232 MR. HARRIS ON SOME ELEMENTARY LAWS OF ELECTRICITY. 



body, even in vacuo, except to flow upon some other body towards which it tends : 

 thus, in the experiment of charging- a jar under an exhausted receiver, the electrical 

 current invariably flows from one coating to the other. If the rod of a charged jar 

 be caused to project into the middle of a large receiver, the charge will not leave the 

 jar ; for the ball in which the rod terminates is still without the influence of the 

 points of attraction toward which the electricity would otherwise tend. In short, 

 electrical currents generally, may be shown to be almost exclusive actions between 

 given points (74. 78.). Independently, however, of these considerations, it is evident, 

 that the excitation of heat is the sole cause of the less effective transmission of the 

 electricity. Thus, a fine wire passed through an exhausted receiver, has its conducting 

 power impaired when heated : now in this instance the atmospheric pressure is ex- 

 tremely diminished, as well for the wire in its cool state, as when subsequently heated. 

 Moreover, the converse of this experiment, the increased conducting power by the 

 application of cold to the wire, is equally demonstrable : thus, a wire under the ordi- 

 nary atmospheric conditions has its conducting power greatly increased by evaporating 

 ether from its surface *. 



53. Although the disposition of electricity on insulated conductors is subject to the 

 laws above deduced (16.), and which are invariable when the surface remains the 

 same, or is perfectly similar in respect of dimensions and form, yet these laws do not 

 appear, under every condition, incidental to the conducting surface. It has been 

 already observed by Volta, that extension in length greatly contributes to increase 

 the capacity of a conductor ; so that of two plane surfaces of equal area, that which 

 has the greatest extension has also the greatest capacity for electricity. I have pur- 

 sued this interesting fact, and have arrived at some further results which seem of 

 importance. 



(o.) Having procured some rectangular plates of equal area, such as represented in 

 fig. 1 9, whose figures varied from a circle, through a square, up to a long parallelo- 

 gram, I submitted them to experiment, according to the methods already described 

 (14.). Each plate was placed in connexion with the electrometer a, fig. 2, and a given 

 quantity of electricity transferred on it, from a jar charged to a known extent, by 

 means of a small insulated transfer plate. After a few trials it became evident that 

 the intensity varied in an inverse ratio of the perimeter of the respective plates, the 

 differences being inconsiderable between the circle and square, but more decided as 

 the area became extended in length. Thus, in the parallelograms a, d, e, fig. 19, the 

 intensities, as corresponding with the dimensions, were as in the following Table : 

 these intensities have been calculated for a distance = 0*5 of an inch between the 

 attracting surfaces (10.) ; and it may be observed, that in these instances, the numerical 

 agreements are sufficiently near. 



* Transactions of the Royal Society of Edinburgh, 1832; also Philosophical Transactions, 1821, p. 425. 



