of my one so that ;his de- er, will 5en the hly be service luated, e cur- he de- GALVANOMETERS. 103 A static galvanometer. . 171- This instrument, supplied for Submarine-minin-' Service IS a hori;iontal detector, and it has two rnvmrtlr „, I f^^^^9^' an astatic system The upper needi:;^";?';^':^ m i:^.^";;;^ moves on a dial plate divided iuto de-rees This ' Iv^ ^ . IS sensitive, and has a resistance of looo dims! ^'^'^'^"^"^^^^r •SINK i;alvan'omktek. 172. In the sine^^alvanometer the coils are made moveable about a vertical axis, and can thus follow the needle whirh ! horizon al, as it deflects, IV^ is therefore constant and -a' j! easily shown that* ' "■ ^'^" "^ 0=/i sin d where ^r is the an.^de of deflection, and ,. is a constant. menji't'^"'' °^ galvanometer is not much used for measure- TANGKNT (iALVANOMETEfi. 173. In the tan^^ent galvanometer the length of the needle .'c small 111 comparison to the diameter of the coils so th^f', 2 •' practically constant. It can be shown in this case ?hat2 " '' C=/i tan d nearly. _^T1,is is the best elass of galvanometer for purposes of measure- THOMSON'S KEKLECrrNCi UALVANOMETKK. 174. This instrument is constructed a=: -i fa.i,r...,+ 1 ::«^ - -c.-- i..s -.sMveness, :::i:^i':xftz:::^i uhicl, is closed bv a plate of .ilass an II.^Sl ' °,"'' '■""'' °^ co,.taiui,,g a s,nai. «,a^ss d .1 right anges to the normal position of the b^am o li'ht h^^ number of divisions d described when a current pa^se s p mpo •- oHigLt'.' ^Heifer"' °' ^'^ "^^'^ '^^^"^^^^ ^>' ^^' reflected bi^am d cc tan 2 d but o" is a small angle, (maximum 5") so that d cc 2 tan o or, since the instrument is a tangent galvanometer, d oc C It is not always convenient to place these instruments so that he magnets will he m the magnetic meridian; a magnet is the e foie attached tothemstrument so as to form an anificial mag- netic meridian in any required direction. This magnet slides up and down a brass rod flxed to the top of the instrument' and provided! '"■"" """' '^' '""^'' ^ ''''' "^°^^"'> screw is ;iso To check the vibrations, an aluminium pallet is sometimes connected to the magnets, and in some cases this pallet is placed These galvanometers have very high resistances,! and are extremely sensitive : they should invariably be used for accurate measurements, but they require great care in workin- Scttiii<( up the instriiincnt. 175. The following instructions should be followed in settin- up a reflecting galvanometer : ^^lun^ cripZI:' ''''''*"'° galvanometer at the K. M. Colleije of Canada, i« of thi« .Ics- m\JI^]' "''*'"'■'' '" *''" 1'"**'^'" °^' •■"•^^•^**"a galvanometer used for .Submarine thei^!;:eS!S^:;ns'a'"'"""*^«''^""""*""^'^'^^ ''■ ''■ '■ ^« 721.; ohn. at :* GALVANOMETER SHUNTS. 105 1. The support should be rigid and not connected with the floor of the room."*^ 2. The instrument should be levelled, the magnets beine ap- proximately in the magnetic meridian. 3. The^cale is placed parallel to the magnets, and at the focal distance! of the mirror from the instrument. 4. The lamp being placed behind the scale, the spot of lif'ht is brought approximately to zero, by raising or lowering the scale, and, by means of the directing magnet. 5. The spot of light is brought accurately to zero by the slow motion screw. This operation is only necessary when it is re- quired to measure the deflectioji of the galvanometer. When making comparative measurements and the spot of light moves away from the zero, it should be brought back by sliding the scale which is arranged for the purpose, and not by means of the directing magnet. | GALVANOMETER SHUNTS. 176. It frequently happens that the current flowing in a circuit is, either too powerful to pass through a given galvanometer or else the reading it gives is too large. In such cases what is called a "shunt," is attached to the galvanometer that is, the current is divided just before entering the galvanometer, and is reunited just after issuing from it. The strength of the current flowing through the galvanometer will evidently depend on the relative resistances of the two parts of the divided circuit and can therefore be regulated at pleasure. ' The problem is : To find the resistance of a shunt such that only ~th of the current will flow through the galvanometer. Let g and s be the resistances of the galvanometer and of the shunt respectively; then, according to § 147, Current flowing through the galvanometer or n s=- S[ + S .C W — I *A stone pillar brought up, if possible, from the rock forms a good support. tThis distance is supplied by the instrument maker. rrhese instructions are applicable to the instrument at the R. M C but will answer, with modihcations, for other reflecting galvanometers. ' '' I ! 3 '! l! I ro6 INSTRUMENTS AND APPLIANCES. galvanometer and shunt to.etle T '" '" "'"'"' "^ ""nt to^rethei as a sin-le resirtance= '^'^ a.u, . ,e sy„,.o„, „,„ ,e usee, to e.p.ess .hi. ,.esi,,„ee. ^^^ "«l.er .f ., ,3 the current flovvi,,, through the «a,va„o,„ete.. flow,„g through the^'alvan^metJn" ' °'' '•'""' °^ ""^ ^-rent These shunts are construeted like resistance eoiis (§ .64, Each galvanometer must evidently have its own , ! f , and their resistances will be X 1 / "^ °"" -' °f ^'-unts, of these shunts can be n„, • ■ ^ ■' '9 '999 "'"P'^^'^ely. Anyone are further ari,';^^,^^?^,;^--'' ^y '" of a pe,. and there of the current beiuR sent t hrou 'h the «l ^ "°"' °'' ""^ "''o'e reststance of the slmnt can br^ade'irorrnfa],:/ ""' ''' "'*= Vompeimling resistance. ■ " ™= ^^™ 'hat that the resistance of a ^alv.nom . may be regarded as reduced to --S-^ ,, , ^ "■"'■'""'"''to This decrease in th ^'+'' ' ''""" ' '^ ^lied. and, in some'c se, '^1,:"",^"^,,;:' " '''"''''' "'= "««' "Trent galvanometer would net's dhnii,,","r"', P'"'='"« 'I"-""k1> the shunt. When this occurs *"'"""''«'' ^y the appli„tion of tl e mtroduced into circu t H '^.^"".'Pe'isatrnt; resistance „„,st be ance in circuit before IVdafte;,ri't?od'''r '" 'Tf ""= ^^ same, so that the total currenr,:,"boT,;'cats'may"[;e"rqut '" ''"^ Compensating-- resistance => a gal- other words, it recdve a blow ?nd if' ■•""",'P'>kive force, or?, work done on the magnet by the insfaln"" '" '^"''"- *at Ihe th^squarc of the quantity of ekctricltTth^pa'ss"""'"' ™™^ '^ ^See Appendix. - ' "^'^^^ /if int red lie ca the t" re^Mi-cI the ■tance= •''>'— :ance. Ivanometer. ■ided with a the current (§ 164). 't of shunts, iJy. Anyone ?, and there the whole that is, the vanometer is applied, il current, foufrh the ion of the must be Jie resist- shunt the ual, rh a <,ral. ce, or, in that the ^'aries as irrent is he niag- liFFECT OI- AN INSTANTANKOUS CUKKKNTON A flALVANOMETKR. loj netic attraction of the Earth.the resistance of the suspensions ind ^therlf "\nh' ''^-t ' ^"^^'!^' '"^^^'"^'* '^ finallvK r^o r^^^^^^ l>> them. If the resistance of the suspensions and that of the air be ne^dected it can be shown* that sin» ^' is a measure of the work done on the needle by the instantaneous current, provided the r:" '^::;:t^ '--' ^^^- ^^^ --^^n- cc!nnn::^s^^o Q^ thof ^u the result ; by applying a shunf the SoT^ f '"'f 'S ^^''" ^'^ can be reduced to any desired extent T^^^^^^^^ be shunted whilst the quantity (?' is passing " galvanometer Q ^_j^ d 180. In the above, however, the resistances of the air and th., r the suspensions were nederfpH if \a + k r ^nd that of a correction to allow for thSe;es^snce^^^7 ^'''^^''^^l^" ^^PP^^' tude of the oscillations o rapTdly decLi I Sn'"'' the ampli- spot of l^ht describes d divisio^ns of he scS inTZ.^''' -f^ t.on, and d, divisions in the succeeding osdllation ink ""'''"''" side, then.^-^, evidently represents the loss n.^if *^f '■'^"'^ and, assuming that the losfis mnrovii f tJ^^lf oscillations, half oscillation, the corV^ectSn^^orLSls^.^^.T ^^^^ *See Appendix. D^d+\(d-dJ ro8 INSTKUMKNTR AXO APPMANCFS. if. I III D is called tlu' *'cnrr,^nt,..V • . . detlection that woulH ta n ir t u'r"''' '"'' '' •'^PP'-"^i'"atcly the or of the suspensions "'" ''"^'■"' "" resistance of the air arTllie^lS:'" '^'" ^'"^^'^^' "^ ^^'^ ^^ -«-tin, galvanometer Q'~D' and ^ =-X .^ •f the ^^alvanometer is shunted whilst Q' is passing. nnlistic galvanometer. '^^^l^^:^^-:^ - ^n-an.e- minamum, of which the follcn"n^!rrLfcri"tion ." "'"^"^ ^" "" used^'bSt^.fSijJ^;^;;!^^;;;-- -f-tin, galvanometers was ing arrangement : ^^'"oved and replaced by the follovv- "F'orty small magnets of varvinir len-thc „.«,-« lavmg been magnetized to sitnr^t inn^ ^vee prepared, and, little spheres, in eac of ^v^ ich al Z' T'" ^"'' "^^ '"^« ^^^'^ same direction. The sober i It magnets pomted in the from a hollow leaden Inl IT^ f completed by segments cut nected so as to form an a "t.f.V T '^''''"' ^^'^'"^ '''^^^^Y con- ed in the ordinary manner."''' ^°^"^'"'^t'«"' -J"ch was suspend- thraToffeSr;Se"resift"^^^^ ^^^ °^^--^' --^ It was found that U e rat o of t^ e firl. *^^^."^°*'^" "^ the needles, only 1.1695, a number wirchi^ffi.? '.Y'"^ *° *'^"' '^^^^"^ ^^•-'^^ a simple correction to 1^^ apD Id w ,f ^^' J^^'"^ -^^ ""'^>- *« ^"ow air."* applied foi the damping effect of the DIFFERENTIAL UAI.VAXUMETER cuJ^tV^S^nrSJ"^ '-^^-^^ coiK and a into two parts, one part ^ofn^H' T" '"*'';"^' *^^^ instrir.--nt, other through 'the second coif ."'t °"' °^ ''^^ ^^"'^' ^'^"^ the reverse effec'ts on t"ema1,ne\here^^^^ ^« P-^-e strength of the two parts of the current I / •^^^^^'^''^^^"^6 in the current be divided into two Pnnol . ^^ 'f observed. If the adjusted so as ^t^o^cHSi^ ^rL^e^i: t^ ''''''^. ^^^f! clearly not mcvc„ ^cvcibt enects, the needle will Cam^''''^^'"^ -"^'^^^^-^^-^-'ty -'I Magnetism by J. R H. Gordon. B.A., I niFFERENTIAI. OAI.VANONfl.TKK. >xitnatt;ly the ncL' of tin; air galvanometer 109 ^n arraiif^^e- educed to a •meters was the follow- 5ared, and, p into two ted in the rments cut i^^idly con- :s suspend- lined, and le needles, icond was y to allow 2ct of the ils, and a Stl-U'l-Ht, i and the produce ice in the . If the ave been edle will tlon, B.A., i m i 183. [.atinicr (lark's diffrrential ^'alvanometer is constnirtcd on the above i)rmri|)le. The j:iistru!nent is provided with bin.linK screws, by means of which a resistance, known or unknown, ran be nitroducod into either of tlie divided circuits, and the instru- ment is so adjusted that, when the; resistances thus introduced are equal the current is e(| illy divided, and further, the eftects of each half of the current on the needle are etjual and opposite. I he instrument is further provided with two shunts of J, ^t'l' one 111 each of the divided circuits, so that, if rer|uired. onlv ' tli of either or both of the divided currents need be sent tliromdi the correspoiuhn- coils. These shunts are introduced int'^o the circuit by msrrtin- a pe,-. and cut out by removin;,' tin same. J t oul> one of the shunts be put into circuit, the part of the current passm^^ throu-h that circuit will be 100 times lar^an- than the other part when the eftects produced on the needle are equal and opposite, or m other words, when the needle does not move- an airan-ement. which is very useful for various measurements as wil be seen in the sequel. A contact key is also attached to the mstrumcnt. n this pattern of differential ;-alvanometer, provision IS further ma.le. by means of pe-s, to enable the whole current to l)e passed throu-h one coil, or throu-h both coils, in the same direction When used in this manner it is, however, no longer a dijjcrcntial galvanometer. IMiir.VITIONS TO liK TAKKN TO PKFA'ENT INJURY To DKLICATK (I AIAANOMKTF.RS. 184. A galvanometer can be damaged, more or less seriously by passing too strong a current through it. Gcnerallv speaking, the more sensitive the galvanometer, the weaker the current that can safely be passed through it. The following are the injuries that may be caused, according to the strength of the current : 1. Injury to the needle suspensions. 2. Demagnetization of the needle. 3. Heating of the wire in the coils, causing destruction of the insulation, and even, if the current be strong enough, melting of the wire. '^ To avoid these injuries the following precautions should be taken : (I.) The measurements sh.ould be taken by a rapid process (short contacts with a key). ' (2.) The galvanometer should be "shunted," so that only a part ot the current will pass through the instrument. HINTS TO OBTAIN' (!00D f! ArA'AXOMKTKR RKADINCJS. 185. To obtain accurate and reliable measurements with a m Is 1 :i i • 1 IIO INSTRUMENTS AND APPLIANCES. U: . jralvanometer requires considerable practice Attenhn,, f^ .1 following- pomts will assist in takin.^ tLse nLsureren". Comparative galvanometer measurements are vitiated • reit:l{|h wiRlS^lrE^lJ^^^li^ ^^"f ^^Wlying the cur- extent in the separate readlngf * ' "^'^"""'^^ ""^ '° ^" ""^^^^v" rei^iL^Sll^-^--S1J-.t.-^^^^^^^ -- - contacts. leaJin^s aie taken by means of short a„^he'Xt/e?:;^urpS•rs^!:e^rc^'l'^M^n"=°"^*-'• STeX?-it;'- '• ™i!-^ -- on'maC'cZti^t-^T,^ beJre;:'^:»r;^3:,r j^' tie"b°;'?™ als'r-°'"?f ■ ^"°"" "« that below 20° the friction of tlenlvobn,' ' ' '■'!''°" '=■ large effect, and beyond 50° a wi ncre'e in'^tbP™''™'''' ^ requned to make a perceptible altemi"' n the deflectio'r T," deflection can be kept within the above limits in tvvo ways : (i.) By usmg a directing magnet. amluS-o/SeciS;,! *'" "'"^"^ ™"P'^' ^-^ consequently the (2.) By shunting the galvanometer. FIGURE OF MKRIT OP A .iALVANOMETER, tln"°«l7whlclf ™e° ™lT 'wilf ^i?!'™°T'^^ '^ "- --'-- deflection, and "iT ,™''e p"rtS,:rr;;ptS,e° to^'rltt- °' galvanometers. It is determined by noti " ho defl.w; '"^^^t'"/ a known difference of potential when tb„ ,"„; ! deflection dne to known and the Ralvano'mrt' sCe'J'tVrk^l^r e'vtenr"" '^ «a,ne'n,t'J=:bstut''ed'Z'rv:ft.''°"""=" "' "^ «" *«---"y a Thus, if Rjp is the figure of merit -ji /iX 1 = i?J^ THERMO-GALVANOMETER. Ill tion to the Jnts. ed : ^S the cur- m unknown alters the current of a galvano- e methods IS of short nconstant, e, a good sedle over (found by Lct. This should lie reason is, ftionately Hirrent is on. The lys : sntly the But Hence /^^^='^.4t g'R' Example— Let the directing magnet be so arranged that one volt gives a deflection of loo divisions through a resistance of looo ohms, a shunt of tj-L^o- being employed, then /vV=iooo X looo X 100 = 100 megohms. This number is also called the "constant" of the galvanometer, but, as it depends on the position of the directing' magnet the term is misleading. ' THERMd-OALVAXOMKTKR. 189. When a current passes through a wire the temperature of the wire is raised, and, if it is fine enough and the current power- ful enough, the temperature will be raised sufliciently to cause fusion of the \.ire. Further, there is a definite strength of current, which will just fuse each kind and size of wire. This evidently gives a means of measuring currents. 190. The box of Firing resistance coils is provided with a couple of clips, between which a very fine wire,* 0.25" long, can be secured, and these clips are so arranged that a current can be passed through the wire and through the box of coils. By altering the resistance in circuit, by means of the box, the cur- rent can be regulated, by trial, so as to just fuse the wire, and, as mentioned above, this current must have some definite value depending on the diameter and substance of the wire. As will be seen, in the sequel, certain comparative measurements can be made by this arrangement, which is called a Thermo-galvano- meter.* :sistance ision of sflecting !i due to ircuit is It. erally a CONDENSER. 191. This uistrunient is the application of the law of condensa- tion stated in § 151, and consists of a large number of leaves of tin-foil, placed like the leaves of a book, but separated from each other by thin sheets of indiarubber. Alternate leaves of tin-foil ^ are connected together so as to form two distinct systems separ- ated by the dielectric (sheets of india-rubber). Each system is I connected to a binding screw, and the condenser can be dis"- I charged l)y means of a peg inserted into a brass bar carrying the i binding screws. These instruments are made of various 'The wire .isually einploye.l in the Service is made of tlie alK.y iridio-platinum and 13 eitlier O.OIW" or ().0(H4 " in diameter. ^ ' I'lunum, /^"--' 112 ' : I If : . J I to I INSTRUMENTS AND APPLIANCES. capacities, an average size being i microfarad. About 300 sheets m^.crofa^d."' '''""' '"' " '°"'^"^^^ "^^^ capicity fs i When using the instrument, one binding screw sliould be connected to the source of electricity, and She other bindinL' screw to Eaith. One plate will therefore be at zero poS al and the other at the potential of the circuit at the poinrof con nection. The peg should be removed when takfng a measure-' men and then replaced to discharge the instrument. The peg should always be m position when the instrument is not in use 192. If a galvanometer (reflecting) be introduced between a condenser and a point in a circuit, the galvanometer will be deflected at the instant of making contact, by the mon^ent.rv current due to the filhng up of the condensed to^l e poteS '^ the pomt in the circuit. Unless the source of electricfty is feeble or the resistance to be overcome very high, the force deflecting fore^XTed '' "^ '' '"' ''' ''''''' °^ ^ ''' '^^ ^^ere- Now from § 136 Hence Q = KPa, KPa ,71 1 1 — Or K'P'a D D' K'p'a ~g + s'D'' if the galvanometer be shunted whilst the condenser, whose capacity IS /^ , is connected. - , wiiose EXAMPLES. So'vlhrf '' ^^'^ ^" ^^' ^' ''^'' ^^t^f"^^ <^J^rk's standard cell at 2. What length of German silver wire o.oi"diam. is required for a resistance coil of 1500 ohms ? What length of silver wire of the same diameter would be necessary at 0%^ and I5°C ? 3. In how many diff-erent ways can 1500 ohms be introduced coi?s'5 ''''"''"'^ "'''''"'' "^'' ^°^^ ^^'''' P''"®"' ^"''' ""^ resistance .u"^' 7T' ^^^"A^enU^'^l^-anometers are preciselv similar. Compare the deflections when connected in continuous circuit with the deflections when connected in divided circuit. 5- A tangent galvanometer is shunted, the resistance of the shunt being 10 ohms, and it is found that the deflection given by I EXAMPLES. 113 3ut 300 sheets capacity is i W' sliould be 'ther binding^ 3ro potential, point of con- r a measure- It. The peg not in use. i between a eter will be ■ momentary potential at :ity is feeble, :e deHecting >o can there- iser, whose lard cell at equired for wire of the introduced resistance Compare t with the one cell whose liquid resistance is o.i ohm is 50°. The resist- ance of the shunt- is then altered to 50 ohms, and 100 ohms resistance is introduced into the circuit ; the deliection is now 21°. Find the resistance of the galvanometer. 6. A tangent galvanometer, whose resistance is 20 ohms, is placed on short circuit with a cell whose E. M. F. is 1.8 volt, and whose liquid resistance is 0.5 ohm. The deflection is 38"\ What current is flowing through this galvanometer when the deflection is 46° ? 7. A tangent galvanometer and a sine galvanometer are suc- cessively placed on short circuit with a cell whose E. M. F. is I.I volt, and whose liquid resistance is i ohm, and the deflections are found to be 18° and 30° respectively. The resistance of the tangent galvanometer is 2 ohms, and that of the sine galvano- meter 30 ohms. What deflection on the sine galvanometer would be given by a current which gives 25° on the tangent galvano- meter ? 8. A shunt of II ohms is connected to a galvanometer whose resistance is 232 ohms. What portion of the current will flow through the galvanometer ? 9. A shunt of 12 ohms is applied to a galvanometer whose resistance is 5081 ohms, the total resistance in circuit is then 100 ohms. The shunt is now removed and 100 ohms resistance added to the circuit. Compare the currents flowing through the gal- vanometer in the two cases. 10. A galvanometer has a resistance of 1050 ohms. It is re- quired to apply a shunt such that ^U^b of the current will flow through the galvanometer. What should the resistance of the shunt be ? 11. Two Daniell's cells give 209 divisions deflection on a shunted reflecting galvanometer. The resistance in the circuit, exterior to the galvanometer, is 3050 ohms, the resistance of the galvano- meter Itself is 7080 ohms, and that of shunt 7 ohms. What is the figure of merit of this galvanometer in megohms, (i) in tern:s of one volt, (2) in terms of one Daniell's cell ? 12. A certain instantaneous current was found to give 232 divisions of deflection for the first swing, and 200 for the second. Another instantaneous current gave 198 and 170, Compare the quantities of electricity that passed. cc of the 1 given by R ( j ■ i ; I , i' 1 i i\\\ !' ik CHAPTER IV. ELECTRICAL MEASUREMENTS. 193. The usual electrical measurements that may be required for Military Engineering purposes can be tabulated as follows ; 1. Measurement of Resistance. a. Resistance, of a conductor, of a galvanometer, of "earths " etc. ' h. Liquid resistance of a cell. 2. Measurement of difference of Potential. The measurement of the electro-motive force of a source of electricity is a special case. 3. Measurement of the strength of a current of electricity. 4. Measurement of capacity. MEASUREMENT OF RESISTANCE. 194. The resistance to be measured may be that : 1. 01 a conductor, generally a wire. 2. Of a galvanometer, i.e. the resistance of the wire composing 3. Of an electro-magnet. 4. Of the insulation of a circuit. The general methods of measuring these various resistances are the same, but m some cases, induced* and secondary* currents are generated by the process of measuring, and precautions must be taken to prevent these currents from falsifying the results. 5. Of the liquid in a cell. The measurement of the liquid resistance of a cell requires special methods, some of which are described in ^^ 216-233. *Soe §§ 893 and 79(), Gauot's PJiyaics, '3- 10 1000 u 100 1000 (( 1000 1000 <. 1000 100 <( 1000 10 The following arrangement of resistances found to give good results : Unknown resistance betvv^een : 1,000,000 and 100,000 100,000 " 10,000 10,000 " 1,000 1,000 " 100 100 " o 199. The firing resistance coils are provided with two resistances of 10 ohms each, and these form the branches AB and AD of the Wheatstone's bridge ; the remainder of the resistances in the box form the branch BC. The binding screws are arranged so as to allow of the required connections being made, and, in such a man- ner as to brmg the key on the box into the diagonal AC. Since r^ and r^, are equal it is clear that when the potential at B is equal to that at Z), * rx= n, 11. so that the unknown resistance is equal to the resistance unplugged in the box of coils when the balance has been arrived at. With thisWheatstone's bridge, therefore, resistances varying from 0.05 to 100 ohms, can be measured directly to within 0.05 ohm, if the gal- vanometer used be sensitive ; but in the Field, the 3-coil galvano- meter (with the lo-ohm coil in circuit) is the Service instrument It will be found in this case, that two resistances can be unplugged' one of which makes the galvanometer just move to the right, and the other to the left ; the mean of these two resistances is within about I p.c. of the true value, when using one Service test cell. 200. The balance is obtained practically as follows : At first no pegs are removed from the box, a deflection will then be obtained unless the unknown resistance Vx is zero. The direction of this deflection should be noted thus : " Left (or right) too little." Trial resistances are then unplugged in the box, until finally that resist- ance has been introduced for which the galvanometer does not deflect. This will be the value of r^ to insert in eqs. I. or II. ii8 KLKCTRICAI. XfF.ASURr.MF.NTS. ii I i I St I • 201. When workin^^ with :i very sensitive pilvaiiometcr, it will probably happen that the exact resistance ,with which there is no Uetlection, cannot be unpin/^'j^'ed. In this case, when ^^reat accuracy is requned, the nearest resistances in excess, and in detect, sliould i)e introduced, and the deflection of tiie /Galvano- meter noted in each case. Let d and d, be the deflections and ^b the nearest resistance in defect, then r., ( [ {'"-^d + d.i Mcimiriuij^ the resistance of a coiled wire. 202. If the wire, of which the resistance is to be obtained, be coiled, a momentary induced current will be produced on makinr^ and breaknif,- the circuit.* This effect will occur, for instance, when measurin^r the resistance of a j^alvanometer, or of the wire of an electro-maf^net. This induced current has its ori/,nn in the branch DC of the Wheatstone's hvkh^c, (see ¥\^. 14) and it will cause a difference of potential between the points B and D, so that there will be a deflection on the /,^llvanoIneter, even when the resistance which gives the balance is unpltig;^^ed. But the induced current is only momentary, and disappears directly the test current has been established. Hence, a key should be placed in the diagonal BD, by means of which the galvanometer can be kept out of circuit until the induced current has disappeared, when the balance can be obtained. The method of working is to first depress the "battery key," and immediately after the "galvanometer key." The Post Oflice pattern box of resistance coils is provided with two keys,t and the binding screws on the instrument are so arranged that these keys may be used for the above purpose. BY LATIMER CLARK's DIFFERENTIAL GALVANOMETER. 203. It was seen, when describing this instrument, that binding screws are provided to allow of resistances being introduced into each of the divided circuits, and, that there will be no deflection when the currents flowing in each branch, through the galvanometer, are equal and opposite. It was also seen that when both shunt pegs are in, or when both are out, these currents will be equal if the resistances introduced are equal ; but that when one shunt peg is left in and the other taken out, the resistance correspond- ing to the peg that has been left in will be 100 times smaller than the other resistance. Hence, if the unknown resistance be placed in one branch, and a box of resistance coils in the other, a resistance can be unplug- *See § 89.3, Ganot's Physics, 9th Edition. tSee § 166. MliASUkKMliNT ()!• KI-lSlSTANCli. letcr, it will li there is no when f^reat -ess, and in :lie fjalvano- lections and 119 ged in the box which will either be equal to, or 100 times \iirgor or smaller, than the unknown resistance, according,' to the arrange- ment of the shunt pegs, and, when this resistance is unplugged, there will be no deflection on the galvanometer. The practical working is as follows : The connections are made as shown in lug. 16. The shunt Fi'j. Hi. )btained, be 1 on making or instance, of the wire DC of the 1 difference e will be a ance which rent is only t has been agonal BD, t of circuit )alance can he "battery The Post two keySjt anged that ITER. bat binding iduced into deflection Ivanometer, 5oth shunt )e equal if one shunt orrespond- les smaller ranch, and )e unplug- pegsbemgm place, infinity is unplugged in the box of coils,* and a short contact is made with the key attached to the instrument ; the direction of deflection of the needle is noted thus, "Left (or right) too much." Trial resistances are then unplugged in the box of coils, until an approximate balance is obtained. The shunt pegs are then adjusted, according to the magnitude thus found for the unknown resistance, and the correct balance obtain- ed. If the resistance be of medium magnitude both shunt pegs should be removed, so as to make the instrument more sensitive. A convenient rule to remember, when the shunt pegs are used for multiplying or dividing, is "Remove the shunt peg on the side of the highest resistance." If the box of coils has a range of i to IT, no ohms, resistances varying from o.or to 1,111,000 ohms can be measured by the above method. MEASURING THE RESISTANCE OF A GALVANOMETER. 304. The resistance of a galvanometer can be found, like that ciy other unknown resistance, by the methods explained in V ly/. etc. But a galvanometer may be used to measure its own resistance by the following method, due to Sir William Thomson. The ordinary Wheatstone's bridge connections are formed, t with this difference, that a contact key replaces the galvanometer in the diagonal BD, and the galvanometer, whose resistance is to be measured, is put in the side DC as the unknown resistance. Fig. *If the box of coils does not contain an infinity yeg, the same result can be ob taineil by disconnecting the box. Or, no resistance is unplu««ud in the first instance, in which case the i.oto should be "Left (or riglit) too little." tSee § 197. \ 5 il' I ■ Si II 120 ELECTRICAL MKASUKEMKNTS. 17.* Now if the resistance n be so adjusted that the potential at B IS equal to the potential at D, namely if the relation Sx=-frb holds, it is clear that no change will be produced in the strength of the current flowing through the galvanometer when the key is depressed ; but this current will either be decreased or increased It these potentials are unequal, and this will be indicated bv an the kT°" '" deflection of the galvanometer on depressing ^ The practical working is precisely the same as that employed m the ordinary Wheatstone's bridge method,t and need not there- tore be further described. If the current flowing through the galvanometer gives too large a deflection, a shunt of known resistance can be applied, in which case the resistance measured is -/J from which -. can be found, l^x -rS " ' since s, the resistance of the shunt, is known. MEASUREMENT OF VERY HIGH RESISTANCES. 205. Ifthe unknown resistance be greater than can be measured by the previous methods recourse must be had to the followinc^ • The unknown resistance is placed in circuit with a reflectintr galvanometer and a test-battery, and the deflection is noted The unknown resistance is then replaced by a known resistance and the deflection is again noted, one cell only of the test-battery being in circuit, and the galvanometer being shunted. Let ^ be the deflection on the reflecting galvanometer, when the unknown resistance rx is in circuit, given by a test-battery composed of n cells, the E. M. F. of each cell of which is P Further let d' be the deflection on the shunted galvanometer* *Compare with Fij?. 14. +See § 200. MEASUREMENT OF RESISTANCE. I^I e measured when the known resistance / is in circuit, LMven liy one cell whose E. M. F. is P. Then nP and (§ 176.) Hence lid' -^» r.,= gs'i -iK + >H') But ig + nf>) can be nef,dected, therefore .l{^'nir'_+gj+r) y.,-- tiff-' This method is employed to lind the insulation resistance of electrical cables. Example.— Let n = 2o, (t=i ohm, ^'•:=7ooo ohms, s— 7 ohms, r =4000 ohms, d=ioo, and ^ = 50 ; then 7000x50x20(4000+7 + 1) . , rx—— — ^- — i =40,080,000 ohms— 7 X 100 "^ 206. It will be observed* that the expression ^'^'^—^^i^'^ is the figure of merit, in terms of the test cells in use, of the galvanome- ter.t Thus, if many measurements of high resistances have to be made, and the galvanometer is not altered in any way, the unknown resistance can be found more simply from the formula * d Example.— The figure of merit of a galvanometer is 100,000,000 ohms. The number of cells in the test-battery is 15, and they give a deflection of 10 divisions through the unknown resistance. Here 15 X 100,000,000 >'■'■— ■ =150 megohms. 10 J h BY TANGENT GALVANOMETER. 207. If o be the deflection given by a cell, whose E. M. F. is P, when the unknown resistance is in circuit, and o' the deflection due to the same cell, when a known resistance / is in circuit, then tan o and tan o' are measures of the two currents. | •■See § 188. 1 That ia aubstituting thi' E. M. F. of one test-cell fur uiie vult. ISee S 17;i I 'h t?.i 122 i;lkctkicai. mkasuuhmknts. Now, as explained in J^ 187, d and «' should lie between 20" and 50"; / can be so chiisen as to make tV any convenient value, and when the unknown resistance is in circuit, the ^Mlvanotneter can be shunted so as to obtain a ffood reading', in which case, ^ tan and // tan /?'= whence r'' can be found. r'+fr + p Readinfjs within the above limits, can also be obtained by usinp a directing magnet, but great care should be taken not to alter the position of this magnet when the known resistance is placed in circuit. Here and fi tan 0= rx+g+p P ft tan ^=-7 — - y ^g-¥p The above method is not accurate.' because the test ceii polar- izes, and P has not therefore the same value throughout, which of course, is essential to the accuracy of the method. A reflecting galvanometer can be used for the above measure- ments, in which case tan- H and tan S niust be replaced by rfand rf'. MEASUREMENT OF RESISTANCE WITH THE THRE'^.-COIL GAL- VANOMETER. 208. Rough measurements of resistance can be made with a 3-coil galvanometer in the following manner : _ 209. The deflection, J, given by one cell (a Leclanche cell for mstance), is found when the looo-ohm coil is in circuit. The unknown resistance is then introduced, and the deflection u obtained. Now c=- I . lOOO-f /' *StH- s i7<;, twecn 20" and nt value, and iiiujinctcr can case, -^- tan d ined by using I not to alter nee is placed st ceil polar- hout, which, )ve measure- d by f/ and d'. F.-COIL GAL- nade with a iche cell for ;ircuit. The deflection «' And But the ratio MEASUKKMIiNT OF KliSISTANCli. rx + 1000+ff g,= a 123 can be found from Table II, since the deflections d and ff are known, hence y^=(iooo+//) (rt— i) The value of /> must be estimated. This method will give results within about 15 p.c.of the truth, when the resistance does not exceed 900 ohms, and is not less than 100 ohms. 210. If the resistance exceed 900 ohms, the second reading should be taken with more than one cell in circuit. For instance, if the resistance does not exceed 10,000 ohms, and is not less than 6,000 ohms, 6 cells should be used. In this case C'= 1000 +fj tp rx+ 1000 + 6/> whence, as in the previous case (§ 209), and neglecting 5/>. ra;=(iooo+,o) (6fl— I) The general equation is J'a; = (l000 + p) (na — i) where n is the number of cells with which the second reading ^ is taken. The following table gives the number of cells to use according to the magnitude of the resistance. rx between 100 and 900 ohms, « = i. " " 900 " 3,000 " M = 2. " " 3,000 " 6,000 " n = 4. " " 6,000 " 10,000 " n = 6. The resistance of high tension fuzes, and the insulation resist- ance of the cables used to fire these fuzes, can be found in this way. The galvanometer should be placed at right angles to the magnetic meridian, so that the index may point accurately to zero. The galvanometer should be gently tapped whilst taking a reading, as the needle is liable to stick. Example. — Let o = 59°, and o = 52° taken with 2 cells in cir- cuit ; then ii fj = 18 ohms, rx=(ioooi 18) (2x1.3 — i)=i630 ohms. r ir* ■J 1 i I:; 12 4 15 ! i i . «|S III.HCTKICAL MI•:AS^RR^fF.^•TS. NolCIl TKST OK VKliV l,(.\V liKSISTANCKS. circuit Sll.?' ' , '"'^" a teratum in the resistance of the Hpf?..'7K .u'^"" t ^^''"'^'^ •" t^^^ ^""-ent sufficiently c.reat to be detected by the A^ilvanometer. ^ "^ ^ and for^^'S'fr '' "''"^Z'''" ^^'^!''^ ^^'"^ continintv of circuit wires a d foi testin^r ,v,re fuzes, when tlie apparatus for making an ac! cuiate measurement is not available. ^ HOUGH TEST OK THE INSULATIOX RKSISTANCF. OF A CABLE. 212. The following applies only to the Land Service cables n<;pH ditio^ro?;rc!br'%r'v;'"^'j'^^^ ^^"^^^ to asc"?ti?^'h: ::' onV fi the cable, rather than its msulation resistance when laid po ^i le le'ak: T 'T "^f'^ '^^ '^^ *" «^^-" conx^ection v ^l' orf Ai ^^'"^ ^''°"^^ ^^^ taken that both ends of the cable one Zlf%V:Te Tlh"" ""'''' T' ^^>'- ^^^ conn^cdoL ar one end ot the cable to the i,ooo ohm coil of a 3-coil L-alvanometer- Jie galvanometer through a key, to one pole of a te ceU Ind last thv'' P° ' °^ '^^1 ^^" ^^ ^^ ^^^°^-t "^^"l^^ted lead, of which the ead is pfared in tr ^T' "v ^^"-^f ^^ ^'■^^^^^^-^^- This sho ? leaa is placed in the water. Now, if there be no deflection on fh^ P^ -to rt'if^thetT"'^^^^:.^ '^>' '''' i-^lati^nf ;"LW6: TnH fb; i ' ^,^- ^^.any deflection, the insulation is defective end of tir'nll '^'''^'" '' ''' '^'' ^'''^''' '^'^ deflection. The free end of the cable i.s now connected to the short lead • the deflec^ tion 111 this case should not be /... than twice the fo mer one 1 he necessary connections are shown in Fig. 18. F!i/. IS. If the second reading be twice the first reading the insulation resistance is rather more than 1,000 ohms, for the cur?e f n h" and heTce'^ '''''' '''' ^'^" ^ ^^ ^"^ ^"-"^ "^ ^^e^seZ d cas^f aced on short Tlie unknown f it does n( e as when the n, because its stance of the Y f,aeat to be nrcuit wires, aking an ac- LK. 2 cables used :ain the con- :e when laid lection with of the cable lections are : Ivanometer; St cell ; and f which the This short :tion on the s praciically s defective, . The free the deflec- Drmer one. insulation 2nt in the md case,t •ent method. or MICASUREMENT OJ- KH SISTANXK. r.,- + i,ooo + r + i> ^ •-' ( r,ooo + r'-fj> I r.r > i,ooo ohms. T25 TO DETECT THE POSITION OK AN IN.SULATlON FAUI/r IN AN ELECTRICAL CABLE. 213. If the above test shows that the insulation resistance is very smal, the mam fault will probably occur at one or two places The position of these faults can be found bv retaining the cor - nections a ready inade for the insulation test; "experimenting with the dry cal^le at hrst and then gradually drawing it through the a'fluh. ' '''^"" "f the galvanometer'clearly indicates If the cable be in position, other methods must be employed. BY FUSION OF FINE WIRE. 214. The thermo-galvanometer attached to the Firing resistance coils can be used, occasionally, for measuring low resistances" when a powerfu current is available, and its passage will not injure he wire etc., to which the resistance to be measured is due. One pole of a battery, capable of fusing fine wire, is connected to the thermo-galvanometer, and the other to the wandering peg and the resistance through which the battery WiW jmt fuse a stan- dard wire * is determined This measurement must be made by a series of trials, and for the hrst trial little or no resistance is in- troduced into circuit. The resistance is then graduallv increased and by i-20th ohm as the sought resistance is approached • the length of wire deflagrated is a guide to the amount by whiih to increase the resistance for the next trial. Let r. be the resistanrP unplugged in the box of coils when the wire is just fused Should too high a resistance be unplugged, a fresh start must be made for the battery polarizes if the wire be not at once fused To further insure accuracy, the contacts should be of equal lengths, of about \ second duration. The unknown resistance, having now been introduced into the circuit, the resistance through which the same wire can be fused is found; let r, be the resistance introduced by the box of coils in this case ; then ^ ^ °^ r.r=rf, — r /, This method is used for in a circuit of subm measuring the resistances of "earths irine mines. 'The best wire to be used is the standard 0.0014" iridio-platiniun wire. 'Jb 126 ELECTRICAL MEASUREMENTS. h;'i N, III m i'V;'; I (( ii DETERMINATION OF THE RESISTANCE OF FINE WIRE AT THE POINT OF FUSION. 216. The resistance of fine wire cat the point of fusion is required, when calculating,' the battery power necessary to fire electrical wire fu2es. This resistance can be determined in the following manner, by means of a box of Firing resistance coils and a battery capable of fusing the wire.* The experimental wire is placed in the clips of the thermo- galvanometer,and the connections are soarranged,that the current will pass through the wire and the box of coils. The resistance that can be unplugged, so that the wire is just fused, is then found, taking the precautions, mentioned in § 214, to prevent polarization. Let this resistance be n, and let rl- be the resistance of the wire at the point of fusion. Double the length of wire is then placed in the clips, and the resistance found through which this length of wire is just fused. Let n be this resistance and it IS clear that in this case the resistance of the wire is 2 ra;. Now the currents in both cases are evidently equal, and, since the same battery was employed, it follows that the resistance in circuit must also have been equal ; hence ri -t rj; + (J = YiJ + 2 Yx + {> or rx = ri — rb If /, expressed in inches, be the length of the wire in the first case {i. e. the distance apart of the clipst) ~ is the resistance of the wire, per inch, at the temperature of fusion. ' MEASUREMENT OF THE LIQUID RESISTANCE OF A CELL. 216. The liquid resistance of a cell cannot be found by any of the methods just described, because the current produced by' the cell under examination interferes with the measurements. Other methods must therefore be adopted, and in all of them the cell supplies the current used to effect the measurement. Generally speaking, it is more difficult to overcome the effects of polariza- tion when measuring liquid than when measuring ordinary resistances, and it is therefore more difficult to obtain accurate results. tGrove's cells will give the best results. tSee § 190. RE AT THE MEASUREMENT OF LIQUID RESISTANCE. BY mange's WHEATSTONE's BRIDGE METHOD. 127 217. The arranf]^ement is very similar to the ordinary Wheat- stone's bridg^e for measuring resistances, and its typical form as follows : In a quadrilateral A B C D, (Fig. 19) formed of conductors, Fig 19, the sides AB and AD contain known resistances r, and r^, the side BC a resistance n that can be varied at pleasure, and the side DC the cell or battery under examination. A galvanometer is placed in the diagonal A 0, and a key in the diagonal BD. It will be noticed, on comparing with the ordinary Wheatstone's bridge (Fig. 14), that the cell takes the place of the unknown resistance, the galvanometer the place of the cell, and the key that of the galvanometer. A second key is placed in DC, so tha't the circuit may be made, or broken at pleasure. On depressing this key the current divides at C, re-uniting again at .1, and the part of the current flowing through the galvanometer is, from ^ 147- When the key in the diagonal BD is depressed, the points B and D become electrically the same, and the part of the current flowing through the galvanometer divides again at A, flowing along /I /) and ABD. In this case the current flowing through the galvanometer is n rf>+g + ■r.. ri>ig+-'Jf-) _L-Zi+''-'^ n>+g+ ''' '^' + /'-v Now the balance is obtained when, on depressing the key in the diagonal BD, there is no change in the deflection of tlu; gal- vanometer. This requires that the currents flowing through the galvanometer in both cases shall be equal. That is 128 ELECTRICAL MEASUREMENTS. C = C ill (■ \ f J 1 i- 1 f ' ■ -■ l i I: I: "J; The above expressions must therefore be equated, and it will be found on simplitication that The binding screws provided on the Post Office pattern box of resistance coils allow of the necessary connections being made for Mance s method, and, as the practical working is almost precisely similar to that of the ordinary Wheatstone's bridjre method, it is unnecessary to describe it. BY THE DEFLECTION OF A GALVANOMETER. 218. Usin^ a tangent galvanometer.— The cell, or battery, of which the liquid resistance is to be found, is placed in simple circuit with a tangent galvanometer and a box of resistance coil- as shown in Fig. 20. Resistances r^^ and /^ are unplugged, anrl the corresponding angles of deflection, d and &, are observed. Now* H tan 0= — //tan o 'x+Vb+g-i-r P Whence f>x ^y'b+g-{-r,) tan d' ~ {ri,-{-g-}-rc) tan o tan o— tan d' The resistance of the galvanometer must therefore be known, or else found. Fi;/. :.'(). liiii i i lllf This method, "similarly to the last, is unsuitable for inconstant batteries. If a reflecting galvanometer be used, tan i) and tan <)' are replaced by d and d' . *See§173. , and it will MEASUKEMENT OF LIQUID RESISTANCE. 129 219. Usin/; a sine f(alvanomctcy.-~A similar method is applicable with a sme f^^alvanometer,* and the resistance can then be found from the formula f,^- t^A+^^slA^^-_(_^J^+^±rc^l^ sin J sin fJ— sin o 220. With either galvanometer a shunt may have to be used, to keep the deflections within proper limits, in this case t^ must be replaced, in the above equations, by ^^,.t To reduce the effect of polarization n, and /(,' should be larw, and the difference of value between them not too great. BY THE FUSION OF FINE WIRE. 221. This method is applicable when the battery, of which the liquid resistance is required, is powerful enough to fuse hne wire. This method can therefore only be used when the liquid resist- ance is very small, and the E. M. F. is large ; as is the case in the voltaic batteries, used for firing electrical fuzes. The standard 0.003" and 0.0014" iridio-platinum wires are very suitable. The thermo-galvanomeLer, attached to the Firing coils, can be used for the purpose. One pole of the cell, or battery, is con- nected to the thermo-galvanometer, and the other to the wander- ing peg, so that the circuit is completed on depressing the key. It is first found what resistance r^ can be unplugged, so that one wire will just be fused, and secondly the corresponding resistance /ft when two wires are just fused. In ihe second case the two wires form a divided circuit, each branch of which lias the same resistance, so that the current is equally divided between the two wires ; from which it follows that the current necessary to fuse two wires is double the current required to fuse one wire. Now the current in the first case is c=--^ and therefore in the second case P where fjj: is the liquid resistance required, y^ is the resistance of one indio-platinum wire at the point offusion;l and ;-,• is the resist- ance of the connections.^ Hence 2C = l*.c = Vb — 2y b — Vr *See § 172, tSee § 17U. tSoegtilJ, ;uulTal)le II. SSee g IHO, u ; I I Hi !l 130 ELECTRICAL MKASL' KEMENTS. In finding the resistances ;>, and Z^, tlie precautions, mentioned in ^ 214, must be taken. It will be observed, that the accuracy of this method depends on the diameter of the wire being constant. HV LATIMER CLARK's DIFFERICNTIAL (lALVANOMETER. 222. In this method the connections and pegs are, in the first mstance, so arranged that the current flows through oidy one coil and then they are altered to enable the current to flow through both coils, m the same direction.* Tl'^> cell is connected to one side of the instrument, and a box c , ance coils to the other. Let ri, be the resistance unplugged j .. box of coils in the first instance, then, for tlie second reading, a resistatice ;-'/, mast be found, such that the deflection of the needle will be the same as before. Clearly the current in the first instance is double the current in the second instance, that is and P (J= C' = P where £ is the resistance of one coil of the differential galvano- meter,t and Vc is the resistance of the connections.^; Hence The necessary connections are shown in Fig. 21. f; to be made zcvo, wlien l»,—r'i,—rr Accurate results cannot be expected by this method, if the cell be mconstant, for the method is based on the assumption that P has the same value for both readin/^^s. i!V kkmpf/s method. 223. In Kempe's method, the nej^'ative pole of the cell or battery, of which the liquid resistance is required, is connected to a reiiect- iiij? fralvanometer. The ;4:alvanometer is connected to one plate of a condenser, the other plate is connected, through a kev, to the positive pole of the battery, and also to '^ earth." A" box of resistance coils is placed between the negative pole and the key. The arrangement is shown in Fig. 22. d galvano- Hence A suitable The first reading is taken when the resistance in the box of coils is infinite, and the corrected'^- deflection D measures the potential P, at the positive pole of the cell. For the second reading, a resistance n is unplugged in the box of resistance coils ; the corrected deflection D' will in the case measure the potential /) at the negative pole of the cell or battery, which is less than P by the fall of potential due to the liquid"' resistance. From Fig. 2^, which exhibits graphically t the f^dl of potential Fiij. :>,:. i m I anging the *Se6) s 180. +See g 146. K]2 m i:r.i-.fTiarAr. NfKAsrK-i: mkkts. in the circuit thn.uKl. the box of ceils, when the resistance r, is unpluf,'-f,'-e(l, It appears that P n f JV'*;/''^'^"'";!'"^' *'■ ^^ T"' '^7^^ ^'"^^ t'l^ ^^"ic condenser is used loi both readni^^s (i.e., I\~K) P_l) p D' Hence whence />,,• can l.e foinul. In practice n can genevallv be so ad- justed as to make In this case, tlierefore D = 2D' px=rh This method will -ive j^^ood results, and is applicable to incon- s ant batteries ; but it requires careful manipulation. Great care sliould be taken to prevent the leads connected to the box of coils from mflucnrin,t,- the .galvanometer. MEASUHEMICNT OF DlFFEliEKCE OF POTENTIAL. 224. The general problem is to measure the difference of potential between any two points in a circuit. Let these two points be denoted by A and B ; let /.« be the potential at .-1 , and pb that at B, then p^-p, is the difference of potential between -1 and B. r -^ These measurements are made by comparing /«-/,, with some known difference of potential, such as that furnished by a stand- ard cell. -^ 225. The cells to be used, to obtain the known difference of potential are either Latimer Clark's standard. Grove's, Bunsen's or Damells.-'- ^ ' BY law's condenser METHOD. 226. One plate of the condenser is connected to the point B t and the other through a key to the point A ■ a reflecting galvano- meter is introduced between A and the condenser, as shown in •SeeSg !r)8 161. +See § 224. iistaiicc ri, is nser is used MKASURKMFNT f)|- niFFF.HF.NrK OF POTI'NTIAI. T.i^ Vv^. 24 When tint key is (h-pressod, the coiKleuser is cluirfjed witli ;i difference of potential, equal to /)^-/.,„ ;ind the corrected* /■''■.'/. J.'>. detiection on the ^galvanometer measures this difference of potential. The difference of potential Pa-P(, is next replaced by a standard cell whose L. M. F. is P\ as shown in Fig. 25, and the deflection ot the galvanometer is measured. Flij. ,/.;. For accuracy, however, both deflections should be nearly the same ; the galvanometer should therefore be shunted, as shown in the figure. Then, if D and D' are the corrected* deflections obtained with /)„-/>j and the standard cell respectively, it ap- pears from § 192, since K=K' , that If no shunt be necessary, the formula is reduced to Pa-pb=jy.P This method will, with care, give good results. BY THE DEFLECTION OF A GALVANOMETER. 227. In this method the measuring apparatus consists of a galvanometer and a box of resistance coils, connected together in simple circuit, and attached to the points A and 7^,t as shown in rig. 26. *See § 180. +See § 224. ' i.\ im 'U i:i.i:( TK'K ,\L mi;as(iki:mknts. A rosistunco ;v, is tl,,,, mi|,lu;^r^r,,,|, so as to -ive a suitable deHec- tioii. attcr wliirh th.- lurasmin- appanitus is discoiinectecl and /■''';/. .'''. K i III 1. I -S I « i!;Mf ^ "'If ill ; j H II attached to a standard rell. when a resistance r^' is found, hy trial, such that the deflection is tlie same as l)efore. ICvidentlv the current, which flowed throu;i,di the j^^alvanonieter in the first 'case, is^ e(|ual to the current flowin;^ in the second case; hence, if Pa'—pf,' he the difference of potential i)etween the poii^ts A and'/?, whilst tlu" measuring' apparatus is connected, and /''tlie K. M. 1-'! of the standard cell. . r Pa -p u ^ V y + '-ft + A^ 'yKrb ^g) r'h +.ir + // or P'n—p'h- rb-Yg .p ia) 228. It will be observed that the introduction of the measuring apparatus alters the resistance of the circuit, and this will affect the difference of potential between A and B. A correction must therefore be applieil, as follows : Let A' be the total resistance in the circuit, and r the resistance between A and H, before the introduction of the measuring apparatus, tjien r pa—pi,=yC R J> .{b) Now, when the measuring apparatus is connected, as a loop, to to the points A and />, the resistance between these points will no longer be r, but becomes '^''^V , and the current is altered to 6»' = and hence 7^ + T^(^+gI^) yy+yb+g ) pa ~Pb= . ----- .C y+yb+g . y(yb-hg) 'y+yb+g R + P \ ''(ybfg)__^,] [r + rf,+g ) ■ic) MIvASURKMKNT Ol" i;i.i:( IKOMOIIVH FOKCU:. IJ5 Eliininatiii^' /' between tlie e.iimtions (h) and (t), and conibiniiiL' with eiiuation (. mentioned in § 224. Hut a simplirtcation can be introduced in the practical working, because the galvanometer and the box of*coils can be placed directly in tlie circuit, and not on a loop. The arrangements will then be as follows. nv law's C()NI)i:\si:r miitiioo. 231. The connections given in ^ 224 will be slightlv altered, as shown in Fig. i-j, there will, however, be no difference in the calculations. Fhj. ..';■. '36 ELECTRICAL ME ASUKKMKNTS. HY THE DEFLECTION Ol" A CALVANOMETER. 232. Tlu- cell, or hattcry, under examiimtion, .'i l)o.\ of resist- ance coils, and a /.'alvanonieter are connected to^'etlicr in simple circuit, as shown in I'ig. 28. A resistance y/, is unphi^'Kcd in the Fig. ;JS. I'll ? i 1 !! i > I ■ j'l^ J f I f^ W- box (,f coils, and is selected so that a suitable defleciion is obtained on the ^'alvanometer. The cell, or battery is then replaced by a standard cell, and a resistance ,-,' is found with which the deflection is the same as before. Then since the cur rents which flow through the galvanometer in both cases are eciual or SIR CHARLES WHKATSTONE's MRTIIOD. 233. In this method the experimental cell, or battery, is placed in simple circuit with a galvanometer and a box of resistance coils. Resistances n and rb are unplugged, so as to give suitable detlections, o and o , on the galvanometer. The experimental cell, or battery, is now replaced by a standard cell, and a resistance /-ft is found by trial, so that die deflection on the galvanometer will be o; the resistance y^ , giving the deflection +iooo jy ft -f-iooo Generally, /> and ft can be neglected, so that This last equation is applicable when the E. M. F. of "firing" cells is required, if the instruments for the more accurate methods be not available. The galvanometer should be placed at right angles to the magnetic meridian, so that the index may point accurately to zero. The galvanometer should be gently tapped whilst taking a reading, as the needle is liable to stick. Example.—Let I*':=i.g6 volt., 0=57", and ^7 = 66" ; then, from Table II, a=o.yi, hence Px=o.yi X 1. 96 — 1.39 volt. BY THE FUSION OF FINE WIRE. 236. If a cell, or battery, be capable of fusing fine fire, an ap- proximation to its E. M. F. cr.n be found when the current required to just fuse the wire, is known. For this purpose the cell, or battery, is connected in simple circuit, with the Firing resistance coils and the thermo-galvano- meter. The resistance through which the cell, or batterv, can just fuse the fine wire, placed in the clips of the thermo-galvano- meter, is then found by trial, as explained in § 214. Then P.r = iri>+,o)C where r{' is the resistance unplugged in the box of coils, and p is the liquM resistance of the cell, or battery, which, if not known, ^1 lii ! i li rfj l! m m M^.' |i 138 ELECTRICAL MEASUREMENTS. can be found by the method described in § 221. The value of C depends on the nature and on the diameter of the fine wire em- FsXunV"blTn''^' """""'"" ''""^"'^ iridio-platinum wires MEASUREMENT OF AN ELECTIUCAh CURRENT. \. fo finl^i?. °^f ""* in View when making measurements of current IS to hnd the strength of the current (in webers) flowing in some given circuit If the electromotive force in that circuit be know" and also the total resistance in circuit, the current can be easily tound from the equation -^ If / and 7^ are not known they can be measured. This con- stitutes one of the methods of measuring currents of electricity. 237. Since the strength of the current flowing in a circuit depends on the resistance in that circuit, if the introduction of the measuring apparatus alters this resistance, the current ntcasnred will not be the current actually sought, and a correction must therefore be applied. This correction can be made as loiiows : Let ^ be the total resistance in circuit before the measuring apparatus is introduced, then sui^ij, r - ^" and Cx IS the current to be measured. Now let r be the additional resistance due to the measurintr apparatus, and C '• the current actually measured then ' C - ^ and hence \i\ THE DEFLECTION OF A GALVANOMETER. 238. I„ this method the unknown current is compared with a known, current, such as that produced by a standard cell in a circuit of known resistance. A galvanometer, shunted if necessarx-, is first introduced into the circuit under examination, and the deflection noted. The galvanometer, with the same shunt as before, together with a box of resistance coils, is now connected to a standard cell capable (if MEASURKMENT OF ELECTRICAL CURRENTS. 139 supplying a current,* and the resistance is so arranged, by means of the box of coils, as to obtain the same deflection as before. Then, clearly, C':r = --- P where P is the E. M. F. of the standard cell, and /> its liquid resistance. ' ^ It will be noticed that the liquid resistance of the cell must be known. As it is difficult to measure this resistance accurately, it follows that this method is subject to the effects of polarisation, and to the inaccuracy of the measurement of the liquid resist- ance, which latter inaccuracy is also due to polarization. MEASUREMENT OF POWERFUL CURRENTS. 239. The following method of measuring powerful currents has been used by Major Armstrong, R.E. : Two points, between which there is a small resistance r, are selected in the circuit carrying the current to be measured The measuring apparatus, consisting of a box of coils and a shunted reflecting galvanometer, is connected to these two points, and a high resistance n is unplugged in the box of coils. A divided circuit is thus formed, as shown in Fig. 29, and clearly a very Fig. ,0). small portion only of the main current will flow through the galvanometer, and this portion, c, can be measured as explained m § 238. It will be observed that the introduction of the measuring apparatus does not practically alter the strength of the main current, and hence, t \^ X — ' . — ,c Example.— Let ri>=io,ooo ohms, gs^y ohms, ^=7,000 ohms, and r=o.2 ohm. It is found that c^ *See § 15i>. +See § 147. 500,000 weber. m n , ! :' i!,?Ji II. s 1 1? hi mi ! ) i i*5 ii- 140 Hence ELECTRICAL MEASUREMENTS. 0-2 ' 7 ' 500,000 = 100 webers. METHOD OF VIBRATIONS. 240. If a magnetic needle be set vibrating, it will oscillate about Its position of equilibrium, and would continue to do so were it not for the resistance of the air and the friction of the pivot 1 he case is strictly analogous to that of a simple pendulum, and hence It can be proved* that, when the amplitude of the oscilla- tions is small, the time of an oscillation is independent of the amplitude. In fact '-"=-1 where F is the force producing the oscillation, applied at each pole of the needle, and ;^ is a constant depending on the mass and on the shape of the needle.t Now an alteration in the strength of the magnetic fieldt will alter the value of F, and hence that of ^ ; so that r- becornes a measure of the strength of the magnetic field. If the coils of a tangent galvanometer be placed east and west • so that the needle is at right angles to the coils, the force due to a current passing through the coils will be in the direction of the magnetic attraction of the Earth, and further, will be constant. When no current is passing F=zMH where H is the horizontal component of the Earth's magnetism and M IS the magnetic intensity of each pole of the needle. Therefore when a current C is passing F=MH + bCM where 6 is a constant depending on the galvanometer. Hence if if be the time of an oscillation when under the influence ot the Earth's magnetism alone, t' when the magnetic effect of the current, C ,.is added, and t" when the current C" is added, then y2 — X MH' t'^ = MH + bUxM' t"= or MH-hbO"M /2 42 .r" -t"2_ii *.See s 55. Ganot's Physics, 9th Edition. +See Appendix. iSee § 61(8, Ganot's Physics, 9th Edition. IM" Wi th MEASUREMENT OF CAPACITY. 141 canVe'Ltd" """ '^ ' '"°"" ^""^"^' ^" ^^^^ ---"^ C'. The practical method of working is as follows: The .galvano- meter is placed with Its coils east and west, so that the needle °s at right angles to the coils. The needle is then set vibS fng under the influence of the Earth's magnetism alone, and the number of vibrations in a given time are counted, from vvhich t can be deduced. The currents C. and V are then success velv passed through the galvanometer, and the times^' and r at found as above. Accurate results cannot be expected bj- this method, owing to e effects of polarization. ^ MEASUREMENT OF THE FUSING CURRENT OF A FINE WIRE. 241. The strength of the current necessary to fuse a given fine wire IS required when calculating the battery- power necessary to fire wire fuzes, and can be found as follows : Some of the wire is placed in the clips of a thermo-galvano- meter and a battery,- capable of fusing the wire, is connected so that the current can pass through the Firing resistance coils, and through the wire. Let n be the resistance unplugged when the wire IS just fused, and r^ the resistance of the wire a^ the Point of fusion , then -^ ■' P C z=z ' If r.p, (> and P are not known, they can be found as explained in §§ 215, 221 and 231 or 233. ^ MEASUREMENT OF THE FIRING CURRENT OF A WIRE FUZE. 242. The firing current of a fuze can be found in the same wav as the fusing current of a fine wire, and the same formula is ao- phcable. Not to waste fuzes, a high resistance can be unpluE^ed in the first instance, after which the resistance in circuit can gradually be reduced until the fuze is fired ; but in this case it is necessary to use Grove's cells, or some other constant element to avoid the effects of polarization. The resistance ;-„ of the wi're- bridget can be estimated from its resistance when cold by allow- ing fo. the increase of temperature. The temperature at which the tuze is fired can be taken at about 600° Fahr. MEASUliEMENT OF CAPACITY. 243. The capacity of a conductor can be measured by compari- son with the capacity of a known condenser. ' *0r the time required for, say 10 vibrations, can beobservod. tOf Grove cells, if possible. m ifti m 1 i-te-^ liiHi ii 142 ELECTRICAI. MEASUREMENTS. As already explained, if a reflecting galvanometer be placed between a cell and a condenser, an instantaneous current will be obtained on depressing the key, and the deflection was shown in § 192 to be proportional to the capacity. Hence if Kx and A" are the capacities of the unknown and known condensers, and D, D the corresponding corrected swings, Aa;=w.A If there be a large difference between the numerical values of D and D' , a shunt should be applied to the galvanometer, in which case evidently The connections are precisely the same as those given in § 231. MISCELLANEOUS TESTS. TESTING THE POWER OF A FIRING BATTERY OR OF A QUANTITY DYNAMO. 244. The most direct way of ascertaining whether a firing bat- tery is powerful enough for the purpose required, is to find the resistance through which it can just fuze one or more fine iridio- platinum wires placed side by side. The number of the wires should be equal to the number of divided circuits to be employed, the diameter of the wire the same as that of the fuzes and the length of each wire the same as that of the bridge of the fuzes.* 1 ne necessary number of wires are placed in the clips of the thermo-galvanometer attached to the Firing coils, and the battery under examination is connected so that the current can pass through the box of coils, and through the wires, on depressing the key. Successive trials are then made, gradually increasing the resistance in circuit, until that resistance is unplugged, through which the wires are 7W fused. This operation must be carried out with all the precautions mentioned in § 214. The resistance thus found should be less than the resistance in the circuit con- necting the charges. When testing a quantity dynamo, very short contacts must be made (about h second), because the current increases if the wire be not fused immediately, and it is only the strength of the current on first making contact that can be relied on when firing charo-es. *Namely 0.25" for the Service fuzei. MISCELLANEOUS TESTS. 143 QUANTITY 246. When no Firing' resistance coils are available, a routjh measurement of the power of the battery can be obtained by lindmg the length of standard iridio-platirmm wire, which it can just fuse. The resistance through which the wire is fused, or in other words the resistance through which the battery is capable of firing the fuzes can then be found from the following^equations :• Rx=p + 2.g6 I; for the 0.003" wire, and Rx=f> + 10.4 I; for the 0.0014" wire where / is the length of wire fused, in inches. 246. A fair idea of the condition of each cell of a firing battery can be obtained, by connecting a wire to one pole and then applying it to the other pole of each cell in succession, commencing with the cell to which the wire is attached. The spark should gradu- ally increase in brightness, and any sudden diminution in passing from one cell to the next indicates that the last cell touched is defective. CONTINUITY TEST EOR AN INSULATED ELECTRICAL CABLE. 247. An electrical conductor can have no solution of continuity, if a current can be passed through it. A test for continuity is, therefore, to connect in simple circuit the cable, a test cell, and a galvanometer. If no deflection be obtained there must be at least one solution of continuity. PRICKER TEST TO LOCATE A SOLUTION OF CONTINUITY. 248. If the above test indicates a solution of continuity, the fault, or faults, can be located in the following manner : One pole of a test-cell is connected to onaend of the cable, and the other pole is connected to "earth." The cable is then punctur- ed at intervals with a strong needle attached to an earth-plate, the needle touching each time the core of the cable. So long as there is no fault in the cable between the needle and the test-cell, the galvanometer will be deflected, but it will cease to deflect directly the fault is passed. The punctured places should be repaired with indiarubber tape and solution. EXAMPLES. 1. The resistance of an electrical cable, having a copper core, is 2.56 ohms at 60° Fahr. What will be its resistance at 0° Fahr ? 2. The resistance of an iron wire at 5" C is found to be 2.654 ohms. The resistance coils, with which the measurement was taken, are made of German silver, and are true at 15.2" 0. Find .he resistance of the wire at 100' C. i I 1 f 144 ELECTRICAL MEASUREMENTS. 111 i 3- An iron wire, of which the resistance is to be measured, is stretched in a room whose temperature is 40^' Fahr The wir^ IS connected to the measurin^^ apparatus, which is in another room whose temperature is 50° Fahr, through 500 yards of in- sulated copper cable exposed to a temperature of - 10^ Fahr Ihe measured resistance of the cable and wire together is ^^ 27 ohms, and that of the cable is 2.72 ohms, at 60° Fahr. Find the resistance of the iron wire at 60'^ Fahr. 4. A resistance has to be accurately measured by means of Wheatstone's bridge. The ratio -^%i>^ is employed, and it is tound that, when 319 ohms are unplugged in the box, the galvano- meter deflects 21 divisions to the right, and that when 320 ohms are unplugged, the deflection is 5 divisions to the left. " What is the true resistance ? 5. The resistances r^ and r,j of a defective Wheatstone's bridL^e are marked 10 and 100, but in reality they are 9.8 and 100% ohms. A resistance is measured, and it is found to be 106 ohms What error, in ohms, is caused by the inaccuracy in the bridge ? 6. Investigate the effect on the measurement of resistance by Wheatstone's bridge, when the unknown resistance contains a ieeble source of E. M. F. 7. vShow that no definite result can be obtained with Wheat- stone's bridge method if y, and r^ are both zero. 8. Show that with Wheatstone's bridge, more accurate "results can be expected when r^ and r^ are each 1000 ohms than when they are each 10 ohms, if r^ lies between 1,000 and 10 000 ohms. 9. Can the liquid resistance of a cell be found, by measuring the deflection given by one cell through a known resistance, and the deflection given by two similar cells through another known resistance ? 10. A cell, a box of resistance coils and a galvanometer are connected in simple circuit. A second circuit is also connected to the cell, and the resistance in this circuit is made equal to that of the galvanometer and that unplugged in the box of coils together. The deflection of the galvanometer is noted. The second circuit is then removed, and the resistance in the box of coils IS altered until the same deflection is obtained. Show that the resistance added is equal to the liquid resistance of the cell. 11. The difference of potential between two points in a circuit IS measured as explained in § 227, and it is found that r/,=ioo ohms, /6= 1,100 ohms, r--ioo ohms, 7^=940 ohms ; also "-=1000 ohms, //^lo ohms, F = i.oy volt. What error, in volts, is caused by not making the correction given in '^ 228 ? EXAMPLES. 145 12. The E. M. F. of a cell is to be measured by Law's con- denser method. A Grove's cell is used as a standard, and the corrected deflection is igo divisions. The corrected deflection with the experimental cell is 100 divisions. What shunt should be applied to the galvanometer, whose resistance '- 7.100 ohms, in order that the deflections may be more nearly equal ? 13. It is found by trial that a certain battery can ju^,l fu/e 3.8 inches of the standard 0.0014" iridio-platinum wire. What is the E.M.F. of the battery, given that its liquid resistance is 0.8 ohm. Through what length of Service cable (7-strand) could the battery fire one No. 14 fuse ? 14. What diminution, in webers, does the introduction of a No. 14 fuze cause in the current flowing through the 2-ohm coil of a 3-coil galvanometer, produced by a Leclanche test-cell whose liquid resistance is 15 ohms ? 15. The resistance between two points in a circuit, through which a powerful current is flowing is 0.5 ohm. A box of coils and a reflecting galvanometer, whose resistance is 5,000 ohms, are connected in simple circuit to these points. The galvanometer has a shunt of -^^^r ohms applied, and a resistance of 8,000 ohms IS unplugged in the box of coils. The deflection of the galvano- meter is then 100 divisions. Find the strength of the current, given that the deflection of the galvanometer, caused by a standard cell whose E.M.F. is 1.07 volt, is 200 divisions, when a shunt of h^ is applied, and the resistance in circuit is 4280 ohms. 16. A circuit is formed of No. 20 B.W.G. copper wire whose conductivity is 88.6 p.c. that of pure copper wire. The difference of potential between two points, 50 yards apart, is found to be 2.83 volts. Find the strength of the current. 17. A flne platinum wire whose resistance is 2 ohms at 5"6'is placed in a glass vessel containing 100 c.c. of distilled water. A thermometer is introduced and a certain current is passed through the wire for 5 minutes and it is found that the temperature of the water is raised from 12" to 18'^ C. Find the strength of the current. I m m m 1 i i' m 3 APPENDIX. Sine and Tangent Galvanometers. As was stated in § 148, when an electrical current passes through a galvanometer the needle is acted upon by two couples one caused by the electrical current, the other due to the magnetic attraction of the Earth, and of any magnet sufficiently near to have a sensible influence on the needle. These two attrac- tions together produce the magnetic field* in which the needle is lying, before the current passes, and the lines of force in this magnetic field are parallel to the initial position of the needle if the directing magnet be not too close. Hence, if M be the inten- sity of magnetism of each pole of the needle, H the stren'^th of the field, / the length of the needle, and o the angle of defle'ction HMl sin o is the moment of the couple due to the magnetic field'. Each force of the couple produced by the current is bCM where 6 is a constant depending on the construction of the gal- vanometer, and Its direction is perpendicular to the coils Therefore, in a sine galvanometer, the moment of the couple due to the current will clearly be bCM.l, and in a tangent galvano- meter, bCM. I cos 0. Henne in a sine galvanometer HMl sin fi=bCMl or C=n sin ^ And in a tangent galvanometer HM I sin d=bCMl cos ^ or C=//. tan o To show thai the ivorlc done by an instantaneous current on the needle of a galvanometer varies as Q'-. The current is nearly instantaneous, it will therefore have entirely ceased before the needle commences to move sensibly Hence the deflecting couple may be taken as acting impulsively" and at right angles to the needle. Now tin; stnMiglh of the current \Sfe§«Si(, (Jaiiot's l'liy,Hi,'.s. illli Ivliti.m. APPKN'DIX. M7 will not f^eneniUy be coiisUiiit, for iustiincc, the instantaneous current due to the fillinf; up of a condenser (hminishes ui strength as the potential of the condenser increases. Let, therefore, C be the current at any time /, then the moment of the deflecting couple is bCMl, where /; is a constant depending on the galvano- meter, M the magnetic intensity of each pole of the needle and / the distance between the poles. Hence the efjuation of motion is mk\dio = bCMl. dt But C can be considered constant for the short interval of time dt, so that CM is the quantity of electricity, dQ, transmitted during the interval dt. Hence mk^.do)=bMl. dQ Integrating and remembering that Q=o when co—o, and writting iJ for o> But the work done on the needle is '-—, hence 2 Q^cc work done on needle. d To show that sin^ - is a measure of the work done by an instan- taneous current on the needle of a galvanometer. The effect of an instantaneous current is, as seen above, to impre?:, an initial angular velocity on the needle. During the rnotion, the forces acting on the needle are : the magnetic attrac- tion of the Earth, (forming a couple,) the resistance of the air and the friction of the pivot. The last two forces will, however, be neglected and their effect allowed for by a subsequent correction.* Let io be the angular velocity at any time t, and 6 the angle described from the position of equilibrium ; the remaining letters having the same signification as in the previous paragraph. Then the equation of motion, obtained by taking moments about the pivot, is d^d mk^ ,1=^ -HM/sin 6 at'' Multiplying both sides of this equation by 2-T,>and integrating mk- dd -,: - =2HMl cos f/-f constant. (dt ) But, ~z=o, when d=d, the angle of deflection. Hence *See § 180. '"* [dt) =2HM/(cos ^-cos <5) LfS Fa.KCTKrr'AI. mkasitricmknts. clft N"^v ^if =^l wlioii 0=„, so that rnk'^ili = //iU/(i-cos^?) =2//A//sin2 f? Hut HMl is a constant so long as he same galvanometer is used, and no alteration is made in its position, or otherwise' o Therefore, sin-' ^ is a measure of the work done on the needle by the instantaneous current. _ To find the time of oscillation of a magnetic needle placed under the influence of a uniform magnetic field. ^^mur me The effect of the magnetic held on the needle, is to apply a force to each pole, parallel to the position of equilibrium. These wo forces form a couple, and if 6 be the angle of deflection of the needle at any time /, measured from the position of enuili- bimm, and / the distance between the poles, F/sin^ or Fl ft since II IS a small angle, is the moment of this couple.' Hence the equation of motion is ^ nt^uct mk^ j-^= -Fid , dt'^ Integrating and remembering that ^^^~ =o, when d=fl. ink^\^^V=Fl{,r'~d'^) Whence it will be found that the time of an oscillation is 2t = 7t Fl or, writing t for the time of an oscillation, where / is a constant. /3— X TABLE IT. THKEK-COIL GALVANOMF.TER. ^ Tabic giom^ the ratio of the currents producing the deflections .? and , the looo-ohm cod being in circuit. Examples. — Let dz Let d- o =bi ; then a =0.64. RESISTANCE. Variation in resistance due to alteration in temperature. The following formula is given by Dr. Mathiessen, in which n is the resistance of the metal, or alloy, at che temperature f C and r° its resistance at o°C. rf=r^{i+at±bt"') The following are the values of a and h : a Most pure metals 0.003824 Mercury 0.0007485 German silver 0.0004433 Platinum silver 0.00031 Gold-silver 0.0006999 -t- 0.00000126 —0.000000398 + 0.000000152 —0.000000062 "Tliis Table gives the mean of a seiiea of experiineats, made by the authoi-. T«= -™-, and o is the deflection due to the current C. 15" KI.I'CTKrcAt. \fKASri«. a f> .2 a s a 8 I* 4) a Olmis. o.oigj; 0.02103 0.02057 0.02104 0.02650 0.02697 0.07244 0.1166 0.1251 0,1701 0.2526 1.2247 0.3140 0.2695 I-I399 0.1544 0.1680 0.1440 0.1469 0.4080 0.4150 0.4067 1.96 0.7654 0.9738 2.257 13.06 2.959 1.85 1.668 9.151 9.9.36 9.718 9.940 12.52 12.74 34.22 55.09 59.10 80.36 1 19. 39 578.6 148.35 127.32 66.10 0.2214 0.2415 0.2064 0.2106 0.5849 0.5950 0.5831 2.810 1.097 1.396 3.236 18.72 0-377 0.388 0.365 0.365 0.365 0.387 0.072 4.243 0.031 2.652 2.391 0.044 0.065 This table gives the resistances of chemically pure metals; the resistances of commercial rnetala are always higher. It has been extracted from a table in "Elec- tricity and Magnetism," by Prof. Fleeming Jenkin, F.R.S.S., etc. The variation in res.at.-.r..ne d»o to temperature are fr=.iu -Rough Notes of a Course of Lectures on apparatus used m Military Telegraphy and Firing Mines." S.M.E., Chatham uli', from .— ... ;:r:"""" • ' ■ i|5J,. ■ijg proximate ntage of vi resistance temp, at 2 i » ^s.s-s C4 0-377 t5 >4 0.388 >6 9 0.365 I 0.J65 0.365 0.387 1 0.072 3 0.031 0.044 0.065 iistances of in "Elec- ariation in Bctures on bam. TAUIJ.: n. ELECTKO-MOTIVE FORCE OF VARIOUS CELLS. 151 Daniell Grovo Bunsen Callan Poggendorf Marie Davy Leclanche Zinc'amalg Sulphuric acid, 7» tol " 22 to 1 Sulphate of zinc 1 part common salt, 4 parts water; Sulphuric acid, 7i tol " Salt water Saturated solution Copper of copper sulphate Nitrate ./ copper saturated Sulphate of copper I Nitric acid (fum- Platinum Nitric acid, sp. gr. „ 1-33 l)e la Rue iZinc Sulphuric acid, 22 to I Sulphate of zinc Dilute sulphuric , Nitric acid Carbon acid " Cast iron Chrome mixture Carbon Sulphuric acid, 22, 1 Paste of aulphatej *° * ii "^ mercury Dilute sulphuric'! „ acid ! I jSolution of sal' Binoxide of man ammoniac , , ganese Volt.. 107» 0078 1000 om) 1060 IflSfJ I {MM 1.810 1-672 1-7.34 1-700 J l-70fi I 2.028 1-524 1-33 1-481 Chloride of silver Silver I Becquerel iZinc amalg Sulphate of zinc Sulphate .,f lead Load Niaudet Duchesnin 1 Plante Latimer Clark, Standard cell 1 -05!) O-.'ir) 1 -()5 I 1-541 Platinum ^Dilute sulphuric Dilute suIphuric'Platinum ' IT!) acid jKjjd jCommonSalt 'Chloi-ide of lime Curlu.u Perchlorideof iron Lead Lead Lead 2.5 Zinc amalg Sulphate of zinc I'aste of sulpliato Mercury 1 •4r.7 of iiieicury I J'orous Cell. '*Thi« table is duo to M. .Niau.l.t. aii.! is .riv.M in •• \ IM,v«;,...i 'i\ ♦■ ■.• tnc.tya«dMug..etism,"by.L K. H. l^.-doM/KA , Camb. ^ ""' ' '•""^'«'-- "" '•'''^- ' 1 Hi f 152 ELECTRICAL MEASUREMENTS. CURRENT. wi^sls-''^"* necessary to fmc the standard iridio-platinum For the 0.003" wh'e ...1.65 weber. " 0.0014" wire 0.8 " Heat produced by a current. Number of units of heat* produced) by a current C in time t, in the portion of the circuit whose re-f -0-2405 C^rt- sistance is r. ' liA H ™fl *Thi8 unit ofheat belongs to tl.e C. {\. .S. systeni, and is the .,uautitv of beat INDEX. Amalgamating zinc plates Arches, demolishing with explosives Arrangement of circuit Astatic galvanometer . . .... M system .... .... B Bags for primers, dimensions of , . Balistic galvanometer . . .... Barrels for ammunition, dimensions of Battery power, calculation of \\\ Batteries, voltaic, tiring with \\\\ " II precautions when using " '' testing .... .... Blasting rock .... Bickford's fuze .... .... " " connecting to instantaneous leader •I II preparation of exposed and II II rate of burning . . .... I" II testing .... .... M instantaneous fuze .... .... " " II making a joint in '.' " " testing .... .... Binding screws .... Bridges, demolishing iron and wooden bridges with Britannia joint .... guncotton or dynamite § page 82 67 . !)7, 98 171 . 149 . . page 72 .... 181 . . page 66 120, 121 106 63 .... 119 . . page 63 71 .... 74, 75 71 71, page 79 .... 114 75 75 .... 114 154 56 100 (Jables, electrical, Service .... Capacity, derinition of .... . . . [ II measureniont of .... .... II unit of .... Cells, voltaic Circuit, II .... .... II different arrangements of .... II testing .... II wires II II laying . . .... .... " " placing in galleries .... .... " II jirecautions .... .... Charges, precautions when making up II preparation of .... .... II weight of, for variouH jiurposes .... ( ■obimns, cutting down with guncotton or dynamite Coml)ined contmuous and divided circuit <"otMp;iris()n of <;rove".s and LecliiiicLu's lliing cells II gunpowder, guncotton and dynamite • II wire and high tension Fh/,oh .... i»3, page 79 136 243 143 81-84, 86, 158-163 92 97, 98 125. 126 . . . 93, 94, 99-101 103 52,103 . . page 63 . . page ()1 . . page 22 page 72 57 97 85 22 44 ! ni i ' I ! i , 154 INDEX. Condensation of electricity .... Condenser ' . '/'[ '" Conductors, definition of .... .... \ \] Contact keys . . \[\[ " " " " " " Continuity of a conductor, test for .'...' Continuous circuit .... . . ' ' " ' ' " ' ' ' ' ' " Copper plates, cleaning .... " ' ' ' It wire, resistance of .... _ | " Current, electrical, definition of " II measurement of .... ' * . ' II by the deflection of a galvanometer " II method of vibrations II powerful currents " to fire an electrical fuze » to fuse a fine wire .... " " II unit of .... / [ r, ... " ., " " and work, relation between . . C^uttmgs, railway or road, demolishing with gunpowder Demolition of arches . , II II bridges 105, It II II II 11 II II II cuttings, railway or road II II embanKments .... II II houses .... II II palisades II II piers (stone or brick) II 11 trees .... .... II 11 walls .... Demolitions, hasty Detectors .... \ Detonation, definition of 11 examples of .... . . II rate at which it travels .... 11 sensitiveness of explosives to " of wet guncotton Detonator for dynamite .... ..." 11 guncotton Detonator, makeshift for dynamite " " " guncotton .... Dielectric, definition of .... DiflFereuce of potential, definition of .. 151 191, 192 129 15G, 157 . . 247 97 page 82 page 79 .. 135 236-242 . 238 . 240 . 239 . 242 241 1.39 152 53 67 56 31, .33, ,35, .37 56 63 53 62 59 66 86 1«9-171 3 9 6 7 14 31, 34, 40, page 70 41, 42, page 70-71 40 41 .... 131 133 . , " " ""it of .... DiiJ'erential galvanometer . . " " Latimer Clark's Divided circuit .... " " theory of .... " fu^.es .... .... Drying wet guncotton \ \ Dynamite .... .... " applicatidii . . .... 11 cap, ilimeii.sidiis of ... . " " testing . . .... M riiarges, preoantioHN whm iiifiking up II dctonator.s .... .... I " maki'sliitt electriciil. . « fro/,en,treatm('ntof . . . . II preeautidns when usiiij,' priuK'i', inucautlons vvlicii niiikiiimip. •I 11 pit'par;iti()ii of .... mea,surement of. ))y Law's condenser method .' . ." 226 by the defiection of a galvanometer 227, 228 14J 182 . . . . 183 97 . . . . 147 45 15 18 20 . . {la^e 70 113 . j)age t 2 31, .34, 40 40 19 . page 60 • page 62 50 M ..: INDl'X. 155 .... 151 . . 191, 192 129 105, 15G, 157 .... 247 97 . . page 82 . . page 79 .... 135 .. 236-242 .... 238 .... 240 .... 239 .... 242 .... 241 1.S9 152 53 .... 67 56 57 56 63 53 62 59 66 86 .. 109-171 3 9 6 7 14 40, page 70 page 70-71 40 41 .... 131 133 226 3r 227, 228 .... 14J 182 . . . . 183 97 . . . . 147 45 irt 18 20 . paj,'t) 70 113 . l)age t 2 31, 34, 40 40 J9 . iiagc. 60 page 62 iiO Dynaniito, sizes and weights of uartridgcs 11 shape of cartridges .... M testing .... .... Dynamo, tension .... .... M quantity Dualine .... .... .... . . page 66 18 112 89, 90, 107, 123, page 82 87, 88, 107, 122, page 82 • • • • • ■ a « 10 E Earth plates . . 11 measuring resistance of . . Electrical fuzes .... " II color of .... II II connecting to circuit wires 'I 11 current to tire and to fuse (low tension fuzes) II II electrical resistance of . II II insulating head of II II precautions when using . Electro-motive force, definition of II II measurement of . . . by the deflection of a galvanometer . . by fusion of standard wire .... II 11 by Law's condenser method . . II II by Sir Charles Wheatstone's method using 3-coil galvanometer II M II II II II II II II II unit of Embankments, demolishing with gunpowder Explosion, definition of .... .... II rate of .... .... .... Farad Figure of merit of a galvanometer Firing battery. Grove II II Leclanche II II makeshift .... PI II testing . . I- key M by means of electricity . . II II M M precautions II slow or instantaneous leade II a tension dynamo .... II a quantity dynamo . . II a voltaic battery .... II resistance coils . . .... Field Service jointing and testing box Fluxes for soldering .... .... Fougasse, firing .... II placing charge ... Fulminate, mercuric .... .... Fusing current of fine wire, measurement of Fuze, Bickford's .... .... .... II definition of .... .... .... Fuzes, dimensions of ... . ... "divided" M electrical .... .... .... II II wire .... .... .... II II high tension .... .... 1 1 for use with dynamite .... .... M II I guncotton . . . . .... II I' II gunpowder . , .... II II Ti slow or instantaneous leader M precautions when using . . II preparing for use 95 ■?09, 214 . 36-44 v.ige 71 .. 102 page 71 page 71 . . 102 page 60 .. 134 230-235 .. 232 .. 235 .. 231 .. 233 . . 2.34 141 53 2 3 143 188 81, 82 83--85 86 244-246 105 104-103 page 66 77 107 106 107 169 107 page 83 54 54 21 241 71 30 page 70 45 36-44 37-41 42 31, 34, 40 31, 33,35,37, 41, 42 . . 31, 32, 37, 39, 42 32 35 .... . . page 60 45 i ' ill ! l! 1=56 ixniix. G II II II n II II II (ilates, blowing in (drove's cell, military pattern, data r, , " " " making up (iralvanometers II astatic . . .... II balistic . . .... . ' * II differential " " Latimer Clark's pattern II effect of an instantaneous current on 11 figure of merit II limits of dedection to obtain good readings measuring resistance of precautions to be taken to prevent iniurv to" sme _ ■' ^ shunts . . .... [[ ] ' ' ' ' ■ ■ ■ ■ II tangent . . .... _ _ ' ' ' ' ' ' ' ' ' II Thompson's reffecting " ., " ., " setting up tlie instriiment « three-coil Glue, marine . . ... " ' Guncotton .... .... [\ \ ' ' ' ' ' " II application . , .... charges, precautions when making up compressed .... .... ' ' ' ' detonators . . . _ ' [ ' " ' ' ' ' 11 dimensions of .... II electrical .... IP testing . , dimenP'ons and weights of various sizes dry, storage of drying of wet guncotton .... ,, granulated magazines . . .... . _ _ ' ' " makeshift detonators for . . . . .... packing cases for ' " ' ' " " precautions when using primer preparation of ' " " '• " indiarubber bags for " ." Precautions when making up II price of _ ^ ' ' " * II storing .... ^ [ ^ " " II test to ascertain dryness ,[[[ II testing _ * II time required to dry I' various shapes 11 wet, detonation of . . (iunpowder application ' charges, precautions wlien making up II bags for .... fui-es ■ ■■ ■■ magazines . . , . . , " ' makeshift fuze .... .... " " precautions when using ]] \ preparation of primer price of ... . .... . _ . ' ' ' ' primers, precautions when making up size of grain .... ' ' ' ' M suited to Military Engineering Land operations testing volume occupied by II II II II II II II II II II II II Hasty demolitions H (50 . . page H'2 82 168-190 171 181 182, 183 183 . . 179, 180 188 185-187 204 .... 184 172 .. 170-178 .... 173 174-175 17i) .... 170 page 83 13-1() 16 page 62 13 31, 33, 35, 37, 41, 42 pages 70 and 71 page 71 ••■• 113, 116-118 page 65 29 15 13 28 35. 41 28 page 59 48, 49 49 page 6'1 . . page 6(> 15 .... 15(6) Ill • • • • ■ . page 65 ■ • • 13 page (i5 •••• .... 14 12 page 61 52 .31, 32, 37, 39, 42 25-27 39 page 59 47 page 65 . . page 61 page 65 11 . .. 110 page 65 68 iiR W^^S IXDHX. r57 High-tension fuzt-H .... " " tlefectb that may occur in " " nieasurin^ resistance of .. ., << M sources of electricity for use with " " testinif .... Hose, Orel's .... " ' ' ; It powder . . Horsley's blasting powder .... ]][[ Houses, blowing down s 42 US 210 89 118 74 73 18 62 Igniting an explosive India-rubber bags for guncotton primers.'.* II " gunpowder .... Instantaneous Bickford's fuze " " « placing in a gallery . . Instantiwieous current, eft'ect of, on a galvanometer Instantaneous leader. .. . .... 'I M connecting to Bickford's fuze . . " II I'ate of burning .... Insulating head of electrical fuzes .... "I joints in electrical cables .... Insulation fault in a cable, to detect position of. . . ,' I' resistance of cables, rough measurement of Insulator, definition of Iridio-platinum bridge of wire fuzes . . . ' Iron wire, resistance of ,[ 3 49, page 72 52, page 72 75 52 .179, 180 73, 74, 75 .... 74, 75 • . page 79 102 100 .... 213 . . 210, 212 130 37 . . page 79 Jointing Bickford's instantaneous fuze 11 circuit wires . . II iron wires .... .... .... II and testing box, for Field Service Joint, Britannia .... II three-way .... II two-way .... K vey, contact II firing . . . II II reversnig 75 .99-101 . 100 . 109 100 101 . 100 15G 105 157 Land mines, preparation of charge for ... . Latimer Clark's differential galvanometer .... II II standard cell .... .... Leaders, precautions when using ti slow and instantaneous .... Leads, gauge of, for electrical measurements Leolanche firing cell .... .... II 'I II making up . . .... II II H cement for . . .... " II 'I data .... .... 11 test cell .... .... .... Lightning conductors . . .... .... M Magazines, dimensions uf roof .... " II II walls . . .... .... II earthen traverses Ijecween store magazines II rtoora of .... .... 52 183 ,. 161 page 62 ,. 70-75 . 153 83 84 page 32 page 81 1()3 page (i8 page 67 page 67 page 69 page 68 I5« INDICX. 1 * P 3 ! , MagazinuN, guncottou . . " » internal dinionHidns of " " root's of ... . .... " " walla of . . " gunjiowuer. . '_/ "' '"'' " " dimensions of passages in . . " " exposed to Artillery fire . * . . ,' \][ " » "ot exposed to Artillery fire " II racks ., „ '■* ••*• •••• ...• . II II site of ... . .... I- tliickness of protection of roof against artillery fire. " " " " 11 walls „ „ " II ventilation of Magnetic needle, eflect of an electrical current on Magneto electric machines for firing charges . Makeshift detonator for guncottoa (for use with" Bickford's'fuze) « tiring battery II insulated wire .... [\ \ M wire fuze for gunpowder ,,,[ '/'[ " " detonator for dynamite . . . - ." " " M guncotton Marine glue " " ' ' ' ' Mercuric fulminate .... . ' ' " " " ' * ' ' " " Metal-lined wooden «jiu,es for ammunition,' dimensions of Military Grove cell II <' II data .... .... ." " " 'Tiakirt;; up . . .... Mines, land, preparation ot ci:arge for . . . .* *..',' I Jf Nitro-glycerine " 11 advantages and defects Non- conductor, definition of Ohm . . ■ Ohm's law II II Ord's hose graphic representation of testing .... Palisades, cutting through by means of guncotton . Piers, demolition of with explosives .... Polarization ot voltaic cells, definition of . . . . . Potential, definition of . . .... Powder hose . . .... • . , . II II placing in a gallery Precautions when preparing charges .... " II II firing arrangements . II II II fuzes .... . . . II II using dynamite " II II guncotton .... II II II gunpowder .... ... Pricker test Primer, definition of ' ' . ' ' II preparation of dynamite . , .... II II II dry guncotton II II II gunpowder , , . . . , " II II to detonate wet guncotton •28 . page ivge HQ . page 70 ■ page 70 ... 25-27 ■ page 67 26 27 page 67 24 page 67 page 67 page 68 .. 148 .. 87-89 35 86 94 39 40 41 page 83 21 page 66 81 page 81 82 17 17 129 142 145 146 74 114 59 m .. 137 132 73 52 page 61 page 62 page 60 page 60 page 59 page 59 . . 248 46 50 48 47 49 II << Quantity dynamo .... .... II II data . , .... II firing.. " method of using " testing .... Quantity of electricity, unit of . . Quarrying; " Gibraltar method .... Quick mfitch . . .... .... 159 87 page 81 107 88 122 140 (>4 65 72 R t( K << « (< • ( <( (< l( II <( Racks, in magazines « page 67 Hails, cutting with guncotton or dynamite .... .... .... .... 55 Recipes, sundry page 82 if esistance coils .... .... .... .... ..,. 1(54 " box of ".■.'.■ 165 " Firing 167 " Post OflBce pattern .... .... .... .... iqq connecting unknown resistance to measuring apparatus . . .... 196 electrical, definition of .... .... .... .... .... ]28 of a conductor, measurement of, by fusion of fine wire . . .... 214 Latimer Clark's differential gal- vanometer .... " " " " " tangent galvanometer " " Wheatstone's bridge method 19^ " with 3-coi] galvanometer " galvanometer, menurrment of .... .... .... of copper and iron wire .... . .... .... " fine wire at the point of fusion, measurement of liquid, measurement of, by deilection of a galvanometer . . .... '* " " " fusion of fine wire .... " " " " Kempe's method " " " " LatimerClark's differential galvanometer Mance's Wheatsone's bridge method .. 203 207 -202 " very high, measuring " " low, rough test of *' unit of .... Reversing key .... Rock, blasting .... .... Selection of arrangement of circuit Service cables . . .... .... " '• resistance of Simple circuit . . Sine galvancjmeter .... Shunts, for galvanometers Solder for circuit wires . . Soldering .... .... .... " fiuxes for .... .... Sources of electricity for use with high tension " " " wirefu/es Standard cells, Latimer Clark's . . " Modified Daniell's cell .... Stockades, cutting an opening in, by means of ( Stone piers, demolishing with explosives. . Storage of guncotton .... .... .... " " gunpowder . . .... Tiimpiiig, effect of .... .... .... " amount roiiuire7 pagos"'-? 79 > ! 1 ! I Go INDKX. Tangent galvanometer Tension dynamo * •lata " " Hring with " " method of usinif " testing.. . Test cells ? ;'" " " requirements of .... '*.".' Testing circuit wires .... .... * ' _ * * " " ' ' ' ' ' " " " complete circuit .... " . ". "'** "" " continuity of a conductor .... " dynamite '* •' high tension fuzes .... .[[[ " guncotton dryness of " gunpowdei " quantity dynamo .... .'..'. [[[[ " tension dynamo .... . . ' ' " " " ' " voltaic batteries " wire fuzes '[' Thermo-galvanometer .'.',,' Thompson's reflecting galvanometer .'.'.'.' Three-coil galvanometer Trees, cutting down with guncotton ordynamite* '.'. '.'..'. " " u Units, electrical tt". ,^ " , ^nomenclature of the Paris Congress of 1881 Uprights or columns, cutting down by means of guncotton or dynamite ... 173 8<> . page 8'2 . . . 107 ... 1'2H . 1.58-163 . . 125 126 . . 247 .. 112 .. 118 .. HI .. 15 ((5) .. 110 .. 122 . . 123 llJt .. 117 189, 190 174-175 .. 170 57 1.S8-144 A .... 144 A 57 Volt w Walls, demolishing with explosives Weber .... .... ' ' "" • • • • Wheatstone's bridge attached to Firing resistance "coils wt;,^ „„•, J ' • " . X " . ^"^* Office pattern coils v\ ire, coiled, measuring resistance of ■ ' fuzes .... .... " " ' ■ ■ ■ ■ ■ ■ • " defects that may occur in .... " makeshift ''" " sources of electricity for use with .... " testing .... , , _ " * " ■ ■ ■ ■ ■ ■ Wires, making electrical joint in .... It 141 61 1.39, 140 . . 199 .. 198 . . 202 .. .3«-41 117 .. 38-41 79 . 117 .99-101 ... 17.3 85) . page 82 . . . 107 JK) ... 123 . 1.^8-163 ... \r,d ... 120 . . 126 .. 247 .. 112 .. 118 .. Ill .. 15(6) .. 110 .. 122 .. 123 119 .. 117 189, 190 174-175 .. 170 67 ite 1.38-144 A .... 144 A 57 141 1.39, 61 140 199 198 202 .3«-41 117 .38-41 79 117 99-101