Experimental Researches in Electricity Part 42

Pos. Neg.

In Air the range was 0.19 0.09 Oxygen 0.19 0.02 Nitrogen 0.18 0.11 Hydrogen 0.14 0.05 Carbonic acid 0.16 0.02 Olefiant gas 0.22 0.08 Coal gas 0.24 0.12 Muriatic acid 0.43 0.08

I have no doubt these numbers require considerable correction, but the general result is striking, and the differences in several cases very great.

1394. Though, in consequence of the variation of the striking distance (1386.), the interval in air fails to be a measure, as yet, of the insulating or resisting power of the gas in the vessel, yet we may for present purposes take the mean interval as representing in some degree that power. On examining these mean intervals as they are given in the third column (1388.), it will be very evident, that gases, when employed as dielectrics, have peculiar electrical relations to insulation, and therefore to induction, very distinct from such as might be supposed to depend upon their mere physical qualities of specific gravity or pressure.

1395. First, it is clear that at the _same pressure_ they are not alike, the difference being as great as 37 and 110. When the small b.a.l.l.s are charged positively, and with the same surfaces and the same pressure, muriatic acid gas has three times the insulating or restraining power (1362.) of hydrogen gas, and nearly twice that of oxygen, nitrogen, or air.

1396. Yet it is evident that the difference is not due to specific gravity, for though hydrogen is the lowest, and therefore lower than oxygen, oxygen is much beneath nitrogen, or olefiant gas; and carbonic acid gas, though considerably heavier than olefiant gas or muriatic acid gas, is lower than either. Oxygen as a heavy, and olefiant as a light gas, are in strong contrast with each other; and if we may reason of olefiant gas from Harris's results with air (1365.), then it might be rarefied to two-thirds its usual density, or to a specific gravity of 9.3 (hydrogen being 1), and having neither the same density nor pressure as oxygen, would have equal insulating powers with it, or equal tendency to resist discharge.

1397. Experiments have already been described (1291. 1292.) which show that the gases are sensibly alike in their inductive capacity. This result is not in contradiction with the existence of great differences in their restraining power. The same point has been observed already in regard to dense and rare air (1375.).

1398. Hence arises a new argument proving that it cannot be mere pressure of the atmosphere which prevents or governs discharge (1377. 1378.), but a specific electric quality or relation of the gaseous medium. Hence also additional argument for the theory of molecular inductive action.

1399. Other specific differences amongst the gases may be drawn from the preceding series of experiments, rough and hasty as they are. Thus the positive and negative series of mean intervals do not give the same differences. It has been already noticed that the negative numbers are lower than the positive (1393.), but, besides that, the _order_ of the positive and negative results is not the same. Thus, on comparing the mean numbers (which represent for the present insulating tension,) it appears that in air, hydrogen, carbonic acid, olefiant gas and muriatic acid, the tension rose higher when the smaller ball was made positive than when rendered negative, whilst in oxygen, nitrogen, and coal gas, the reverse was the case. Now though the numbers cannot be trusted as exact, and though air, oxygen, and nitrogen should probably be on the same side, yet some of the results, as, for instance, those with muriatic acid, fully show a peculiar relation and difference amongst gases in this respect. This was further proved by making the interval in air 0.8 of an inch whilst muriatic acid gas was in the vessel _a_; for on charging the small b.a.l.l.s _s_ and S positively, _all_ the discharge took place through the _air_; but on charging them negatively, _all_ the discharge took place through the _muriatic acid gas_.

1400. So also, when the conductor _n_ was connected _only_ with the muriatic acid gas apparatus, it was found that the discharge was more facile when the small ball _s_ was negative than when positive; for in the latter case, much of the electricity pa.s.sed off as brush discharge through the air from the connecting wire _p_ but in the former case, it all seemed to go through the muriatic acid.

1401. The consideration, however, of positive and negative discharge across air and other gases will be resumed in the further part of this, or in the next paper (1465. 1525.).

1402. Here for the present I must leave this part of the subject, which had for its object only to observe how far gases agreed or differed as to their power of retaining a charge on bodies acting by induction through them. All the results conspire to show that Induction is an action of contiguous molecules (1295. &c.); but besides confirming this, the first principle placed for proof in the present inquiry, they greatly a.s.sist in developing the specific properties of each gaseous dielectric, at the same time showing that further and extensive experimental investigation is necessary, and holding out the promise of new discovery as the reward of the labour required.

1403. When we pa.s.s from the consideration of dielectrics like the gases to that of bodies having the liquid and solid condition, then our reasonings in the present state of the subject a.s.sume much more of the character of mere supposition. Still I do not perceive anything adverse to the theory, in the phenomena which such bodies present. If we take three insulating dielectrics, as air, oil of turpentine, and sh.e.l.l-lac, and use the same b.a.l.l.s or conductors at the same intervals in these three substances, increasing the intensity of the induction until discharge take place, we shall find that it must be raised much higher in the fluid than for the gas, and higher still in the solid than for the fluid. Nor is this inconsistent with the theory; for with the liquid, though its molecules are free to move almost as easily as those of the gas, there are many more particles introduced into the given interval; and such is also the case when the solid body is employed. Besides that with the solid, the cohesive force of the body used will produce some effect; for though the production of the polarized states in the particle of a solid may not be obstructed, but, on the contrary, may in some cases be even favoured (1164. 1344.) by its solidity or other circ.u.mstances, yet solidity may well exert an influence on the point of final subversion, (just as it prevents discharge in an electrolyte,) and so enable inductive intensity to rise to a much higher degree.

1404. In the cases of solids and liquids too, bodies may, and most probably do, possess specific differences as to their ability of a.s.suming the polarized state, and also as to the extent to which that polarity must rise before discharge occurs. An a.n.a.logous difference exists in the specific inductive capacities already pointed out in a few substances (1278.) in the last paper. Such a difference might even account for the various degrees of insulating and conducting power possessed by different bodies, and, if it should be found to exist, would add further strength to the argument in favour of the molecular theory of inductive action.

1405. Having considered these various cases of sustained insulation in non-conducting dielectrics up to the highest point which they can attain, we find that they terminate at last in _disruptive discharge_; the peculiar condition of the molecules of the dielectric which was necessary to the continuous induction, being equally essential to the occurrence of that effect which closes all the phenomena. This discharge is not only in its appearance and condition different to the former modes by which the lowering of the powers was effected (1320. 1343.), but, whilst really the same in principle, varies much from itself in certain characters, and thus presents us with the forms of _spark_, _brush_, and _glow_ (1359.). I will first consider _the spark_, limiting it for the present to the case of discharge between two oppositely electrified conducting surfaces.

_The electric spark or flash._

1406. The _spark_ is consequent upon a discharge or lowering of the polarized inductive state of many dielectric particles, by a particular action of a few of the particles occupying a very small and limited s.p.a.ce; all the previously polarized particles returning to their first or normal condition in the inverse order in which they left it, and uniting their powers meanwhile to produce, or rather to continue, (1417.--1436.) the discharge effect in the place where the subversion of force first occurred.

My impression is, that the few particles situated where discharge occurs are not merely pushed apart, but a.s.sume a peculiar state, a highly exulted condition for the time, i.e. have thrown upon them all the surrounding forces in succession, and rising up to a proportionate intensity of condition, perhaps equal to that of chemically combining atoms, discharge the powers, possibly in the same manner as they do theirs, by some operation at present unknown to us; and so the end of the whole. The ultimate effect is exactly as if a metallic wire had been put into the place of the discharging particles; and it does not seem impossible that the principles of action in both cases, may, hereafter, prove to be the same.

1407. The _path of the spark_, or of the discharge, depends on the degree of tension acquired by the particles in the line of discharge, circ.u.mstances, which in every common case are very evident and by the theory easy to understand, rendering it higher in them than in their neighbours, and, by exalting them first to the requisite condition, causing them to determine the course of the discharge. Hence the selection of the path, and the solution of the wonder which Harris has so well described[A]

as existing under the old theory. All is prepared amongst the molecules beforehand, by the prior induction, for the path either of the electric spark or of lightning itself.

[A] Nautical Magazine, 1834, p 229.

1408. The same difficulty is expressed as a principle by n.o.bili for voltaic electricity, almost in Mr. Harris's words, namely[A], "electricity directs itself towards the point where it can most easily discharge itself," and the results of this as a principle he has well wrought out for the case of voltaic currents. But the _solution_ of the difficulty, or the proximate cause of the effects, is the same; induction brings the particles up to or towards a certain degree of tension (1370.); and by those which first attain it, is the discharge first and most efficiently performed.

[A] Bibliotheque Universelle, 1835, lix. 275.

1409. The _moment_ of discharge is probably determined by that molecule of the dielectric which, from the circ.u.mstances, has its tension most quickly raised up to the maximum intensity. In all cases where the discharge pa.s.ses from conductor to conductor this molecule must be on the surface of one of them; but when it pa.s.ses between a conductor and a nonconductor, it is, perhaps, not always so (1453.). When this particle has acquired its maximum tension, then the whole barrier of resistance is broken down in the line or lines of inductive action originating at it, and disruptive discharge occurs (1370.): and such an inference, drawn as it is from the theory, seems to me in accordance with Mr. Harris's facts and conclusions respecting the resistance of the atmosphere, namely, that it is not really greater at any one discharging distance than another[A].

[A] Philosophical Transactions, 1834, pp. 227, 229.

1410. It seems probable, that the tension of a particle of the same dielectric, as air, which is requisite to produce discharge, is a _constant quant.i.ty_, whatever the shape of the part of the conductor with which it is in contact, whether ball or point; whatever the thickness or depth of dielectric throughout which induction is exerted; perhaps, even, whatever the state, as to rarefaction or condensation of the dielectric; and whatever the nature of the conductor, good or bad, with which the particle is for the moment a.s.sociated. In saying so much, I do not mean to exclude small differences which may be caused by the reaction of neighbouring particles on the deciding particle, and indeed, it is evident that the intensity required in a particle must be related to the condition of those which are contiguous. But if the expectation should be found to approximate to truth, what a generality of character it presents! and, in the definiteness of the power possessed by a particular molecule, may we not hope to find an immediate relation to the force which, being electrical, is equally definite and const.i.tutes chemical affinity?

1411. Theoretically it would seem that, at the moment of discharge by the spark in one line of inductive force, not merely would all the other lines throw their forces into this one (1406.), but the lateral effect, equivalent to a repulsion of these lines (1224. 1297.), would be relieved and, perhaps, followed by a contrary action, amounting to a collapse or attraction of these parts. Having long sought for some transverse force in statical electricity, which should be the equivalent to magnetism or the transverse force of current electricity, and conceiving that it might be connected with the transverse action of the lines of inductive force, already described (1297.), I was desirous, by various experiments, of bringing out the effect of such a force, and making it tell upon the phenomena of electro-magnetism and magneto-electricity[A].

[A] See further investigations of this subject, 1658-1666.

1709-1735.--_Dec. 1838._

1412. Amongst other results, I expected and sought for the mutual affection, or even the lateral coalition of two similar sparks, if they could be obtained simultaneously side by side, and sufficiently near to each other. For this purpose, two similar Leyden jars were supplied with rods of copper projecting from their b.a.l.l.s in a horizontal direction, the rods being about 0.2 of an inch thick, and rounded at the ends. The jars were placed upon a sheet of tinfoil, and so adjusted that their rods, _a_ and _b_, were near together, in the position represented in plan at fig.

116: _c_ and _d_ were two bra.s.s b.a.l.l.s connected by a bra.s.s rod and insulated: _e_ was also a bra.s.s ball connected, by a wire, with the ground and with the tinfoil upon which the Leyden jars were placed. By laying an insulated metal rod across from _a_ to _b_, charging the jars, and removing the rod, both the jars could be brought up to the same intensity of charge (1370.). Then, making the ball _e_ approach the ball _d_, at the moment the spark pa.s.sed there, two sparks pa.s.sed between the rods _n_, _o_, and the ball _c_; and as far as the eye could judge, or the conditions determine, they were simultaneous.

1413. Under these circ.u.mstances two modes of discharge took place; either each end had its own particular spark to the ball, or else one end only was a.s.sociated by a spark with the ball, but was at the same time related to the other end by a spark between the two.

1414. When the ball _c_ was about an inch in diameter, the ends _n_ and _o_, about half an inch from it, and about 0.4 of an inch from each other, the two sparks to the ball could be obtained. When for the purpose of bringing the sparks nearer together, the ends, _n_ and _o_, were brought closer to each other, then, unless very carefully adjusted, only one end had a spark with the ball, the other having a spark to it; and the least variation of position would cause either _n_ or _o_ to be the end which, giving the direct spark to the ball, was also the one through, or by means of which, the other discharged its electricity.

1415. On making the ball _c_ smaller, I found that then it was needful to make the interval between the ends _n_ and _o_ larger in proportion to the distance between them and the ball _c_. On making _c_ larger, I found I could diminish the interval, and so bring the two simultaneous separate sparks closer together, until, at last, the distance between them was not more at the widest part than 0.6 of their whole length.

1416. Numerous sparks were then pa.s.sed and carefully observed. They were very rarely straight, but either curved or bent irregularly. In the average of cases they were, I think, decidedly convex towards each other; perhaps two-thirds presented more or less of this effect, the rest bulging more or less outwards. I was never able, however, to obtain sparks which, separately leaving the ends of the wires _n_ and _o_, conjoined into one spark before they reached or communicated with the ball _c_. At present, therefore, though I think I saw a tendency in the sparks to unite, I cannot a.s.sert it as a fact.

1417. But there is one very interesting effect here, a.n.a.logous to, and it may be in part the same with, that I was searching for: I mean the increased facility of discharge where the spark pa.s.ses. For instance, in the cases where one end, as _n_, discharged the electricity of both ends to the ball _c_, fig. 116, the electricity of the other end _o_, had to pa.s.s through an interval of air 1.5 times as great as that which it might have taken, by its direct pa.s.sage between the end and the ball itself. In such cases, the eye could not distinguish, even by the use of Wheatstone's means[A], that the spark from the end _n_, which contained both portions of electricity, was a double spark. It could not have consisted of two sparks taking separate courses, for such an effect would have been visible to the eye; but it is just possible, that the spark of the first end _n_ and its jar, pa.s.sing at the smallest interval of time before that of the other _o_ had heated and expanded the air in its course, and made it so much more favourable to discharge, that the electricity of the end _o_ preferred leaping across to it and taking a very circuitous route, rather than the more direct one to the ball. It must, however, be remarked, in answer to this supposition, that the one spark between _d_ and _e_ would, by its influence, tend to produce simultaneous discharges at _n_ and _o_, and certainly did so, when no preponderance was given to one wire over the other, as to the previous inductive effect (1414.).

[A] Philosophical Transactions, 1834, pp. 584, 585.

1418. The fact, however, is, that disruptive discharge is favourable to itself. It is at the outset a case of tottering equilibrium: and if _time_ be an element in discharge, in however minute a proportion (1436.), then the commencement of the act at any point favours its continuance and increase there, and portions of power will be discharged by a course which they would not otherwise have taken.

1419. The mere heating and expansion of the air itself by the first portion of electricity which pa.s.ses, must have a great influence in producing this result.

1420. As to the result itself, we see its effect in every electric spark; for it is not the whole quant.i.ty which pa.s.ses that determines the discharge, but merely that small portion of force which brings the deciding molecule (1370.) up to its maximum tension; then, when its forces are subverted and discharge begins, all the rest pa.s.ses by the same course, from the influence of the favouring circ.u.mstances just referred to; and whether it be the electricity on a square inch, or a thousand square inches of charged gla.s.s, the discharge is complete. Hereafter we shall find the influence of this effect in the formation of brushes (1435.); and it is not impossible that we may trace it producing the jagged spark and the forked lightning.

1421. The characters of the electric spark in _different gases_ vary, and the variation _may_ be due simply to the effect of the heat evolved at the moment. But it may also be due to that specific relation of the particles and the electric forces which I have a.s.sumed as the basis of a theory of induction; the facts do not oppose such a view; and in that view the variation strengthens the argument for molecular action, as it would seem to show the influence of the latter in every part of the electrical effect (1423. 1454.).

1422. The appearances of the sparks in different gases have often been observed and recorded[A], but I think it not out of place to notice briefly the following results; they were obtained with b.a.l.l.s of bra.s.s, (platina surfaces would have been better,) and at common pressures. In _air_, the sparks have that intense light and bluish colour which are so well known, and often have faint or dark parts in their course, when the quant.i.ty of electricity pa.s.sing is not great. In _nitrogen_, they are very beautiful, having the same general appearance as in air, but have decidedly more colour of a bluish or purple character, and I thought were remarkably sonorous. In _oxygen_, the sparks were whiter than in air or nitrogen, and I think not so brilliant. In _hydrogen_, they had a very fine crimson colour, not due to its rarity, for the character pa.s.sed away as the atmosphere was rarefied (1459.)[B]. Very little sound was produced in this gas; but that is a consequence of its physical condition[C]. In _carbonic acid gas_, the colour was similar to that of the spark in air, but with a little green in it: the sparks were remarkably irregular in form, more so than in common air: they could also, under similar circ.u.mstances as to size of ball, &c., be obtained much longer than in air, the gas showing a singular readiness to cause the discharge in the form of spark. In _muriatic acid gas_, the spark was nearly white: it was always bright throughout, never presenting those dark parts which happen in air, nitrogen, and some other gases. The gas was dry, and during the whole experiment the surface of the gla.s.s globe within remained quite dry and bright. In _coal gas_, the spark was sometimes green, sometimes red, and occasionally one part was green and another red: black parts also occur very suddenly in the line of the spark, i.e. they are not connected by any dull part with bright portions, but the two seem to join directly one with the other.

[A] See Van Marum's description of the Teylerian machine, vol. i. p.

112, and vol. ii. p. 196; also Ency. Britan., vol. vi., Article Electricity, pp. 505, 507.

[B] Van Marum says they are about four times as large in hydrogen as in air. vol. i. p. 122.

[C] Leslie. Cambridge Phil. Transactions, 267.

1423. These varieties of character impress my mind with a feeling, that they are due to a direct relation of the electric powers to the particles of the dielectric through which the discharge occurs, and are not the mere results of a casual ignition or a secondary kind of action of the electricity, upon the particles which it finds in its course and thrusts aside in its pa.s.sage (1454.).

1424. The spark may be obtained in media which are far denser than air, as in oil of turpentine, olive oil, resin, gla.s.s, &c.: it may also be obtained in bodies which being denser likewise approximate to the condition of conductors, as spermaceti, water, &c. But in these cases, nothing occurs which, as far as I can perceive, is at all hostile to the general views I have endeavoured to advocate.

_The electrical brush._