Magnetism on a permanent Electric Current. 311 



ratio of these two areas is taken, for the reason that the 

 earth-inductor coils are turned through 180° when used. 

 H = horizontal intensity of earth's magnetism at position of 

 earth inductor. 



sin -g = a quantity relating to effect on the galvanometer used 



with test-coil, produced by withdrawing the latter from 

 the magnetic field. 



sin -=- = a similar quantity relating to the galvanometer and 



o 



the earth inductor. 



k = constant of tangent-galvanometer. 



a. = reading of tangent-galvanometer when measuring pri- 

 mary current through the metal strip. 



w = effective width of metal strip. 

 t = effective thickness of metal strip. 



d = difference in readings on the Thomson-galvanometer scale 

 caused by reversing magnet in the main experiment. 



d'= difference in readings on same scale caused by reversing 

 current in determining sensitiveness of the Thomson gal- 

 vanometer. 



©= reading of tangent-galvanometer when measuring current 

 used to determine sensitiveness of Thomson galvanometer. 



p=z proportion of the above current which passes through the 

 Thomson galvanometer. 



r== total resistance of circuit containing Thomson galvanometer 

 during main experiment. 

 The above formula reduces to the form 



<I> 



,, Tr 7460 sin— tan a.d f 11 

 M x V _ 2 



~l<y ~~ 7~¥ r 



tdpr tan © sin -= 



It will be seen that h and w have disappeared. The elimi- 

 nation of w is a very important fact, as this would be an ex- 

 ceedingly difficult quantity to determine with accuracy. As 

 the case stands, it is not at all important to preserve the form 

 of the metal strip after its thickness has been determined. This 

 makes the adjustments of the side connexions (see fig. 1), 

 leading to the Thomson galvanometer, a matter of consider- 

 able ease. 



The following pages give some details of the study of the 

 various metals examined. 



Gold. 



The experiments which furnished the results already pub- 

 lished were made with gold leaf so thin as to be transparent. 



