1881. J Determination of the Ohm in Absolute Measure. 105 



the Cavendish. Laboratory, I determined last June to repeat the 

 measurement by the method of the Committee, which has been 

 employed by no subsequent experimenter, and sought permission from 

 the Council of the British Association to make the necessary altera- 

 tions in the apparatus. In this way I hoped not merely to obtain an 

 independent result, but also to form an opinion upon the importance 

 of certain criticisms which have been passed upon the work of the 

 Committee. 



The method, it will be remembered, consists in causing a coil of 

 insulated wire, forming a closed circuit, to revolve about a vertical 

 axis, and in observing the deflection from the magnetic meridian of 

 a magnet suspended at its centre, the deflection being due to the 

 currents developed in the coil under the influence of the earth's 

 magnetism. The amount of the deflection is independent of the 

 intensity of the earth's magnetic force, and it varies inversely as the 

 resistance of the circuit. The theory of the experiment is explained 

 very fully in the reports of the Committee,* and in Maxwell's 

 "Electricity and Magnetism," section 763. For the sake of distinct- 

 ness, and as affording an opportunity for one or two minor criticisms, 

 a short statement in the original notation will be convenient : — 



H= horizontal component of earth's magnetism. 



7 = strength of current in coil at time t. 



Gr=total area inclosed by all the windings of the wire. 



iv= angular velocity of rotation. 



0= wrangle between plane of coil and magnetic meridian. 

 M= magnetic moment of suspended magnet. 



0=angle between the axis of the magnet and the magnetic 

 meridian. 



K= magnetic force at the centre of the coil due to unit current 



in the wire. 

 L= coefficient of self-induction of coil. 

 K= resistance of coil in absolute measure. 

 MHt= force of torsion of fibre per unit of angular rotation. 



The equation determining the current is — 



L^ + R7=HGwcosa*+MKwcos(«*-0) . . . (1), 

 at 



whence 



+ KM(Rcos(0— 0)+Lasin(0— 0))} . (2). 

 * Collected in one volume. London. 1873. 



i 2 



