452 ' Messrs. Howland and Hutchinson on the 



.'. The force 



-_^ 



and for the two, 



where the sign of Ihe entire expression has been changed, 

 since the poles of the upper and lower needles are opposite. 

 Or 



Needle constant : — 



For the disk-galvanometer windings have in the same way, 

 for the lower needle, force due to current I in one turn 



= 27rl r 



{a' + P)"^ 



= 27rI.C. 

 For the four turns, 



X' = 87rI.C. 



Upper needle. — The force is got in the same way as for the 

 disk, omitting the integration, i. e. we must multiply the general 



term of B by ^ and replace 27r -^ by I. This gives 



a replacing c, and p, r. 

 For the total force, 



or 



X/ = 87rI.D. 



