1479.  Before I proceed further in tracing the probable cause of the difference between the positive and negative brush discharge, I wish to know the results of a few experiments which are in course of preparation:  and thinking this Series of Researches long enough, I shall here close it with the expectation of being able in a few weeks to renew the inquiry, and entirely redeem my pledge (1306.).

Royal Institution, Dec. 23rd, 1837.

THIRTEENTH SERIES.

S 18. On Induction (continued). P ix. Disruptive discharge (continued)—­Peculiarities of positive and negative discharge either as spark or brush—­Glow discharge—­Dark discharge. P x. Convection, or carrying discharge. P xi. Relation of a vacuum to electrical phenomena. S 19. Nature of the electrical current.

Received February 22,—­Read March 15, 1838.

P 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 principally, 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 balls and terminations of different diameters, and the following are some of the principal results.

1482.  If two balls 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 balls, or other masses 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