124 



we have 



Dr. S. W. Richardson on the Magnetic 



Bxo-xN 



M7 



Ri 



or 



And if 



R 2 + r 2 

 B - J^^ + %* 



H = the field strength in the ring X, 

 7i=the number of turns in the primary of X, 

 ^ = the mean diameter of the ring, 



. 4ra 



Note. — This is the condition for no quantity through the 

 galvanometer. The condition for no current through the 

 galvanometer cannot be satisfied owing to the variability 

 of the permeability with the field strength. It wiil, how- 

 ever, be seen later that this did not materially affect the 

 experiments. 



Practice of the Method. 



The sensitiveness of the method was increased by means of 

 a secohmmeter making about 3 alternations per second. 



The secohmmeter D, a diagram of which is shown in 

 fig. 2, consisted of two ebonite rings fitted on to a steel axle 

 S. To each ebonite ring was attached externally a cylindrical 

 brass ring divided along the black lines/,/'. 



Close to the edges of the brass rings were grooves. In 



these grooves fitted contacts of copper gauze b, b, d, d. Two 



other pairs o£ contacts of copper gauze a, a, c, c, pressed 



against the central portions of the brass rings. The contact 



* Cf. Maxwell, vol. ii. p. 395. 



