series of primary currents was successively passed through the primary coil and reversed, the throw of the ballistic galvanometer being noted in each case. In the first set of observations the ring was kept at the ordinary temperature of the air, 15° C., and in the second set it was immersed in liquid air, and the following table shows the results, both for the high and for the low temperature observations.
After taking a complete magnetisation curve at the ordinary temperature, the ring was immersed in liquid air, bringing its temperature down to about —185° C., and a complete series of observations taken again in the same manner, previously having first carefully
Table I.—Magnetisation Curve of Annealed Soft Iron (Sankey’s Transformer Iron).
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A t 15° C. A t —186° C. (in liquid air). V M agnetising force. H. Induction. B. Perm eability. ix. M agnetising ! force. | H . Induction. B. Perm eability. A4- ' 0 -725 1000 1379 0 *841 1000 1189 0*971 2000 2060 1*174 2000 1704 1*174 3000 2555 1*407 3000 2132 1*378 4000 2903 1*595 4000 2508 1*595 5000 3135 1*886 5000 2651 1 *840 6000 3261 2*145 6000 2797 2*10 7000 3333 2*440 7000 2869 2*58 8000 3101 2*99 8000 2675 3*35 9000 2687 3*83 9000 2350 4*47 30000 2237 5*08 10000 1968 6*27 11000 1754 7 *05 11000 1560 8*99 12000 1335 9*72 12000 1234 12*35 13000 1053 13*11 13000 992 i 17 *22 14000 813 17*90 14000 782 22 *1 14400 652 21 *35 14300 670
demagnetised the ring as described by an alternating current. The ring was then taken out of the liquid air, allowed to warm up again to the ordinary temperature, and another complete set of observations taken at the ordinary temperature. In this manner a series of eighteen complete sets of observations were taken, about half of them being at lo° C. and half of them at —185° C. In cooling the ring in liquid air, it was found to be important to cool it slowly by holding it some time in the dense gaseous air lying over the liquid air. If suddenly plunged into liquid air the iron becomes hardened. It was found that after the first five sets of observations, which were some-
