1885.] Magnetisation in the Length of Rods of Iron, fyc. 207 



which had been before used for the purpose, ranging in diameter from 

 1*40 to 6*25 mm. Their length was in every case 100 mm., and the 

 apparatus was capable of measuring with tolerable certainty an 

 elongation or retraction equal to a ten-millionth part of this length. 



By using thinner iron rods and greater magnetising forces than 

 those previously employed, the following curious and interesting fact 

 was established. If the magnetisation be carried beyond a certain 

 critical point the consequent elongation, instead of remaining station- 

 ary at a maximum, becomes diminished, the diminution increasing 

 with the magnetising force. If the force is sufficiently increased, a 

 point is arrived at where the original length of the rod is totally 

 unaffected by magnetisation : and if the magnetisation be carried still 

 further, the original length of the rod will be reduced. It also 

 appeared that the position of the critical point in steel depended in a 

 very remarkable manner upon the hardness of temper of the metal ; 

 considerable light is thus thrown on the apparently anomalous results 

 obtained by Joule and by Mayer. Further experiments disclosed 

 strong reason for believing that the value of the critical magnetising 

 force in a thin iron rod was greatly reduced by stretching ; this would 

 explain the fact that Joule obtained opposite effects with stretched 

 and unstretched wires. 



By ascertaining the relative values of the temporary moments 

 induced by gradually increasing external magnetising forces, an 

 attempt was made to connect the point of maximum elongation with a 

 definite phase of the magnetisation of the several rods in which the 

 elongation had been observed. 



Though more experiments must be made before it is possible to 

 generalise from them with perfect safety, the results so far obtained 

 by the author indicate the laws given below. The elongations and 

 magnetisations referred to are temporary only ; before the beginning 

 of an experiment the rod was permanently magnetised by passing 

 through the magnetising coil a current equal to the strongest sub- 

 sequently used. In iron the greatest elongation due to permanent 

 magnetisation was generally found to be about one-third of the total 

 elongation, while in nickel the permanent retraction amounted only to 

 about one twenty-fifth part of the whole. 



I. Iron. 



1. The length of an iron rod is increased by magnetisation up to a 

 certain critical value of the magnetising force, when a maximum 

 elongation is reached. 



2. If the critical ,value of the magnetising force is exceeded, the 

 elongation is diminished until with a sufficiently powerful magnetising 

 force the original length of the rod is unaffected, and if the force is 



