136 A DIRECT COMPARISON OF ELECTROSTATIC WITH ELECTROMAGNETIC FORCE. 



Experiments 3, 4, 5, 6, 7 were rejected on account of the micrometer 

 being touched during the observation of equilibrium. These experiments gave 

 an average value of t> = 27*39. 



The value of v derived from these experiments is considerably smaller than 

 that which was obtained by MM. Weber and Kohlrausch, which was 31 '074 

 Ohms, or 310,740,000 metres per second. 



Their method involved the determination of the electrostatic capacity of a 

 condenser, the electrostatic determination of its potential when charged, and 

 the electromagnetic determination of the quantity of electricity discharged through 

 a galvanometer. 



The capacity of the condenser was measured by dividing its charge repeat- 

 edly with a sphere of known radius. Now, since all condensers made with 

 solid dielectrics exhibit the phenomena of " electric absorption," this method 

 would give too large a value for the capacity, as the condenser would become 

 recharged to a certain extent after each discharge, so that the repeated division 

 of the charge would have too small an effect on the potential. The capacity 

 being overestimated, the number of electrostatic units in the discharge would 

 be overestimated, and the value of v would be too great. 



In pointing out this as a probable source of error in the experiments of 

 MM. Weber and Kohlrausch, I mean to indicate that I have such confidence 

 in the ability and fidelity with which their investigation was conducted, that 

 I am obliged to attribute the difference of their result from mine to a phe- 

 nomenon the nature of which is now much better understood than when their 

 experiments were made. 



On the other hand, the result of present experiments depends on the 

 accuracy of the experiments of the Committee of the British Association on 

 Electric ^Resistance. The B. A. unit is about 8 '8 per cent, larger than that 

 determined by Weber in 1862, and about 1'2 per cent, less than that derived 

 by Dr Joule from his experiments on the dynamical equivalent of heat by 

 comparing the heating effects of direct mechanical agitation with those of electric 

 currents. 



I believe that Sir William Thomson's experiments, not yet published, give 

 a value of v not very different from mine. His method, I believe, also depends 

 on the value of the B. A. unit. 



The lowest estimate of the velocity of light, that of the late M. Foucault, is 



298,000,000 metres per second. 



