126 POPULAR LECTURES AND ADDRESSES. 



power of a certain thread or wire. But, as we 

 have seen in electro-magnetic measure, the resist- 

 ance of the same thread or wire is measured by 

 another velocity. The mysterious quantity " v " 

 is the square root of the product of the two 

 velocities. Or it is the one velocity which mea- 

 sures in electro-magnetic measure the resistance, 

 and in electro-static measure the conductivity, of 

 one and the same conductor ; which must be of 

 about 29 ohms resistance, because experiment 

 has proved " v " to be not very different from 

 290,000 kilometres per second. 



I have spoken to you of how much we owe to 

 Sir Charles Bright and Mr. Latimer-Clark for the 

 suggestion of names. How much we owe for the 

 possession of names, is best illustrated by how 

 much we lose how great a disadvantage we are 

 put to in cases in which we have not names. 

 We want a name for the reciprocal of resistance. 

 We have the name "conductivity," but we want 

 a name for the unit of conductivity. I made a 

 box of resistance coils thirty years ago, and 

 another fifteen years ago, for the measurement of 



