60 ON MAGNETISM. 



ed by the action of A in the circumstances considered 

 in the case (a) of Article 20, A being presented broad- 

 side-on to B which is end-on. It is to be remarked that 

 no new torsion of the suspension-thread is introduced 

 by the rotation of B, because the suspension-bar, which 

 is carried by the uprights on F, is made to rotate 

 through the same angle as B. 



The experiment can now be varied by reversing 

 A, end for end, and B will be deflected the opposite 

 way, as in Figure 25 : the mean of the two deflexions 

 will be free from any error in the reading of Figure 

 23 : and, generally, information will be obtained which 

 may assist to remove accidental errors. Also the mag- 

 net may be placed in the position A', whose distance 

 from A can be measured with great accuracy : if in 

 one case the center of B was too near to A, in the other 

 case it will be equally too far from A ; and the mean 

 of the results will be free from the effect of error in the 

 position of the center of B. 



From the mean of these deflexions, the true deflex- 

 ion 6 will be found with great accuracy. And now we 

 remark that the needle B is kept in a definite angular 

 position by the opposition of two forces whose angular 

 moments must be equal. One is, the angular moment 

 produced by A in the case () of Article 20 : its value is 



The other is, the angular moment produced by the 

 Earth, as in Article 21 : its value is 



