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Mr. S. Bidwell. On Changes produced by [Mar. 25, 



cases diminished, and if the weight were considerable, magnetisation 

 caused retraction instead of elongation. From these facts he appears 

 to have formed the conclusion that, under a certain critical tension, 

 (differing for different specimens of iron, but independent of the 

 magnetising force) magnetisation would produce no effect whatever 

 upon the dimensions of the wire. In one of his experiments* a cer- 

 tain iron wire loaded with a weight of 408 lbs. was found to be 

 slightly elongated when magnetised ; the weight was then increased 

 to 740 lbs. with the result that magnetisation was accompanied by a 

 slight retraction. In both cases the magnetising currents varied over 

 a considerable range, and the smaller ones were without any visible 

 effect. Commenting upon these results, Joule conjectured that " with 

 a tension of about 600 lbs. [which number I suppose is selected as 

 being roughly the mean of 408 and 740] the effect on the dimensions 

 of the wire would cease altogether in the limits of the electric currents 

 employed in the above experiments, "f 



In reference to this surmise % I ventured the following remark : — 

 "If he had actually made the experiment, he would perhaps have 

 found that the length of the wire was increased by a weak current, 

 that a current of medium strength would have had no effect whatever, 

 and that one of his stronger currents would have caused the wire to 

 retract." I had, in fact, reason to believe that the effect of tension 

 was to diminish the "critical magnetising force" (which produces 

 maximum elongation) so that the retraction which is found to occur 

 in all iron rods when a sufficient magnetising force is employed, is 

 observed with smaller magnetising currents when the rod is stretched 

 than when it is free,§ but want of suitable apparatus prevented me 

 from submitting this idea to the test of direct experiment. 



I have lately modified the instrument, which is described in my 

 former paper, in such a manner that it can be used for observing the 

 effects of magnetisation upon rods and wires under traction. The 

 working part of it is shown in diagrammatic section in the annexed 

 figure. The coil CO contains 876 turns of copper wire, 1*22 mm. in 

 diameter, wound in 12 layers on a brass tube with boxwood ends. To 

 the lower end A of the tube is fitted a brass plug or stopper, having 



* Eeprint, p. 254. 



+ These currents produced deflections ranging from 6° to 58° on his tangent 

 galvanometer, which " consisted of a circle of thick copper wire one foot in diameter, 

 and a needle half an inch long furnished with an index." 



X J oule's conjecture is sometimes quoted as if it were an experimental fact. See 

 Chrystal's article on Magnetism, " Enc. Brit.," vol. xv, p. 269. 



§ My belief was principally founded upon the fact that while the critical mag- 

 netising force appeared in all the cases which I had examined to be about twice that 

 corresponding to the " turning-point" in the magnetisation curve, the turning-point 

 itself occurred at an earlier stage when the wire was stretched than when it was 

 unstretched. 



