378 



Mr. J. W. Gemmell. 



[Dec. 10, 



resultant of these two forces is a force R, parallel to the bar, and of 



value 2F/- 



(r» + o*)* 



v * that is / 



A '[force, equal and parallel to this, but in the opposite direction, 

 may be similarly shown to act on the pole n of the needle. Thus the 

 needle is acted on by a " couple," the value of which is — 



r>7^» M Mm n 



2bf cos 0, that is — — cos 9. 



To balance this " couple " we have, as before, another " couple," 

 mH sin|0. Hence, equating these, we get — 



M=(r2 + a2)fHtan^ (2.) 



Now, in these experiments, r is 100 cm., a 7*6 cm., and H was 

 found to be 0*155. Substituting these values in equation (2), we 



* B2= 2 1 -£L V (1 * cosS*N) j but cos SsK = cosHSs^-sm^$sTX= r lz^: . hence 

 F/ a - M 



