Changes produced by Magnetisation in the Length of Rods. 109 



" On the Changes produced by Magnetisation in the Length of 

 Rods of Iron, Steel, and Nickel." By Shelford Bidwell, 

 M.A., LL.B. Communicated by Professor Frederick 

 Guthrie, F.R.S. Received April 1. Read April 23, 1885. 



The earliest systematic experiments on the effects produced by 

 magnetisation upon the length of iron and steel rods were those of 

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

 The experiments were made with bars 36 inches long, which were 

 placed inside a solenoid 38 inches long ; and the variations in the 

 length of the bars when currents of electricity were passed through 

 the solenoid were measured by means of a delicate micrometer, each 

 division of which indicated a change of Yz'wt^s inch. 



Using bars of iron and soft steel, he found that their length was 

 increased by magnetisation, the elongation varying up to a certain 

 point as the square of the intensity of the magnetisation, temporary or 

 permanent, of the bar, and he remarked that the elongation was, for 

 the same magnetisation, greater in proportion to the softness of the 

 metal. 



When the metal was hard steel it appeared that " the bar was 

 slightly increased in length every time that contact with the battery 

 was broken." On passing the first current through the magnetising 

 coil the length was unaffected, but when the circuit was broken after 

 the passage of this current there occurred a small elongation 

 equivalent to a fifth of a micrometer division, and each succeeding 

 make and break of the current was accompanied by a further small 

 elongation. 



These experiments were made with currents of gradually increasing 

 strength ; Joule appears never to have tried what would be the effect 

 of applying the same current twice in succession. Had he done so 

 there is reason to believe, as will appear hereafter, that effects of a 

 somewhat different character would have been observed. He attri- 

 buted the increase in length when the current was interrupted " to 

 the state of tension in the hardened steel," adding that he " found 

 that soft iron wire presented a similar phenomenon when tightly 

 stretched." 



The phenomena were, however, not exactly identical in the two 

 cases. From the account which he proceeds to give of his experi- 

 ments with stretched wires, it appears that when the tension of the 

 wire exceeded a certain limit, the effects produced by the current 

 were exactly the opposite of those which occurred when the wire was 

 unstretched ; magnetisation, instead of causing the wire to lengthen 

 temporarily, caused it to shorten, while it resumed its original length 



