Electric Currents of great Strength. 169 



linders, and w x mass of moving parts before w was added ; the 

 dimensions being in millimetres. 

 From these expressions we obtain 



F= ^ 



The constants G and g were calculated from the actual 

 measurements of the coils, which could be made with great 

 accuracy, since all the parts were large. 



The constants were as follows : — 



mean radius r= 153*3 millims. 

 Gg = 1631-45 „ 



F 



— — = Q5Q'626 ,, 



The constant was also determined by running the same 

 current through the electrodynamometer and a tangent gal- 

 vanometer of one turn of copper wire, whose radius was r, 



rT 



and whose constant was equal to ~ 



tnir 



In this case 



„2rr2 XT' 



sine/), 



C 2 = 



r 2 T 2 



4nV 2 



tan 2 = 



F 



Gg 



F 



Gg~ 



r 2 T 2 



4n 2 7r 4 



tan 2 

 sin<£ 



i 



and 



where 



T= horizontal force of earth's magnetism. 



r= radius of galvanometer-coil, 



6= deflection of galvanometer, 



= deflection of electrodynamometer. 



The result obtained in this way closely agreed with that ob- 

 tained by the previous method. 



No difficulty was experienced from the heating produced 

 by currents of even eighty vebers when the current was 

 allowed to run for a long period through the instrument : as 

 long as the stream of water was maintained around the mer- 

 cury-cups, even a small immersion of the point of the axis of 

 the movable part of the instrument did not result in heating. 

 By this instrument, therefore, the whole current could be 

 measured without shunting. At first the metal pivots which 



