244 -C 'TION MECHANICS. 



independently of any absolute measurement of the terrestrial magnetic 

 force, we get a certain velocity which gives a certain result. Thus, 

 it is that velocity is the proper measure of the resistance of a 

 metallic circuit to the flow of a current through it. Going 

 now to electrostatics, the resistance of an insulator to the 

 transmission of electricity along it, may be measured in a curious 

 manner in connection with the velocity. It may be measured by the 

 reciprocal of a velocity, or in other words, the conducting power 

 of a wire may be measured, with reference to the electrostatic 

 phenomena, by a velocity. Thus, imagine a globe in the centre of 

 this room, at a great distance from the walls. Imagine that globe to be 

 two metres in diameter and one metre in radius ; let it be electrified^ 

 and hung on a fine silk thread, perfectly dry, so as to insulate perfectly. 

 There we have a perfectly insulated globe in the middle of this room. 

 Now if you apply an excessively fine wire, say a wire one ten- 

 thousandth of an inch in diameter, to the globe, and bring that to a 

 plate of metal connected with the walls of the room, or you may 

 suppose the walls of the room to be metallic, so that we may have no 

 confusion owing to the imperfect conductors. When you apply this 

 very fine wire connecting the insulated globe with the walls of the 

 room, the globe instantly loses its electricity. By instantly, I mean in 

 so short a time as would be impossible to measure by any method we 

 could apply I mean a time as small as one-millionth of a second 

 the globe would lose its electricity, if we had connected to it ten or 

 twenty yards of the finest wire we could imagine. But suppose the 

 wire a million times finer (if we can suppose that) than we can apply, 

 the same thing would happen. Or take a cotton thread, and get such 

 a globe as I have been imagining, surrounded with metallic walls, that 

 moist cotton thread will gradually diselectrify it ; in a quarter of a 

 minute the globe will have lost perhaps half its electricity, in another 

 quarter half of the remainder, and so on. If the resistance of the 

 conductor I have supposed is constant, the loss will follow the com- 

 pound interest law so much per cent, per second of the charge will be 

 lost. Now imagine a conductor of perfectly constant resistance to be 

 put between the ideal globe and the walls of the room, and imagine the 

 globe to be connected with one of these electrometers of which I 

 shall say a word in conclusion by an excessively fine wire going into 



