340 Lord Rayleigh's Comjiarison of Methods for the 



intensity (H) by an absolute tangent-galvanometer. The dif- 

 ference of potential at the two points of derivation is thus known 

 in terms of the included absolute resistance (R) and H. The 

 circuit is continued through a sensitive galvanometer and the 

 coil of the inductor, and is closed only when the latter coil is 

 nearly in the plane of the meridian. When balance is obtained, 

 the electromotive force of induction n . tto 2 . H . o> is equal to 

 RH tan a/G, where G is the constant of the tangent-galvano- 

 meter and a the angle of deflection. The result, from which 

 H disappears, if it may be assumed to be the same in the two 

 places, is thus 



R = ?i7ra 2 G . (o cot a, 



or, if A be the mean radius of the galvanometer-coil, 



a 2 

 R=2n7r 2 £ocot«- r , 



A 



from which the value of the resistance-coils is obtained in abso- 

 lute measure. One advantage of this method, which it shares 

 with VI. below, is that the resistance immediately expressed 

 may be that of well-constructed coils of German silver or of 

 platinum-silver at a known temperature. 



This method is nearly free from the secondary objections to 

 III. discussed above. The self-induction of the revolving wire- 

 circuit does not enter, as no appreciable current is allowed to 

 form itself; but there would appear to be a possibility of dis- 

 turbance from mutual induction between the wire-circuit and 

 secondary circuits in the ring. It would certainly be neces- 

 sary to prevent the flow of currents round the ring by the 

 insertion of an insulating layer; and even with this precaution 

 some control in the way of a variation of speed would almost 

 be necessary. Again, it is a question whether disturbance, 

 from thermo-electricity for instance, may notarise at the place 

 where the contacts are made and broken. 



It is to be hoped that a complete series of observations may 

 be made by this method, which certainly possesses considerable 

 merits; but at best it remains open to the objection mentioned 

 under II., with which in this respect it stands upon a level, ?'. e. 

 that errors may enter from the measurements of both coils, the 

 error of A entering singly into the result, and that of a enter- 

 ing doubly. 



In respect of requiring absolute measurements of angle, 

 there is nothing to choose between II., III., IV., and V. 



V. Weber's Method hy Damping. 

 This is the method followed by Kohlrausch * in his investi- 

 * Pogg. Ann, Ergaiizungsband vi. ; Phil. Mag. 1874, April and May. 



