INDUCTANCE AND CAPACITY 389 



The value of jr derived from (19) when equated to that in 



(18) gives 



Tp [L N L X L M L P ] + ~T7 [ RpLiM + RN!JX + RxL/N RuLr] ~\- 



i B [R N Rx - R M R P ] = 0. (20) 



By supposition (20) must hold for all values of t and, therefore, 

 for the steady state, so 



R N R X - R M R P = (21) 



which is the ordinary condition for the balance of the Wheatstone 

 bridge. In order that (20) may be true for all values of t the 



coefficients of - r- and - 7 must also be zero, so 

 at 2 at 



L N L X - L M L P = (22) 



R P L M -\- RN^/X -f- RxLiN RM!JP = 0. (23) 



As no assumption has been made concerning the relation of 

 i B to 2, equation (20) holds when the bridge is supplied with 

 sinusoidal as well as with variable currents. 



The Secohmmeter. 14 To increase the sensitiveness of the 

 bridge methods for the measurement of self -inductance and 

 capacity which depend upon the use of variable currents, Ayrton 

 and Perry devised the secohmmeter, by which the impulses on the 

 galvanometer needle can be made to follow one another so 

 rapidly that the instrument takes up a steady deflection. The 

 arrangement is essentially a double commutator. One of the 

 commutators reverses the battery connections while the other 

 reverses the galvanometer terminals so that the impulses on the 

 needle of the instrument are always in the same direction. 

 Referring to Fig. 228 the shaded portions of the two commutators 

 are made of an insulating material. The unshaded portions are 

 conducting segments. The brushes aa', bb f , and cc r , dd', are so 

 placed that the circuits are manipulated in the proper sequence. 

 The secohmmeter is driven at a constant speed by a small 

 motor. 



In using this device the speed must not be so high that suffi- 

 cient time is not allowed for the establishment of the steady 



