248 SECTION MECHANICS, 



Association, of which our President was one, and I also had the honor 

 to be a member, proposed a method of measurement which was carried 

 out chiefly by Professors Clerk-Maxwell, Balfour Stewart and Jenkins, 

 who laid down what is called the British Association unit to which the 

 name, according to the advice of Mr. Latimer Clark, of " Ohm," was 

 given in commemoration of one of the great founders of electro-magnetic 

 science, Ohm being the man who gave us the first law of currents in 

 connection with electro-motive force, it was considered appropriate that 

 his name should be given to the electric unit, but I may mention as a 

 matter of great importance and interest in physical science, a revision 

 of the measurement of the British Association unit is being undertaken. 

 There is now an endeavour 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 percent- 

 age of being exactly a thousand metres per second. One per cent, 

 away from that amount, it may, Iput that it is two or three per cent, or 

 four per cent, or one-third per cent, is of course possible as anyone may 

 judge by looking at the difficulties that will have to be met with in 

 making the experiments. I will just say in connection with the electro 

 measurement 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 

 electro-magnetic experiments taken in connection with other experi- 

 ments of his on the dynamical equivalent of heat, show some dis- 

 agreement from the British Association measurement of their unit of 

 resistance. There is something to be reconciled here. Joule on the 

 one side holds that the British Association unit, the Ohm, is a little too 

 much or rather too little, I forget which, but on the other side, in 

 Germany, Kohlrausch holds the Ohm to be a little on the other side 

 of the exact thousand million centimetres per second. I believe if you 

 eliminate doubt by the method of averages, Kohlrausch and Joule's 

 experiments would show the British Association to be very nearly 

 right, but I do not approve of that method of removing doubts, and we 

 shall not be satisfied until both Joule and Kohlrausch are satisfied. 



I will now mention a number of experiments with electrometers 

 which, I am afraid, are of little interest to anyone in the world, but 

 myself. Here is the first attempt at a quadrant electrometer, but it is 



