440 POPULAR LECTURES AND ADDRESSES. 



mechanism at a constant potential will lose a 

 constant quantity of electricity per unit of time, 

 losing in equal times equal quantities. In that 

 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 electro- 

 static measurement. So, then, we have the very 

 curious result that according to the electrostatic 

 law of the phenomena we can measure, in terms 

 of electrostatic principles, the conducting power 

 of a wire by a 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 

 the capacity of the Leyden jars in that way, and 

 in future when any one goes to buy a Leyden 

 jar of an optician, let him tell the optician to 

 give him a jar of one or two metres capacity 

 or whatever it may be, and require him to find 

 out how to produce it. I give that as a hint to 

 any one interested in electrostatic apparatus, or in 





