128 



Mr. S. Bidwell. On the Changes produced by 



steel with currents of less than the maximum strength, and a smaller 

 magnetising force would perhaps have produced elongation, unless 

 indeed the permanent magnetisation induced by the first current 

 equalled or exceeded the critical value. This was almost certainly 

 the case with his yellow-tempered steel, which was permanently 

 shortened by the first magnetisation, while all the other specimens 

 were permanently elongated. These considerations are consistent 

 with all the phenomena exhibited by Mayer's steel bars. 



In working with a rod of steel which had been neither annealed 

 nor hardened, I obtained some very curious effects of which I am not 

 at present prepared to offer a complete explanation. I therefore 

 describe the experiments exactly as they were performed, without 

 attempting to account for the results. 



'Experiment 1. — A current of 2 amperes was passed through the 

 coil, whereupon the rod elongated 3 scale divisions. Without 

 breaking the circuit, the current was reduced by inserting resistance 

 to 0*6 ampere. The rod underwent a further elongation of 3 divi- 

 sions, making the total elongation equal to 6 divisions. On breaking 

 the circuit the rod retracted 6 divisions, returning to its original 

 length ; but when the circuit was again closed, the resistance still 

 being inserted and the current consequently 0*6 ampere, the resulting 

 elongation was only 3 divisions. 



It appears therefore that a strong magnetising force subsequently 

 diminished causes a greater temporary elongation than the diminished 

 force is capable of producing if applied in the first place. 



Experiment 2. — A current of 2 amperes being passed through the 

 coil, an elongation of 3 scale divisions was produced. The current 

 was reduced to 026 ampere, when a further elongation of 1 division 

 occurred. On breaking the current the rod returned to its original 

 length. Once more a current of 026 ampere was passed through the 

 coil, but no movement whatever occurred. 



From this it appears that the temporary elongation of a steel rod 

 when once produced may be maintained by a magnetising force which 

 is itself too small to cause any perceptible elongation whatever. 



Something of the same kind, though in a smaller degree, was 

 observed by Mayer in rods of iron. 



Both these experiments were repeated many times, the results 

 being invariably of the same character, and there is no doubt what- 

 ever as to the reality of the effects described. 



On a small scale, I have repeated some of Joule's experiments with 

 stretched wires, and found, as he did, that when a wire was loaded 

 with a certain weight, the effect of magnetisation was not elongation 

 but retraction. No measurements, however, were attempted, my 

 apparatus not being well adapted for the purpose. 



It appeared, upon consideration, that the results of this class 



