ELECTRICAL MEASUREMENT. 245 



the instrument, and suppose the electrometer to indicate a certain 

 degree a potential, as we now call it that subject of electric measure- 

 ment really discovered by Cavendish in his measurement of electric 

 capacity. Now suppose, then, we are measuring the electric value the 

 potential of the charge in the globe by an electrometer, then we shall see 

 the electrometer indications decreasing and the potential gradually going 

 down according to the logarithmic, or compound interest law, in the 

 circumstances I have supposed. But instead of this being carried out, 

 let us suppose the following conditions, which we can imagine, although 

 it would be impossible for any mechanician to execute it. Let the 

 globe by some imaginary means be gradually diminished in its 

 diameter. Suppose, in the first place, the insulation to be exceedingly 

 perfect, and suppose the resistance of the conducting wire to be enor- 

 mously great, so that in the course of a minute or two there is but 

 little loss of potential. Now let this globe, which is supposed to be 

 shrinkable or extendable at pleasure, be shrunk from the metre radius 

 to 90 centimetres radius, what will the effect be ? The effect will be 

 that the potential will increase in the ratio of 90 to 100. Shrink the 

 globe to half its dimensions the potential will be double, and so on. 

 That follows from the result of the mathematical theory that the 

 electrostatic capacity of a globe is numerically equal to its radius. 

 Now, while the globe is charged let its radius be diminished. Let the 

 globe shrink at such a speed that the potential shall remain constant. 

 There, then, you can imagine a globe losing a constant quantity of 

 electricity per unit of time, because it is kept now at a constant poten- 

 tial. A globe kept by this wonderful shrinking mechanism at a 

 constant potential will lose a constant quantity of electricity per unit 

 of time, losing in equal times equal quantities ; and the globe going 

 on shrinking and shrinking so as to keep a constant potential, the 

 velocity with which the surface approaches the centre measures the 

 conducting power of the wire in absolute electrostatic measurement. 

 So, then, we have the very curious result that according to the electro- 

 static law of the phenomena we can measure in terms of electrostatic 

 principles the conducting power of a wire by the velocity. Although 

 I have put an altogether ideal case to you, it would be very wrong for 

 me to allow you to suppose that this is an ideal kind of measurement ; 

 in point of fact, we measure regularly in electrostatic measurement 



