216 MB. S. BIUWELL ON CHANGES PRODUCED BY MAGNETISATION IN 



would under higher forces exhibit the same peculiarity as that of cobalt. A com- 

 parison of the three curves seems to favour such a conjecture, but I cannot test it 

 with the apparatus at present at my disposal. w 



No result was anticipated from the experiment with the bismuth ; but with high 

 forces it was found to exhibit elongation. The effect, though small, was quite 

 unmistakable, and even roughly measurable. It was certainly not due to heat, the 

 elongation occurring instantly when the current was turned on, and disappearing as 

 suddenly when the current was stopped. Nor do I think it could be accounted for 

 by the presence of iron as an impurity. The results may be stated as follows : 



Magnetic force. Elongation. 



280 .:.:.....,. .... Suspected. 



470 .,-..-,. Quite perceptible. 



680 . f , 0'25 scale div. = 0'3 ten-millionths. 



842 1'25 scale div. = T5 ten-millionths. 



Thus it appears that when the experiment was discontinued the elongation was 

 increasing more rapidly than the field. 



No alteration in the rod of manganese steel could be detected with forces up to 

 700 units. With 850 units there was a perceptible elongation, estimated at about 

 one-tenth of a scale division, or one fifty-millionth part of the length of the rod. 



Before the experiment there was no indication of any attraction between the steel 

 and a delicately-pivoted compass needle, 22 mm. long, at a distance of two or three 

 mm. After the magnetisation the steel had acquired sufficient polarity to draw the 

 needle 20 from the magnetic meridian when held about 3 mm. from the opposite pole 

 of the needle. 



It must be admitted that it is not easy to form a plausible conjecture as to the 

 physical causes of these magnetic extensions and retractions. It has been customary 

 to explain the extension of soft iron by supposing its component molecules to be 

 small magnets of elongated form, which, under the influence of a magnetising force, 

 tend to set themselves in one direction ; but to this view there are obvious objections. 

 In any case, it seems probable that there are at least two influences, or sets of 

 influences, at work, the one tending to cause extension, the other retraction. In the 

 early stages of the magnetisation of iron the former predominates, but as the 

 magnetisation advances it is overcome by the latter. 



One of the influences tending to produce retraction must certainly be of a purely 

 mechanical nature. 



Suppose a uniformly magnetised rod to be transversely divided through the middle. 

 The two halves, if placed end to end, will be held together by their mutual attraction, 



* [I hope to have the opportunity of repeating all my experiments with more powerful currents than 

 those hitherto used. May 14th, 1888.] 



