Current and Potential Difference Analogies, 207 



value of 



the other the mean value of 



V\, v 2 , v 3 being the instantaneous values of the potentials. 

 Then we take the difference, and so get the mean value of 



(v 1 —v 2 ){v 2 —v 3 ), 



which is equal to r times the mean watts given to ab 

 (fig. 4). 



With Mr. Blakesley's method two measurements are made, 

 one with a split dynamometer (fig. 5) giving the mean 

 value of 



a l a 3, 



and the other with an ammeter 

 giving the mean value of 



a l5 a 2 , a 3 being the instantaneous 

 values of the currents. Then the 

 difference is taken, and so the 

 mean value of 



a i(«3~ «i) or ot&z 



is obtained, and this is equal to - times the mean watts. 



r 

 given to ab (fig. 5). 



It is important to notice that as no instrument is inserted 

 in the non-inductive circuit ab (fig. 5), this method of 

 Mr. Blakesley's has exactly the same accuracy as the electro- 

 meter method. 



III. 



The electrometer measurements may, as we pointed out 

 some years ago,^be varied ; and by making the connexions 

 as seen in fig. 6, we can obtain from the two readings the 

 mean values of 



and 





