DIFFERENTIAL GALVANOMETER. 231 



needle is very great, and the damping may be such that the system 

 stops without appreciable oscillation in its position of equilibrium. 



The plane of the limbs of the horse-shoe magnet is horizontal, 

 as well as that of the coil. The system of needles turns about a 

 horizontal axis, and it is provided with a straw which serves as index 

 against a graduated scale. By a system of levers the deflection of 

 the index may be multiplied. 



This apparatus is solidly constructed, and is thus adapted for 

 industrial applications. 



853. DIFFERENTIAL GALVANOMETER. The differential galvano- 

 meter invented by A. Becquerel* is an instrument constructed of 

 two coils, which act in contrary directions on a magnetised needle. 

 The currents which traverse the two coils may be either independent, 

 or may form two branches of the same circuit. The deflection of 

 the needle is then proportional to the difference of the actions of the 

 currents, and the experiment consists, in general, in modifying the 

 intensity of one of them, so as to bring the needle to zero. Let 

 I and I' be the intensities of the two currents, G and G' the constants 

 of the coils ; the deflection 8 of the needle will be given by the 

 equation 



GI-GT = H tan 8. 



The deflection 8 is zero when 



GI = GT; 



if the instrument has been constructed so that the constants G and G' 

 are equal, it follows that 



i = r. 



The two coils are usually symmetrical, and of resistance R and R 1? 

 which are equal to each other. 



In order to verify a differential galvanometer, the same current 

 should be passed through the two coils in opposite directions ; the 

 deflection should be zero if the galvanometer constants are equal. 

 The current is then divided between the two coils ; the needle 

 should remain at zero if the resistances are equal. 



A simple method of at once realising these two conditions consists 

 in taking a strand of two equal wires, which is then wound on the 



* BECQUEREL. Ann. de Chim. et de Phys. [2], Vol. xxxn. 1826. Traits 

 d' Elect, et de Magn., Vol. in., p. 74. 



