Galvanometer Deflection. 



411 



of windings, partly of tliin and partly of thick wire, is about 

 2668, making up a resistance of about 68 ohms. The chan- 

 nels are cut in the same piece of boxwood, along the axis of 

 which is drilled a cylindrical hole in which hangs the magnet.. 

 The ends of this cylindrical cavity are closed by two caps, one 

 of which is fitted with a plano-convex lens, the other with a 

 piece of plane-parallel glass. The torsion-head is fitted with 

 a "Weber's suspension-screw in the usual way. The whole ox 

 the upper part is supported on a foot, with placing screws in 

 such a way that the coils can be turned about a vertical axis. 



The magnet consisted of a piece of silvered glass fitted to a 

 brass frame, with weights for increasing the moment of inertia. 

 To the back of this was fitted a thin piece of magnetized watch- 

 spring about 10 millims. long and 2 millims. broad. Under 

 the earth's force the period of the needle's oscillation was 22'2 

 sec. This was raised to 49*2 sec. by properly weakening the 

 field. 



The deflection of the magnet was measured by means of a 

 scale and telescope in the ordinary way, the position of rest 

 being deduced from three elongations. The deflection of the 

 coils was measured by means of the faint image of the scale 

 from the plate-glass cap which closes the cylindrical core. 



The battery used was six cells of Smee ; and the primary 

 circuit was made and broken by means of an electric tuning- 

 fork lent me by Mr. Dew Smith, driven by the primary cur- 

 rent itself. The induction-coil was of the ordinary construc- 

 tion, the resistance of the secondary being about 2714 ohms. 



By making an observation with the planes of the plate-glass 

 cap and mirror very nearly parallel, the value of the correction 

 for the deviation of magnetic axis of needle in this position 

 from the true plane of the coils was found. Using this cor- 

 rection, a series of observations with different deflections of 

 the coils gave the following result : — 



ct. 



9. 



C. 



O J 



O i 





+ 1 28 



1 55 



•1175 



+ 4 



5 13 



•1172 



+ 5 



6 29 



•1156 



+ 5 16 



6 54 



1195 



-4 34 



6 3 



1225 



Considering the means employed in obtaining them, these 

 values of C, differing from the mean by less than 4 per cent., 

 agree as well with each other as was to be expected. The two 

 disturbing elements were the inconstancy of the battery and 



