208 MEASUREMENT OF CURRENTS. 



The absolute sensitiveness (828) of the method of torsion 



MG 



S. c- 



is constant ; the relative sensitiveness in 



\ 



S =0 



r 



is proportional to the torsion, or to the current itself. 



831. METHOD OF OSCILLATIONS. Let us suppose that the 

 needle occupies a principal position, and that the direction of 

 equilibrium is parallel to the axis of the coil ; this adjustment is 

 correct if, when the current passes, no deflection is produced. The 

 needle is made to oscillate rapidly, first under the influence of the 

 earth alone, and then under the combined action of the earth and 

 of the current. 



In this relative position of the coil and of the needle, small 

 oscillations do not produce any appreciable induction currents ; we 

 may accordingly apply the pendulum formula. If n and N are the 

 numbers of oscillations, reduced to infinitely small angles (692) for 

 the same time in the two cases, we have 



N 2 H + GI 



~^~ ~1T~ ; 



whence 



This is the method used by Biot and Savart ;* it is sometimes 

 useful in comparing two currents, but, like the preceding method, it 

 has the inconvenience of requiring some time for the experiment. 



As regards the sensitiveness, we have 



* BIOT and SAVART. Ann. de Chim. et de Phys. [2], Vol. xv., p?"222. 1820. 

 BiOT. Precis ellm. de Physique, 2nd Edit., Vol. n., p. 715. 1825. 



