58 



Mr. E. C. Bimington on a Method of 



the battery and galvanometer are on together need be known, 

 as is the case with the secohmmeter. 



To find the conditions under which a telephone may replace 

 the galvanometer. 



In order that the galvano- 

 meter may be replaced by a 

 telephone it is necessary that 

 the current through it at any 

 moment shall be zero. Con- 

 sequently v 1 must equal v 2 

 always. Therefore the resist- 

 ance of Gr may be anything. 

 Let it be infinite, and let x 

 be the current in A, D, C, 

 and B— r, y the current in 7 

 denser at any moment t after the battery- circuit is broken. 



Let x be the current in D, y the current in r, and q the 

 charge on the condenser at the moment of breaking the 

 battery- circuit, and suppose the break is not prolonged by a 

 spark at the contact. 



Then 



and q the charge on the con- 



(A + B—i')w—ry = 0, . 



0, 



do 



From (2), 



= x 



A + B-r 



(1) 

 (2) 



(3^ 



and therefore from (3), 



A + B dq 



Now from (1), 



dt 

 dx C + D 



dt, 



W 



or 



C*dx C + Df' 



. x 

 log — = 

 *x 



C + D 



C + D 



x = x e L 

 Again, since the resistance of G may be anything, let it 



