Part 45

1371. The whole effect produced by a charged conductor on a distant conductor, insulated or not, is by my theory assumed to be due to an action propagated from particle to particle of the intervening and insulating dielectric, all the particles being considered as thrown for the time into a forced condition, from which they endeavour to return to their normal or natural state. The theory, therefore, seems to supply an easy explanation of the influence of _distance_ in affecting induction (1303. 1364.). As the distance is diminished induction increases; for there are then fewer

## particles in the line of inductive force to oppose their united resistance

to the assumption of the forced or polarized state, and _vice versa._ Again, as the distance diminishes, discharge across happens with a lower charge of electricity; for if, as in Harris's experiments (1364), the interval be diminished to one-half, then half the electricity required to discharge across the first interval is sufficient to strike across the second; and it is evident, also, that at that time there are only half the number of interposed molecules uniting their forces to resist the discharge.

1372. The effect of enlarging the conducting surfaces which are opposed to each other in the act of induction, is, if the electricity be limited in its supply, to lower the intensity of action; and this follows as a very natural consequence from the increased area of the dielectric across which the induction is effected. For by diffusing the inductive action, which at first was exerted through one square inch of sectional area of the dielectric, over two or three square inches of such area, twice or three times the number of molecules of the dielectric are brought into the polarized condition, and employed in sustaining the inductive action, and consequently the tension belonging to the smaller number on which the limited force was originally accumulated, must fall in a proportionate degree.

1373. For the same reason diminishing these opposing surfaces must increase the intensity, and the effect will increase until the surfaces become points. But in this case, the tension of the particles of the dielectric next the points is higher than that of particles midway, because of the lateral action and consequent bulging, as it were, of the lines of inductive force at the middle distance (1369.).

1374. The more exalted effects of induction on a point _p_, or any small surface, as the rounded end of a rod, when it is opposed to a large surface, as that of a ball or plate, rather than to another point or end, the distance being in both cases the same, fall into harmonious relation with my theory (1302.). For in the latter case, the small surface _p_ is affected only by those particles which are brought into the inductive condition by the equally small surface of the opposed conductor, whereas when that is a ball or plate the lines of inductive force from the latter are concentrated, as it were, upon the end _p_. Now though the molecules of the dielectric against the large surface may have a much lower state of tension than those against the corresponding smaller surface, yet they are also far more numerous, and, as the lines of inductive force converge towards a point, are able to communicate to the particles contained in any cross section (1369.) nearer the small surface an amount of tension equal to their own, and consequently much higher for each individual particle; so that, at the surface of the smaller conductor, the tension of a particle rises much, and if that conductor were to terminate in a point, the tension would rise to an infinite degree, except that it is limited, as before (1368.), by discharge. The nature of the discharge from small surfaces and points under induction will be resumed hereafter (1425. &c.)

1375. _Rarefaction_ of the air does not alter the _intensity_ of inductive

## action (1284. 1287.); nor is there any reason, as far as I can perceive,

why it should. If the quantity of electricity and the distance remain the same, and the air be rarefied one-half, then, though one-half of the

## particles of the dielectric are removed, the other half assume a double

degree of tension in their polarity, and therefore the inductive forces are balanced, and the result remains unaltered as long as the induction and insulation are sustained. But the case of _discharge_ is very different; for as there are only half the number of dielectric particles in the rarefied atmosphere, so these are brought up to the discharging intensity by half the former quantity of electricity; discharge, therefore, ensues, and such a consequence of the theory is in perfect accordance with Mr. Harris's results (1365.).

1376. The _increase_ of electricity required to cause discharge over the same distance, when the pressure of the air or its density is increased, flows in a similar manner, and on the same principle (1375.), from the molecular theory.

1377. Here I think my view of induction has a decided advantage over others, especially over that which refers the retention of electricity on the surface of conductors in air to the _pressure of the atmosphere_ (1305.). The latter is the view which, being adopted by Poisson and Biot[A], is also, I believe, that generally received; and it associates two such dissimilar things, as the ponderous air and the subtile and even hypothetical fluid or fluids of electricity, by gross mechanical relations; by the bonds of mere static pressure. My theory, on the contrary, sets out at once by connecting the electric forces with the particles of matter; it derives all its proofs, and even its origin in the first instance, from experiment; and then, without any further assumption, seems to offer at once a full explanation of these and many other singular, peculiar, and, I think, heretofore unconnected effects.

[A] Encyclopædia Britannica, Supplement, vol. iv. Article Electricity, pp. 76, 81. &c.

1378. An important assisting experimental argument may here be adduced, derived from the difference of specific inductive capacity of different dielectrics (1269. 1274. 1278.). Consider an insulated sphere electrified positively and placed in the centre of another and larger sphere uninsulated, a uniform dielectric, as air, intervening. The case is really that of my apparatus (1187.), and also, in effect, that of any ball electrified in a room and removed to some distance from irregularly-formed conductors. Whilst things remain in this state the electricity is distributed (so to speak) uniformly over the surface of the electrified sphere. But introduce such a dielectric as sulphur or lac, into the space between the two conductors on one side only, or opposite one part of the inner sphere, and immediately the electricity on the latter is diffused unequally (1229. 1270. 1309.), although the form of the conducting surfaces, their distances, and the _pressure_ of the atmosphere remain perfectly unchanged.

1379. Fusinieri took a different view from that of Poisson, Biot, and others, of the reason why rarefaction of air caused easy diffusion of electricity. He considered the effect as due to the removal of the _obstacle_ which the air presented to the expansion of the substances from which the electricity passed[A]. But platina balls show the phenomena _in vacuo_ as well as volatile metals and other substances; besides which, when the rarefaction is very considerable, the electricity passes with scarcely any resistance, and the production of no sensible heat; so that I think Fusinieri's view of the matter is likely to gain but few assents.

[A] Bib. Univ. 1831, xlviii. 375.

1380. I have no need to remark upon the discharging or collecting power of flame or hot air. I believe, with Harris, that the mere heat does nothing (1367.), the rarefaction only being influential. The effect of rarefaction has been already considered generally (1375.); and that caused by the heat of a burning light, with the pointed form of the wick, and the carrying power of the carbonaceous particles which for the time are associated with it, are fully sufficient to account for all the effects.

1381. We have now arrived at the important question, how will the inductive tension requisite for insulation and disruptive discharge be sustained in gases, which, having the same physical state and also the _same pressure_ and the _same temperature_ as _air_, differ from it in specific gravity, in chemical qualities, and it may be in peculiar relations, which not being as yet recognized, are purely electrical (1361.)?

1382. Into this question I can enter now only as far as is essential for the present argument, namely, that insulation and inductive tension do not depend merely upon the charged conductors employed, but also, and essentially, upon the interposed dielectric, in consequence of the molecular action of its particles (1292.).

1383. A glass vessel _a_ (fig. 127.)[A] was ground at the top and bottom so as to be closed by two ground brass plates, _b_ and _c_; _b_ carried a stuffing-box, with a sliding rod _d_ terminated by a brass ball _s_ below, and a ring above. The lower plate was connected with a foot, stop-cock, and socket, _e_, _f_ and _g_; and also with a brass ball _l_, which by means of a stem attached to it and entering the socket _g_, could be fixed at various heights. The metallic parts of this apparatus were not varnished, but the glass was well-covered with a coat of shell-lac previously dissolved in alcohol. On exhausting the vessel at the air-pump it could be filled with any other gas than air, and, in such cases, the gas so passed in was dried whilst entering by fused chloride of calcium.

[A] The drawing is to a scale of 1/6.

1384. The other part of the apparatus consisted of two insulating pillars, _h_ and _i_, to which were fixed two brass balls, and through these passed two sliding rods, _k_ and _m_, terminated at each end by brass balls; _n_ is the end of an insulated conductor, which could be rendered either positive or negative from an electrical machine; _o_ and _p_ are wires connecting it with the two parts previously described, and _q_ is a wire which, connecting the two opposite sides of the collateral arrangements, also communicates with a good discharging train _r_ (292.).

1385. It is evident that the discharge from the machine electricity may pass either between _s_ and _l_, or S and L. The regulation adopted in the first experiments was to keep _s_ and _l_ with their distance _unchanged_, but to introduce first one gas and then another into the vessel _a_, and then balance the discharge at the one place against that at the other; for by making the interval at _a_ sufficiently small, all the discharge would pass there, or making it sufficiently large it would all occur at the interval _v_ in the receiver. On principle it seemed evident, that in this way the varying interval _u_ might be taken as a measure, or rather indication of the resistance to discharge through the gas at the constant interval _v_. The following are the constant dimensions.

Ball _s_ 0.93 of an inch. Ball S 0.96 of an inch. Ball _l_ 2.02 of an inch. Ball L 0.62 of an inch. Interval _v_ 0.62 of an inch.

1386. On proceeding to experiment it was found that when air or any gas was in the receiver _a_, the interval _u_ was not a fixed one; it might be altered through a certain range of distance, and yet sparks pass either there or at _v_ in the receiver. The extremes were therefore noted, i.e. the greatest distance short of that at which the discharge _always_ took place at _v_ in the gas, and the least distance short of that at which it _always_ took place at _u_ in the air. Thus, with air in the receiver, the extremes at _u_ were 0.56 and 0.79 of an inch, the range of 0.23 between these distances including intervals at which sparks passed occasionally either at one place or the other.

1387. The small balls _s_ and S could be rendered either positive or negative from the machine, and as gases were expected and were found to differ from each other in relation to this change (1399.), the results obtained under these differences of charge were also noted.

1388. The following is a Table of results; the gas named is that in the vessel _a_. The smallest, greatest, and mean interval at _u_ in air is expressed in parts of an inch, the interval _v_ being constantly 0.62 of an inch.

Smallest. Greatest. Mean. _ | Air, _s_ and S, pos. 0.60 0.79 0.695 |_Air, _s_ and S, neg. 0.59 0.68 0.635 _ | Oxygen, _s_ and S, pos. 0.41 0.60 0.505 |_Oxygen, _s_ and S, neg. 0.50 0.52 0.510 _ | Nitrogen, _s_ and S, pos. 0.55 0.68 0.615 |_Nitrogen, _s_ and S, neg. 0.59 0.70 0.645 _ | Hydrogen, _s_ and S, pos. 0.30 0.44 0.370 |_Hydrogen, _s_ and S, neg. 0.25 0.30 0.275 _ | Carbonic acid, _s_ and S, pos. 0.56 0.72 0.640 |_Carbonic acid, _s_ and S, neg. 0.58 0.60 0.590 _ | Olefiant gas, _s_ and S, pos. 0.64 0.86 0.750 |_Olefiant gas, _s_ and S, neg. 0.69 0.77 0.730 _ | Coal gas, _s_ and S, pos. 0.37 0.61 0.490 |_Coal gas, _s_ and S, neg. 0.47 0.58 0.525 _ | Muriatic acid gas, _s_ and S, pos. 0.89 1.32 1.105 |_Muriatic acid gas, _s_ and S, neg. 0.67 0.75 0.710

1389. The above results were all obtained at one time. On other occasions other experiments were made, which gave generally the same results as to order, though not as to numbers. Thus:

Hydrogen, _s_ and S, pos. 0.23 0.57 0.400 Carbonic acid, _s_ and S, pos. 0.51 1.05 0.780 Olefiant gas, _s_ and S, pos. 0.66 1.27 0.965

I did not notice the difference of the barometer on the days of experiment[A].

[A] Similar experiments in different gases are described at 1507. 1508.--_Dec. 1838._

1390. One would have expected only two distances, one for each interval, for which the discharge might happen either at one or the other; and that the least alteration of either would immediately cause one to predominate constantly over the other. But that under common circumstances is not the case. With air in the receiver, the variation amounted to 0.2 of an inch nearly on the smaller interval of 0.6, and with muriatic acid gas, the variation was above 0.4 on the smaller interval of 0.9. Why is it that when a fixed interval (the one in the receiver) will pass a spark that cannot go across 0.6 of air at one time, it will immediately after, and apparently under exactly similar circumstances, not pass a spark that can go across 0.8 of air?

1391. It is probable that part of this variation will be traced to

## particles of dust in the air drawn into and about the circuit (1568.). I

believe also that part depends upon a variable charged condition of the surface of the glass vessel _a_. That the whole of the effect is not traceable to the influence of circumstances in the vessel _a_, may be deduced from the fact, that when sparks occur between balls in free air they frequently are not straight, and often pass otherwise than by the shortest distance. These variations in air itself, and at different parts of the very same balls, show the presence and influence of circumstances which are calculated to produce effects of the kind now under consideration.

1392. When a spark had passed at either interval, then, generally, more tended to appear at the _same_ interval, as if a preparation had been made for the passing of the latter sparks. So also on continuing to work the machine quickly the sparks generally followed at the same place. This effect is probably due in part to the warmth of the air heated by the preceding spark, in part to dust, and I suspect in part, to something unperceived as yet in the circumstances of discharge.

1393. A very remarkable difference, which is _constant_ in its direction, occurs when the electricity communicated to the balls _s_ and S is changed from positive to negative, or in the contrary direction. It is that the range of variation is always greater when the small bulls are positive than when they are negative. This is exhibited in the following Table, drawn from the former experiments.

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 balls 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 balls _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 passed 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 assist 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 pass 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 assume 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 shell-lac, and use the same balls 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 circumstances, 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.