1363. Many philosophers have examined the circ.u.mstances of this limiting action in air, but, as far as I know, none have come near Mr. Harris as to the accuracy with, and the extent to, which he has carried on his investigations[A]. Some of his results I must very briefly notice, premising that they are all obtained with the use of air as the _dielectric_ between the conducting surfaces.

[A] Philosophical Transactions, 1834, p. 225.

1364. First as to the _distance_ between the two b.a.l.l.s used, or in other words, the _thickness_ of the dielectric across which the induction was sustained. The quant.i.ty of electricity, measured by a unit jar, or otherwise on the same principle with the unit jar, in the charged or inductive ball, necessary to produce spark discharge, was found to vary exactly with the distance between the b.a.l.l.s, or between the discharging points, and that under very varied and exact forms of experiment[A].

[A] Philosophical Transactions, 1834, p. 225.

1365. Then with respect to variation in the _pressure_ or _density_ of the air. The quant.i.ties of electricity required to produce discharge across a _constant_ interval varied exactly with variations of the density; the quant.i.ty of electricity and density of the air being in the same simple ratio. Or, if the quant.i.ty was retained the same, whilst the interval and density of the air were varied, then these were found in the inverse simple ratio of each other, the same quant.i.ty pa.s.sing across twice the distance with air rarefied to one-half[A].

[A] Philosophical Transactions, 1834, p.229.

1366. It must be remembered that these effects take place without any variation of the _inductive_ force by condensation or rarefaction of the air. That force remains the same in air[A], and in all gases (1284. 1292.), whatever their rarefaction may be.

[A] Philosophical Transactions, 1834, p. 237, 244.

1367. Variation of the _temperature_ of the air produced no variation of the quant.i.ty of electricity required to cause discharge across a given interval[A].

[A] Philosophical Transactions, 1834, p. 230

Such are the general results, which I have occasion for at present, obtained by Mr. Harris, and they appear to me to be unexceptionable.

1368. In the theory of induction founded upon a molecular action of the dielectric, we have to look to the state of that body princ.i.p.ally for the cause and determination of the above effects. Whilst the induction continues, it is a.s.sumed that the particles of the dielectric are in a certain polarized state, the tension of this state rising higher in each particle as the induction is raised to a higher degree, either by approximation of the inducing surfaces, variation of form, increase of the original force, or other means; until at last, the tension of the particles having reached the utmost degree which they can sustain without subversion of the whole arrangement, discharge immediately after takes place.

1369. The theory does not a.s.sume, however, that _all_ the particles of the dielectric subject to the inductive action are affected to the same amount, or acquire the same tension. What has been called the lateral action of the lines of inductive force (1231. 1297.), and the diverging and occasionally curved form of these lines, is against such a notion. The idea is, that any section taken through the dielectric across the lines of inductive force, and including _all of them,_ would be equal, in the sum of the forces, to the sum of the forces in any other section; and that, therefore, the whole amount of tension for each such section would be the same.

1370. Discharge probably occurs, not when all the particles have attained to a certain degree of tension, but when that particle which is most affected has been exalted to the subverting or turning point (1410.). For though _all_ the particles in the line of induction resist charge, and are a.s.sociated in their actions so as to give a sum of resisting force, yet when any one is brought up to the overturning point, _all_ must give way in the case of a spark between ball and ball. The breaking down of that one must of necessity cause the whole barrier to be overturned, for it was at its utmost degree of resistance when it possessed the aiding power of that one particle, in addition to the power of the rest, and the power of that one is now lost. Hence _tension_ or _intensity_[A] may, according to the theory, be considered as represented by the particular condition of the particles, or the amount in them of forced variation from their normal state (1298. 1368.).

[A] See Harris on proposed particular meaning of these terms, Philosophical Transactions, 1834, p. 222.

1371. The whole effect produced by a charged conductor on a distant conductor, insulated or not, is by my theory a.s.sumed 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 a.s.sumption 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 acc.u.mulated, 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 quant.i.ty 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 a.s.sume 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 quant.i.ty 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 a.s.sociates 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 a.s.sumption, seems to offer at once a full explanation of these and many other singular, peculiar, and, I think, heretofore unconnected effects.

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

1378. An important a.s.sisting 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 s.p.a.ce 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 pa.s.sed[A]. But platina b.a.l.l.s show the phenomena _in vacuo_ as well as volatile metals and other substances; besides which, when the rarefaction is very considerable, the electricity pa.s.ses 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 a.s.sents.

[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 a.s.sociated 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 gla.s.s vessel _a_ (fig. 127.)[A] was ground at the top and bottom so as to be closed by two ground bra.s.s plates, _b_ and _c_; _b_ carried a stuffing-box, with a sliding rod _d_ terminated by a bra.s.s ball _s_ below, and a ring above. The lower plate was connected with a foot, stop-c.o.c.k, and socket, _e_, _f_ and _g_; and also with a bra.s.s 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 gla.s.s was well-covered with a coat of sh.e.l.l-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 pa.s.sed 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 bra.s.s b.a.l.l.s, and through these pa.s.sed two sliding rods, _k_ and _m_, terminated at each end by bra.s.s b.a.l.l.s; _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 pa.s.s 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 pa.s.s 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 pa.s.s 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 pa.s.sed occasionally either at one place or the other.

1387. The small b.a.l.l.s _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 circ.u.mstances 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 pa.s.s a spark that cannot go across 0.6 of air at one time, it will immediately after, and apparently under exactly similar circ.u.mstances, not pa.s.s 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 gla.s.s vessel _a_. That the whole of the effect is not traceable to the influence of circ.u.mstances in the vessel _a_, may be deduced from the fact, that when sparks occur between b.a.l.l.s in free air they frequently are not straight, and often pa.s.s otherwise than by the shortest distance. These variations in air itself, and at different parts of the very same b.a.l.l.s, show the presence and influence of circ.u.mstances which are calculated to produce effects of the kind now under consideration.

1392. When a spark had pa.s.sed at either interval, then, generally, more tended to appear at the _same_ interval, as if a preparation had been made for the pa.s.sing 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 circ.u.mstances of discharge.

1393. A very remarkable difference, which is _constant_ in its direction, occurs when the electricity communicated to the b.a.l.l.s _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.