METHOD OF TORSION. 207 



the action of the current I, then if C is the coefficient of the wire, we 

 have 



IMGsin/3 = C0. 



If the needle occupies a principal position, and its original 

 direction is parallel to the plane of the coil, 



(3) 



MG 



Suppose that this wire having an initial torsion, the direction 

 of the needle at the fiducial position makes an angle a with the 

 meridian ; if is the angle through which the wire must be 

 turned to bring the needle back to the meridian, we have 



MH sina = 



This defect of adjustment may be eliminated by making a 

 second observation with the current reversed. The observed 

 torsions and & satisfy the ratios 



IMG sin/3 + MH si 



IMG sin j3 - MH sin a = C (ff - + a) , 



which give, when added 

 (4) 



The mean of the torsions observed on the right and the left is 

 equal to that which would be obtained with a wire originally without 

 torsion. 



This method, which was devised by Ohm,* has more particularly 

 the disadvantage of requiring a manipulation which disturbs the 

 needle whenever the wire is touched, to bring it back to the fiducial 

 position ; we must wait then until the needle has come to rest, which 

 necessitates great loss of time. It can only be applied to constant 

 currents. If we want to use it in comparing variable currents, a 

 series of alternate tests would enable us to eliminate the variations, 

 provided they are not too rapid. 



* OHM. Pogg. Ann., Vol. iv., p. 79. 1825. 



