348 



COMPARISON OF RESISTANCES. 



952. When the diagonals r and R are not conjugate, the in- 

 duced currents obey more complex laws. The swing of the needle, 

 when the branch R containing the battery is closed, may be of the 

 opposite sign to the permanent deflection. In order to get over 

 this difficulty, which makes the observations much longer, care is 

 taken to close first the battery and then the galvanometer.* The 

 needle remains then at zero, whatever are the coefficients of in- 

 duction, if the equilibrium of the bridge for the permanent regime 

 is established, and it is always displaced in the direction of the 

 ultimate deflection. 



This result is obtained by working two independent keys, one 

 on the branch which contains the battery and the other on that 

 of the galvanometer. This double operation may be performed 



M 



Fig. 1 86. 



automatically by means of a special key with two successive con- 

 tacts (Fig. 1 86) formed of three superposed and parallel elastic 

 plates. When the knob M is depressed, the contact of A and B 

 closes the battery circuit ; a moment after, the contact of C and D 

 closes the galvanometer circuit. 



953. CONDITIONS OF SENSITIVENESS OF WHEATSTONE'S BRIDGE. 

 The value of / given by equation (18) 



shows that the sensitiveness of the method, other things being equal, 

 is inversely as A (947). 



It may first of all be asked whether it is immaterial in which 

 of the diagonals the battery and the galvanometer are placed. 



* Sir W. THOMSO^. Phil. Mag. [4], Vol. xxiv., p. 149. 1862. 



