90 POPULAR LECTURES AND ADDRESSES. 



British Association unit ; Lord Rayleigh and 

 Mrs. Sidgwick found it 0*9542, which is an 

 exceedingly close agreement, being within -~^ per 

 cent, of the result of Dr. Werner Siemens. A 

 result differing by nearly one per cent, had been 

 obtained by Matthiessen and Hockin a good many 

 years before, when the precautions necessary to 

 reproduce the mercury standard with absolute 

 accuracy were not so well known as, in the course 

 of a few years after their work, they came to be 

 known. The final conclusion of Lord Rayleigh's 

 work was, that the Siemens mercury unit is 0*9413, 

 of what the Conference at Paris agreed to define 

 as the ohm ; and that is the resistance measured 

 by 1,000,000,000 centimetres per second. I am 

 afraid that conveys a strange idea, but it is per- 

 fectly true as to the absolutely definite meaning 

 of resistance. I shall have occasion to refer to 

 the subject later, when I hope to explain this 

 mysterious velocity of io 9 centimetres per second. 

 In the course of the thirty years from the time 

 when telegraphy began to demand definite 

 measurement, a great deal of accurate measure- 



