126 



Mr. S. Bidwell. On the Changes jwoduced; by 



quently used. The relations of the magnetising force and temporary 

 moment appear in Table III and in the last curve of fig. 3. In this 

 experiment the distance between the steel rod and the galvanometer 

 magnet was 15 cm. 



In the light of these experiments I have endeavoured to find an 

 explanation of the anomalous results obtained by Joule and by Mayer 

 with hard steel. It will be remembered that Joule, using gradually 

 increasing currents, found that (after the first current, which produced 

 no effect whatever while it circulated, but was followed by a small 

 elongation when it had ceased) his hard steel bar was slightly 

 elongated both when the current was made and when it was broken, 

 the length of the bar being thus continuously increased. Though I 

 have made many attempts, using steel rods in different conditions, to 

 obtain Joule's results, I have never succeeded in finding a rod which 

 behaved in the manner described by him. Below I give Joule's table, 

 and also a diagram in which I have plotted his results, the abscissae 

 representing the magnetic intensity of the bar, and the ordinates the 

 corresponding elongations. Both are on an arbitrary scale : — 



Table IV (Joule's). 



Deflection of Magnetic Elongation 



galvanometer. intensity. of bar. 



39° 0' I'll 



1-36* 0-2 



52 35 4-09 1-0 



2-85 1-3 



60 15 5-10 1-8 



3*52 1-9 



69 45 5-91 ........ 2-5 



' 4-20 27 



It clearly appears that the elongations due to permanent magne- 

 tisation and to permanent + temporary {i.e., total) magnetisation lie 

 upon separate curves. And, since the total curve is below the perma- 

 nent curve, it follows that the temporary magnetisation has a nega- 

 tive or retracting effect. Taking ordinates equal to the differences 

 between those of the permanent and total curves, I have plotted the 

 curve for temporary magnetisation, which, of course, lies on the 

 negative side of the horizontal axis, and starts from the point repre- 

 senting a magnetic intensity of I'll. Following the analogy of 

 other experiments, I have continued the curve above the horizontal 

 axis representing this part of it by a dotted line ; it is probable that 



* It is very extraordinary that the " magnetic intensity " of the bar should be 

 greater after the current had been cut off than it was when the current was flowing. 

 Joule makes no reference to the fact. 



