62 Dr. S. Bidwell. On the Magnetic Change* of [May 16, 



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e maximum retraction occurs in a much weaker field. The curve(-/) 

 Hnnealed strip indicates a maximum retraction greater in degree 

 Breached in about the same field. The ascending limbs of 



Fie. 2. 



Boiled cobalt, (c) unannealed, (d) annealed. 



the two curves are nearly parallel, but it is probable that the curve for 

 the annealed strip would in very strong fields become asymptotic, never 

 meeting the axis of H ; an observation made with a field of 1750, 350 

 units beyond the limit of the diagram, showed that the retraction was; 

 still 9 ten-millionths of the length. 



The results of two experiments made with nickel before and after 

 annealing are given in curves (e) and (/), fig. 3. The piece used was a 

 thick wire 9 cm. in length and 0'35 cm. in diameter. After the first 

 series of observations had been made, the wire was annealed in exactly 

 the same manner as the cobalt, and again tested, with the result shown 

 in curve (/). The modification effected in the form of the retraction 

 curve is quite similar to that which annealing produces in the curve of 

 magnetisation, and is probably due to nothing more than increased 

 magnetic susceptibility. No such simple explanation can of course be 

 given of the changes which are brought about in the behaviour of iron 

 and cobalt. The fact that the two nickel curves intersect in a field of 

 1150 units may be merely an effect of temperature. The magnetic 

 retraction of nickel is, as I have remarked in former papers, sensibly 

 affected by small changes of temperature, and the intersection would be 

 accounted for if the annealed wire happened to be a few degrees warmer 

 than the unannealed. 



As a consequence of the reciprocal relation between the changes of 



