Page:Proceedings of the Royal Society of London Vol 60.djvu/106

of the current passing through the primary coil, and it was found that the mean magnetising force to which the iron was exposed was closely expressed by the value 20-219, multiplied by the ampere current flowing through the primary coil. This coil had its secondary circuit connected up to the galvanometer, as above described, and a series of observations were taken with this coil by reversing a constant magnetising current passing through the primary coil, and observing the throw of the ballistic galvanometer connected with the secondary cii’cuit. The ring coil, together with the platinum thermometer, was embedded, as above described, in a mass of paraffin wax, and the whole mass, after having been cooled down to the temperature of liquid air, was slowly allowed to heat up again. Observations were taken with two different magnetising forces over the range of temperature from —185° C. up to the ordinary temperature, and from the calculated induction in the ring determined for each magnetising force, the permeability was found corresponding to each particular force and temperature. The results of these observations are given in Table IY, and are delineated in fig. 2,' in the form of two curves marked unannealed iron.

Table IY.—Variation of Magnetic Permeability of Unannealed Swedish Iron with Temperature.

Temperature measured in platinum degrees by standard thermometer Pi. Permeability.

Magnetising Magnetising Temperature. force, T78. force, 9*79. 0° 917 1210 % - 20 885 1212 - 40 857 1212 - 00 832 1208 - 80 913 1230 -100 993 1240 -120 1067 1255 -140 1153 1265 —100 1230 1280 -180 1262 1290 -200 1272 1293

The results of the observations, as indicated in fig. 2 in the curves marked Unannealed Iron, show that for this unannealed iron the permeability increases as the temperature falls, and is exactly the reverse in the case of the same quality of iron carefully annealed. The difference, also, between the two materials is very marked