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VII. On the. Changes produced by Magnetisation, in the Dimensions of Rings and Hods 



of Iron and of some other Metals. 



By SHELFORD BID WELL. MA., F.R.S. 



Received February 9, Bead March 1, 1888. 



IN a paper communicated to the Royal Society in 1885* I described some effects 

 of magnetisation upon the dimensions of iron rods. It was there shown that the 

 elongation which such a rod at first undergoes when magnetised (a phenomenon which 

 had been carefully studied by JOULE and others) does not, as had been believed, 

 remain unchanged at a maximum when the magnetising force exceeds that which is 

 sufficient to produce so-called saturation. On the contrary, it is found that, when the 

 magnetising force is continually increased beyond this limit, the elongation becomes 

 gradually less until the rod is at last actually shorter than it was in the unmagnetised 

 condition. 



Though there could be little doubt as to the general qualitative reality of the effects 

 described, the experimental evidence was nevertheless not free from certain defects 

 which I was not at the time able to overcome. 



In order to obtain a magnetic field of the highest intensity possible with a limited 

 battery power, a solenoid was used which was not much longer than the rods them- 

 selves, and the field, therefore, was not quite uniform. The effect of the ends of the 

 rods was also uncertain, and might have played some material part in the pro- 

 duction of the phenomena in question. Another serious element of uncertainty 

 arose from the fact that the rods with which the experiments were made were 

 permanently magnetised. Since it was impossible to remove a rod from the instru- 

 ment and demagnetise it before every one of a series of observations, it was thought 

 desirable that the amount of residual magnetism which it contained should at least be 

 the same throughout the series. This object was attained by passing through the 

 coil at the beginning of an experiment a current equal to the strongest which it was 

 intended to use subsequently, the permanent magnetism thus imparted to the iron 

 having been found to be quite unaffected by the passage through the coil of currents 

 weaker or not stronger than the first. The effect of this residual magnetism upon the 

 elongations and retractions was not easily ascertainable. Lastly, the magnetisation 

 might with advantage have been carried considerably further if sufficient battery 

 power had been available. 



'Roy. Soc. Proc.,' vol. 40, 1886, p. 110. 



25.6.88 



