1417.  But there is one very interesting effect here, analogous 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 passes.  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 pass through an interval of air 1.5 times as great as that which it might have taken, by its direct passage 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, passing 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 passes, 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 quantity which passes 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 passes by the same course, from the influence of the favouring circumstances just referred to; and whether it be the electricity on a square inch, or a thousand square inches of charged glass, 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.

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