Page:Proceedings of the Royal Society of London Vol 60.djvu/101

86 The results of these observations are given in Table II, and these observations are set out in the curve marked soft annealed iron in fig. 2.

Table II.—Variation of the Magnetic Permeability of Soft Annealed Swedish Iron with Temperature. Magnetising force = 1‘77 C.G.S. Temperature measured in platinum degrees by standard thermometer Pj. Profs. J. A. Fleming and J. Dewar. On the

mperature. Perm eability. 0° 2835 - 20 2815 - 40 2770 - 60 2727 - 80 2675 -100 2622 -120 2560 -140 2497 -160 2438 -180 2381 -200 2332

The results show that as the temperature rises up from —185° C., or — 200° on the platinum scale temperature, up to the ordinary temperature, the permeability of the soft iron for the particular magnetising force selected increases perfectly uniformly, the curve of increasing permeability with temperature being nearly a straight line.

In the next place, we have examined the hysteresis of the same soft iron ring at different temperatures and for different maximum inductions. These observations were carried out by taking a complete series of hysteresis curves with the ballistic galvanometer, gradually increasing the inductions from zero to 12,000. After the complete hysteresis curves were obtained, their areas were carefully integrated with an Amsler planimeter, and the values, reduced so as to express the hysteresis loss in watts per lb. per 100 cycles per second, and these values plotted in terms of the maximum value of the magnetic induction per square centimetre of the iron core corresponding to each particular hysteresis loss. Nothing would be gained by giving the full details of all the observations by which these hysteresis curves were obtained. They were exceedingly numerous, and the tedious nature of the ballistic observations made it a matter of prolonged observation to secure the whole series necessary, but the final results are shown in Table III. The curve in fig. 3 represents the increase of hysteresis loss with induction, and the observations which