Part 44 (2/2)

Received February 22,--Read March 15, 1838.

-- ix. _Disruptive discharge (continued)._

1480. Let us now direct our attention to the general difference of the positive and negative disruptive discharge, with the object of tracing, as far as possible, the cause of that difference, and whether it depends on the charged conductors princ.i.p.ally, or on the interposed dielectric; and as it appears to be great in air and nitrogen (1476.), let us observe the phenomena in air first.

1481. The general case is best understood by a reference to surfaces of considerable size rather than to points, which involve (as a secondary effect) the formation of currents (1562). My investigation, therefore, was carried on with b.a.l.l.s and terminations of different diameters, and the following are some of the princ.i.p.al results.

1482. If two b.a.l.l.s of very different dimensions, as for instance one-half an inch, and the other three inches in diameter, be arranged at the ends of rods so that either can be electrified by a machine and made to discharge by sparks to the other, which is at the same time uninsulated; then, as is well known, far longer sparks are obtained when the small ball is positive and the large ball negative, than when the small ball is negative and the large ball positive. In the former case, the sparks are 10 or 12 inches in length; in the latter, an inch or an inch and a half only.

1483. But previous to the description of further experiments, I will mention two words, for which with many others I am indebted to a friend, and which I think it would be expedient to introduce and use. It is important in ordinary inductive action, to distinguish at which charged surface the induction originates and is sustained: i.e. if two or more metallic b.a.l.l.s, or other ma.s.ses of matter, are in inductive relation, to express which are charged originally, and which are brought by them into the opposite electrical condition. I propose to call those bodies which are originally charged, _inductric_ bodies; and those which a.s.sume the opposite state, in consequence of the induction, _inducteous_ bodies. This distinction is not needful because there is any difference between the sums of the _inductric_ and the _inducteous_ forces; but princ.i.p.ally because, when a ball A is inductric, it not merely brings a ball B, which is opposite to it, into an inducteous state, but also many other surrounding conductors, though some of them may be a considerable distance off, and the consequence is, that the b.a.l.l.s do not bear the same precise relation to each other when, first one, and then the other, is made the inductric ball; though, in each case, the _same ball_ be made to a.s.sume the _same state._

1484, Another liberty which I may also occasionally take in language I will explain and limit. It is that of calling a particular spark or brush, _positive_ or _negative_, according as it may be considered as _originating_ at a positive or a negative surface. We speak of the brush as positive or negative when it shoots out from surfaces previously in those states; and the experiments of Mr. Wheatstone go to prove that it _really begins_ at the charged surface, and from thence extends into the air (1437.

1438.) or other dielectric. According to my view, _sparks_ also originate or are determined at one particular spot (1370.), namely, that where the tension first rises up to the maximum degree; and when this can be determined, as in the simultaneous use of large and small b.a.l.l.s, in which case the discharge begins or is determined by the latter, I would call that discharge which pa.s.ses _at once_, a positive spark, if it was at the positive surface that the maximum intensity was first obtained; or a negative spark, if that necessary intensity was first obtained at the negative surface.

1485. An apparatus was arranged, as in fig. 129. (Plate VIII.): A and B were bra.s.s b.a.l.l.s of very different diameters attached to metal rods, moving through sockets on insulating pillars, so that the distance between the b.a.l.l.s could be varied at pleasure. The large ball A, 2 inches in diameter, was connected with an insulated bra.s.s conductor, which could be rendered positive or negative directly from a cylinder machine: the small ball B, 0.25 of an inch in diameter, was connected with a discharging train (292.) and perfectly uninsulated. The bra.s.s rods sustaining the b.a.l.l.s were 0.2 of an inch in thickness.

1486. When the large ball was _positive_ and inductric (1483.), negative sparks occurred until the interval was 0.49 of an inch; then mixed brush and spark between that and 0.51; and from 0.52 and upwards, negative brush alone. When the large ball was made _negative_ and inductric, then positive spark alone occurred until the interval was as great as 1.15 inches; spark and brush from that up to 1.55; and to have the positive brush alone, it required an interval of at least 1.65 inches.

1487. The b.a.l.l.s A and B were now changed for each other. Then making the small ball B inductric _positively_, the positive sparks alone continued only up to 0.67; spark and brush occurred from 0.68 up to 0.72; and positive brush alone from 0.74 and upwards. Rendering the small ball B inductric and _negative_, negative sparks alone occurred up to 0.40; then spark and brush at 0.42; whilst from 0.44 and upwards the noisy negative brush alone took place.

1488. We thus find a great difference as the b.a.l.l.s are rendered inductric or inducteous; the small ball rendered _positive_ inducteously giving a spark nearly twice as long as that produced when it was charged positive inductrically, and a corresponding difference, though not, under the circ.u.mstances, to the same extent, was manifest, when it was rendered _negative_[A].

[A] For similar experiments on different gases, see 1518.--_Dec. 1838._

1489. Other results are, that the small ball rendered positive gives a much longer spark than when it is rendered negative, and that the small ball rendered negative gives a brush more readily than when positive, in relation to the effect produced by increasing the distance between the two b.a.l.l.s.

1490. When the interval was below 0.4 of an inch, so that the small ball should give sparks, whether positive or negative, I could not observe that there was any constant difference, either in their ready occurrence or the number which pa.s.sed in a given time. But when the interval was such that the small ball when negative gave a brush, then the discharges from it, as separate negative brushes, were far more numerous than the corresponding discharges from it when rendered positive, whether those positive discharges were as sparks or brushes.

1491. It is, therefore, evident that, when a ball is discharging electricity in the form of brushes, the brushes are far more numerous, and each contains or carries off far less electric force when the electricity so discharged is negative, than when it is positive.

1492. In all such experiments as those described, the point of change from spark to brush is very much governed by the working state of the electrical machine and the size of the conductor connected with the discharging ball.

If the machine be in strong action and the conductor large, so that much power is acc.u.mulated quickly for each discharge, then the interval is greater at which the sparks are replaced by brushes; but the general effect is the same[A].

[A] For similar experiments in different gases, see 1510-1517.--_Dec.

1838._

1493. These results, though indicative of very striking and peculiar relations of the electric force or forces, do not show the relative degrees of charge which the small ball acquires before discharge occurs, i.e. they do not tell whether it acquires a higher condition in the negative, or in the positive state, immediately preceding that discharge. To ill.u.s.trate this important point I arranged two places of discharge as represented, fig 130. A and D are bra.s.s b.a.l.l.s 2 inches diameter, B and C are smaller bra.s.s b.a.l.l.s 0.25 of an inch in diameter; the forks L and R supporting them were of bra.s.s wire 0.2 of an inch in diameter; the s.p.a.ce between the large and small ball on the same fork was 5 inches, that the two places of discharge _n_ and _o_ might be sufficiently removed from each other's influence. The fork L was connected with a projecting cylindrical conductor, which could be rendered positive or negative at pleasure, by an electrical machine, and the fork R was attached to another conductor, but thrown into an uninsulated state by connection with a discharging train (292.). The two intervals or places of discharge _n_ and _o_ could be varied at pleasure, their extent being measured by the occasional introduction of a diagonal scale. It is evident, that, as the b.a.l.l.s A and B connected with the same conductor are always charged at once, and that discharge may take place to either of the b.a.l.l.s connected with the discharging train, the intervals of discharge _n_ and _o_ may be properly compared to each other, as respects the influence of large and small b.a.l.l.s when charged positively and negatively in air.

1494. When the intervals _n_ and _o_ were each made = 0.9 of an inch, and the b.a.l.l.s A and B inductric _positively_, the discharge was all at _n_ from the small ball of the conductor to the large ball of the discharging train, and mostly by positive brush, though once by a spark. When the b.a.l.l.s A and B were made inductric _negatively_, the discharge was still from the same small ball, at _n_, by a constant negative brush.

1495. I diminished the intervals _n_ and _o_ to 0.6 of an inch. When A and B were inductric _positively_, all the discharge was at _n_ as a positive brush: when A and B were inductric _negatively_, still all the discharge was at _n_, as a negative brush.

1496. The facility of discharge at the positive and negative small b.a.l.l.s, therefore, did not appear to be very different. If a difference had existed, there were always two small b.a.l.l.s, one in each state, that the discharge might happen at that most favourable to the effect. The only difference was, that one was in the inductric, and the other in the inducteous state, but whichsoever happened for the time to be in that state, whether positive or negative, had the advantage.

<script>