96 Mr. S. Bidwell. On the Effect of Magnetisation [May 31, 



FIG. 2. 



The curves marked " longitudinally " relate to circumferential changes, along 

 the lines of magnetisation. 



Those marked " transversely " relate to changes in the width, perpendicularly to 

 the magnetisation. 



little or no effect being produced by annealing. Under gradually 

 ascending forces both rings first become narrower, and then recover 

 their original width, and ultimately become wider than when un- 

 magnetised. 



The only previous experiments that I know of relating to magnetic 

 changes of dimensions in directions perpendicular to the magnetisa- 

 tion are those of Joule,* who used a piece of iron gas-piping 1 yd. 

 long and in. in mean diameter, having an insulated wire inserted 

 into it, and bent over the sides, so as to form a magnetising coil of 

 1^ convolutions. The greatest current he used seems to have been 

 about 12 amperes, and the magnetising force therefore about 8 c.g.s. 



* Joule's ' Scientific Papers,' p. 263. 



