124 POPULAR LECTURES AND ADDRESSES. 



influence it by electro-magnetic force. Now take 

 the galvanometer and turn the mouse-mill ; let the 

 length of each bar of the mouse-mill be a centi- 

 metre ; but that would be a flea-mill rather than a 

 mouse-mill say, let each bar be 100 centimetres; 

 turn the mouse-mill round fast enough to cause 

 your galvanometer to be deflected 45. Then one 

 hundred times the velocity of the bars is equal 

 to the resistance in the circuit. Double resist- 

 ance requires double velocity ; half resistance 

 requires half velocity to give the prescribed 45 

 deflection. There, then, is the rationale of 10,000 

 kilometres per second, or 1,000,000,000 centime- 

 tres per second being the measure of resistance. 

 While we thus measure resistance in electro- 

 magnetic measure by velocity, we measure a con- 

 ductivity in electrostatics by a velocity. I have 

 given a very simple explanation of this also in a 

 statement quoted by Sir William Siemens in his 

 presidential address to the British Association at 

 Southampton in 1882. The velocity at which the 

 surface of a globe must shrink towards the centre, 

 to keep its potential constant, when it is connected 



