84 Profs. J. A. Fleming and J. Dewar. On the 



what variable, the annealed iron ring was brought into a completely 

 stable condition, in which the curve of magnetic induction plotted in 

 terms of magnetising force taken at the low temperature was different 

 from that taken at 15 C. by a perfectly constant amount, the observa- 

 tions at the low temperature always lying on one curve, and those at 

 the higher temperature always lying closely on the other curve. In 

 the diagram in fig. 1 the two magnetisation curves are shown, the 

 firm line curve being the magnetisation curve at 15 C., and the 

 dotted curve being the magnetisation curve taken at 185 C. in the 

 liquid air. The figures in Table I are the mean values obtained from 

 the curves plotted from the thirteen sets of closely consistent observa- 

 tions. These curves show that the permeability of soft annealed iron 

 is reduced when it is cooled to about 200 below zero, for the whole 

 range of magnetic forces between zero and 25 C.G.S. units. The 

 permeability curves for the two states are likewise similarly shown on 

 the same chart. The maximum permeability for this iron corresponds 

 with a magnetising force of about 2 C.Gr.S. units; the maximum 

 permeability at the ordinary temperatures for this iron is 3400, being 

 reduced to 2700 when the iron is cooled to the temperature of liquid 

 air. The percentage reduction in permeability becomes less as the 

 magnetising force is increased beyond or reduced below this critical 

 magnetising force. These experiments were repeated, as above 

 stated, many times very carefully with this ring of annealed soft 

 Swedish iron, and also with a second ring of the same kind, and 

 have invariably shown the same results, viz., that the permeability 

 of soft annealed iron is decreased by being cooled to this low tem- 

 perature within the range of magnetising forces from to 25. It 

 will be seen that the highest induction reached in the case of this 

 iron is 14,500 C.Gr.S. units, corresponding to a magnetising force of 

 25. This iron is of very high magnetic quality, and is of the same 

 character as that which is much used for the construction of alterna- 

 ting current transformers in commercial use. * 



A series of experiments was then made with the same transformer, 

 keeping the magnetising forces constant, but allowing the iron to rise 

 gradually in temperature up from the temperature of liquid air to 

 15 C. In these experiments the transformer was embedded in a 

 mass of paraffin wax with a platinum wire resistance thermometer 

 also embedded in the same mass in close contact with the ring coil. 

 The paraffin wax encasing the ring coil and thermometer having 

 been cooled down to the temperature of liquid air by immersing it in 

 a large bath of the liquid air, it was then lifted out and placed in a 

 vacuum-jacketed test-tube, so as to heat up with extreme slowness, 

 and a series of observations taken by reversing a constant magneti- 

 sing force at intervals, and observing at the same instant the tem- 

 perature of the ring coil as given by the platinum thermometer. 



