Magnetisation in the Length of Metal Hods. Ill 



altogether distinct. The permanent magnetisation so produced was 

 found by Mayer to impart a small permanent elongation to rods of 

 soft and blue-tempered steel, and a small permanent retraction when 

 the steel was tempered yellow. Mayer's paper also contains some 

 new facts relating to details of minor importance. 



In 1882 Professor Barrett published an account in " Nature," 

 vol. xxvi, p. 585, of some experiments which he had made not only 

 on iron but also on bars of nickel and cobalt, with a view of 

 ascertaining the effect of magnetisation on their length; cobalt, he 

 discovered, behaved like iron, but the elongations were smaller; 

 nickel, however, retracted under the influence of magnetisation, the 

 amount of its retraction being twice as great as the elongation of iron 

 under similar circumstances. 



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

 summarised as follows : — 



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

 w r hich varies up to a certain limit 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 

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

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

 tisation 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 be 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 obtained by Joule and Mayer might be 

 comparable with those of my own experiments, I have made an 

 attempt to estimate the magnetising forces with which they worked. 



In the first series of Joule's experiments — those in which he ob- 

 served the elongation of iron and steel rods not under traction, he 

 used a coil of the following dimensions : — 



Length of coil 38 in. = 96 "5 cm. 



Internal diameter .... 1*5 „ = 3 "8 ., 

 Length of wire 110 yds. =10,058 „ 



