446 POPULAR LECTURES AND ADDRESSES. 



it was considered appropriate that his name 

 should be given to this electric unit. 



I may mention as a matter of great importance 

 and interest in physical science, that a revision 

 of the measurement of the British Association 

 unit is being undertaken. An endeavour is now 

 being made to measure with the greatest possible 

 accuracy what is the value of the " Ohm " in 

 terms of the absolute scale of centimetres per 

 second. It will certainly come within a small 

 percentage of being exactly ten thousand kilo- 

 metres per second. One per cent, away from 

 that amount, it may be : but that it may be 

 two or three per cent, or four per cent, or one- 

 third per cent, is of course possible ; as any one 

 may judge by looking at the difficulties which will 

 have to be met with in making the experiments. 

 I will just say in connection with this electric 

 measurement of the Ohm, that it touches on 

 another point of measurement, that of heat. 

 Joule in a quite independent set of experiments 

 which I can only name, showed another way of 

 arriving at similar results, and Joule's thermo- 



