120 POPULAR LECTURES AND ADDRESSES. 



tions described above, without any reference to 

 the sun or other natural chronometer. 



I have tried your patience, I fear, too long, but 

 I have now only reached the threshold of my 

 subject. We now must commence the considera- 

 tion of electrical units of measurement. I need 

 not go round defining quantities electrostatically 

 and electro-magnetically ; you will find it all in 

 Everett, and in the British Association volume of 

 collected Reports by the first Committee on 

 Electric Measurement. It is not for me to tell 

 you of an ohm, a volt, a micro-farad, and so on ; 

 but there are two or three points that I should 

 like to notice, and one is, the limitation of the 

 so-called practical system. The absolute system 

 goes from beginning to end in a perfectly con- 

 sistent manner, with the initial conditions carried 

 out all through ; one of which, in the electro- 

 magnetic system, is that the electromotive force 

 produced by the motion at unit speed, across the 

 lines of force of a field of unit intensity, of a unit 

 length of conductor, is unity. That you must 

 carry out if the system is to be complete and 



