ELECTRICAL UNITS OF MEASUREMENT. 125 



to the earth by a wet thread, measures the con- 

 ducting power of that wet thread. Double con- 

 ducting power will require double velocity of 

 shrinkage, that is, the globe must shrink twice 

 as fast not to lose its potential. With a very 

 long semi-dry thread the globe may shrink 

 slowly. Suppose we have a globe insulated in 

 the air of this room for electrical experiment, and 

 connected with the ground by a silk thread. If 

 you have an electrometer to show the potential, 

 you will see it gradually sink. You might 

 imagine that dust in the air would carry off 

 electricity, but in truth practically the sole loss 

 is by this semi-dry silk thread. When you see 

 the potential sinking, imagine you see the globe 

 shrinking slowly, so as to keep its potential 

 constant, while it is gradually losing its electric 

 charge little by little : the velocity with which the 

 surface must shrink towards the centre to keep 

 the potential constant measures the conducting 

 power of the silk thread in electrostatic measure. 

 Thus we learn how it is a velocity which mea- 

 sures in electrostatic measure the conducting 



