376 



Mr. J. W. GemmelL 



[Dec. 10, 



total, and those marked B the residual magnetisation. Beginning at 

 zero, we pass to the right, gradually increasing the magnetising force 

 until we reach our limit. Then returning, we pass through zero, and 

 with the opposite magnetising force proceed to the left limit, from 

 which we again return to zero. 



The first experiments were made upon the wires, the results of 

 which are given in Curves I, II, III, and IV. These wires were after- 

 wards annealed and retested, with the results shown in Curves V to 

 VIII inclusive. The remaining Curves IX, X, and XI, contain the 

 results obtained from the bars. 



The results given by the curves maybe reduced to absolute measure 

 by means of the figures on each, which were obtained in the following 

 manner : — 



Let— 

 H 



M 



m 

 F 

 / 



e = 



the horizontal component of the earth's magnetic xorce. 



the magnetic moment of the wire or bar. 



the magnetic moment of the magnetometer needle. 



the strength of a pole of the wire or bar. 



the strength of a pole of the needle. 



the distance of the centre of the wire or bar from that of 

 the needle. 



half the distance between the poles of the wire or bar. 

 half the distance between the poles of the magnetometer 

 needle. 



the angle of deflection of the magnetometer needle. 



The position of the wire, with regard to the magnetometer in the 

 experiments on the wires, is shown in fig. 2. 



Fig. 2. 



H 1 



T ! 









-\ 



N 















j H 









The pole N of the wire attracts the pole 6- of the needle with 



a force 



w 



w 



(r + o) 



(r-a)' 



and the pole S repels the pole s with a force 



2, b being so small in comparison with r that the poles of the 



