to Reversals of Magnetization in Soft Iron. 217 



entirely in heating the iron, the rise of temperature will be 

 obtained by dividing the number of ergs by Joule's equivalent 

 and the heat-capacity of the iron per unit volume. Taking 

 •11 for the specific heat, 7*7 for the density, and 4*16 x 10 7 

 as Joule's equivalent, we have 



l/(-ll x 7-7 x 4-16 x 10 7 ) = 2°'84 x 10" 8 C. per erg. 

 Whence we get the rise of temperature corresponding to the 

 above values :— I. . . . 0°30 x 10~ 4 C. 

 II. . . . 1-89 



III. . . . 3>87 



IV. . . . 5-25 



The temperatures thus calculated are plotted in fig. 27 

 in encircled points. It is there seen that the curve thus 

 obtained lies above that obtained by direct thermometric 

 measurement. The difference will of course be partly due 

 to experimental imperfection, and partly to uncertainties in the 

 assumed constants, and finally to the real cause, if there is any. 



One apparent term neglected in the calculation, from the 

 want of numerical result in literature, is the heat taken by 

 the cotton covering of the iron wire. This cotton was found by 

 weighing separately to be '72 per cent, of the iron by weight. 

 The effect of introducing this correction will be slightly to lift 

 up the points in the figure 27. But even if we assume its 

 specific heat to be as much as that of water, it will not be 

 enough to make up the difference. 



Another point to be remembered is that the magnetizing 

 field in the direct thermometric experiment was deduced from 

 the square root of the readings of an idiostatic galvanometer, 

 which gives the mean square of an alternating current ; while 

 in the ballistic and magnetometric experiments the field was 

 directly determined. Now when the magnetization is effected 

 gradually or by steps, the hysteresis depends upon the 

 maximum field to which the iron is subjected (or the limits of 

 cycle), so that in comparing the energy in hysteresis with the 

 heating effect caused by alternating field, we ought to have 

 taken the maximum fields in the course of alternations instead 

 of the mean square root. It was found during the experi- 

 ment that when an E.M.F. of 100 volts with high resistance 

 was used, the current, when alternating, was little above 80 

 per cent, of what it w^as when the alternator w T as stopped. 

 From this w r e may infer that the mean square root is not very 

 far from the maximum value w r e are after ; and if we imagine 

 each point in the curve of heating observed in fig. 27 to be 

 somewhat shifted towards the right, they would not be far 

 from corresponding to what actually took place. 



The effect of temperature in changing the magnetic quality of 



