under the Operation of Feeble Magnetic Forces. 235 



the arrangements were unaltered, except that an additional 

 reversing-key was introduced, by which the poles of the 

 Daniell cell could be interchanged. The total resistance of 

 the circuit, independently of the box, was 7 ohms. The 

 length of the core — or, rather, of the part exposed to the 

 magnetizing force* — being about 100 diameters, is scarcely 

 sufficient for an accurate determination : but from the observed 

 position necessary for the compensating-coil we can get at 

 least a rough estimate of the susceptibility for small forces. 

 Thus, on December 28th, there was compensation for small 

 forces when the distances of the needle from the mean plane of 

 the compensating-coil and from the operative pole of the iron 

 core were respectively 17'2 centim. and 9*3 centim. The 

 magnetic force at the needle, due to unit current in the com- 

 pensating-coil, is 



2tt x 74 x 9 2 _ . 



{9 2 + 17'2 2 }* ' 



The magnetizing force in the interior of the helix for unit 

 current is 



4tt x 65 = 817. 



If k be the susceptibility, the strength of the pole is 



J;7rx-330 2 x817x&; 



and since the distance of this from the needle is 9*3 centim., 

 we have, to determine k, 



5-15 x9-3 2 _ 

 i7rx-330 2 x817 ; 



so that 



/*=1+47T& = 81. 



This is probably an underestimate. 



In order to obtain results comparable with those of Stoletow 

 and Rowland, the iron was submitted to a series of cycles of 

 positive and negative force. According to Ewing, the 

 behaviour is simplest when the iron is first treated to a 

 process of " demagnetization by reversals/' This was effected 

 in situ as a preliminary to the experiments of January 4th, 

 the resistance in the box being increased by small steps from 

 a few ohms to a thousand ohms ; while at each stage the 

 battery was reversed several times. It must be remarked, 

 however, that the iron was all the while under the influence 

 of the earth's vertical force ; so that the resulting condition 

 was certainly not one of demagnetization. But even as thus 



* At the upper end the iron projected beyond the coil. 



