112 Mr. A. P. Chattock on determining experimentally 



F, F are the fixed coils of the dynamometer; S the revolving 

 coil to be afterwards suspended within them, and which makes 

 contact once in every revolution with M and N; B a battery 

 sending a current C through the coils F by the commutator L. 

 G is an astatic mirror-galvanometer of about the same resist- 

 ance as the coil S; and R is the resistance between M and N", 

 adjustable by the slider M. 





Let i be the intensity of the magnetic field, at the place 

 where S revolves, due to unit current in the fixed coils F; 

 H the horizontal intensity of the earth's magnetism at the 

 same place; a the effective area of the coil S; and k a factor 

 depending on the relative positions of the two coils when the 

 revolving coil is in contact with M and N through G. iak is 

 then the geometrical factor referred to above. 



Suppose the coil S rotates w r ith an angular velocity to, while 

 a current of strength C flows through the fixed coils. An 

 E.M.F. is set up in S equal to 



Giacok + Hacokf. 



If the resistance R between M and N be now adjusted till no 

 current flows through G, this E.M.F. must be equal to the 

 difference of potential between M and N due to the current, 

 viz. CR ; that is, 



Giorook + Hacott = CR. 



Com mutating the current at L and adjusting R, we get 



- Giacok + Kcrcok' = - CR'; 

 and from these two equations, 



R + R x 



iak — 



2ft> 



