ELECTRICAL MEASUREMENT. 441 



the furnishing of laboratories. There is no likeli- 

 hood that the optician will understand what is 

 meant, but perhaps if you teach him a little he 

 will soon come to understand it, and I hope in ten 

 years hence, in every optician's shop where Leyden 

 jars are sold, there will be a label put on each 

 jar telling that the capacity is so many centi- 

 metres. It could be done to-morrow. We have 

 all the means of doing it, only all have not the 

 knowledge. 



The relation between electrostatic measurement 

 and electro-magnetic measurement is very in- 

 teresting, and here from the supposed uninterest- 

 ing realms of minute and accurate measurement 

 we are led to the depths of science, and to 

 look at the great things of Nature. These old 

 measurements of Weber led to an approximate de- 

 termination of the particular velocity, " v ", at which 

 the electro-magnetic resistance is numerically 

 equal to the electrostatic conducting power of 

 a wire. The particular degree of resistance of a 

 wire which shall be such that the velocity which 

 measures the resistance in electro-magnetic 



