

Magnetic Permeability, CJT., of Iron at Low Temperatures. 88 



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 

 observa/tions. 



After taking a complete magnetisation curve at the ordinary tem- 

 perature, the ring was immersed in liquid air, bringing its tempera- 

 ture 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). 



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- 



