266 Mr. S. Bidwell. On the Changes produced by [Apr. 23, 



Joule, an account of which is published in the "Phil. Mag." of 1847. 

 Joule's experiments have many times been repeated, and his general 

 results confirmed. In particular, Prof. A. M. Mayer carried out a 

 series of very careful observations with apparatus of elaborate con- 

 struction and great delicacy.* The conclusions at which he arrived 

 were in accord with those of Joule, so far as regards iron : in the case 

 of steel there was some apparent discrepancy, which, however, might 

 to a great extent be accounted for by differences in the quality of the 

 metal used, and in the manner of conducting the experiments. In 

 1882 Prof. Barrett published in " Nature " an account of some experi- 

 ments which he had made, not only on iron but also on bars of nickel 

 and cobalt, with the view of ascertaining the effect of magnetisation 

 upon their length. 



The knowledge on the subject up to the present time may be sum- 

 marised as follows : — 



1. Magnetisation causes in iron bars an elongation, the amount of 

 which varies up to a certain point as the square of the magnetising 

 force. When the saturation point is approached the elongation is less 

 than this law would require. The effect is greater in proportion to 

 the softness of the metal. 



2. When a rod or wire of iron is stretched by a weight, the elongat- 

 ing effect of magnetisation is diminished ; and if the ratio of the 

 weight to the section of the wire exceeds a certain limit, magnetisa- 

 tion causes retraction instead of elongation. 



3. Soft steel behaves like iron, but the elongation for a given 

 magnetising force is smaller (Joule). Hard steel is slightly elongated, 

 both when the magnetising current is made and when it is interrupted, 

 provided that the strength of the successive currents is gradually 

 increased (Joule). The first application of the magnetising force 

 causes elongation of a steel bar if it is tempered blue, and retraction 

 if it is tempered yellow : subsequent applications of the same external 

 magnetising force cause temporary retraction, whether the temper of 

 the steel is blue or yellow (Mayer). 



4. The length of a nickel bar is diminished by magnetisation, the 

 maximum retraction being twice as great as the maximum elongation 

 of iron (Barrett). 



In order that the results of Joule and Mayer might be comparable 

 with those obtained by the author, he made an attempt to estimate 

 the magnetising forces with which they worked. From data con- 

 tained in their papers, it was calculated that the strongest magnetising 

 force used by Joule was about 126 units, while the strongest used by 

 Mayer did not on the highest probable estimate exceed 118 units. In 

 the author's experiments the magnetising force was carried up to 

 about 312 units. The metal rods, too, were much smaller than any 

 * " Phil. Mag.," 1873, vol. xlvi, p. 177. 



