Magnetic Permeability, fyc., of Iron at Low Temperatures. 81 



derived from our standard thermometer, denoted by 196'7. This 

 would show, therefore, that the temperature coefficient as usually 

 defined is O000884 between 35 and 0.* 



These observations are specially interesting as giving additional 

 proof that in the case of a metal of known purity the variation of 

 resistivity, as the metal is continuously cooled, is such as to indicate 

 that it would in all probability vanish at the absolute zero of tem- 

 perature. In the case of mercury, we are able to obtain a metal in a 

 state of almost perfect chemical purity, and which, when continuously 

 cooled, passes into the solid condition under circumstances which are 

 entirely favourable to the prevention of stresses in the interior of the 

 metal, due to cooling. These measurements, therefore, afford a 

 further confirmation of the law which we have enunciated as a 

 deduction from experimental observations, that the electrical resis- 

 tivity of a pure metal vanishes at the absolute zero of temperature. 



"" On the Magnetic Permeability and Hysteresis of Iron at 

 Low Temperatures." By J. A. FLEMING, M.A., D.Sc., 

 F.R.S., Professor of Electrical Engineering in University 

 College, London, and JAMES DEWAR, LL.D., F.R.S., 

 Fnllerian Professor of Chemistry in the Royal Institution, 

 &c. Received May 27, Read June 11, 1896. 



Although considerable attention has been paid to the changes 

 produced in the magnetic properties of iron, particularly its magnetic 

 permeability and hysteresis, at ordinary and at higher temperatures, 

 but little information has been obtained up to the present on the 

 behaviour of iron and steel as regards magnetic properties when 

 cooled to very low temperatures. By the employment of large 

 quantities of liquid air we have been able to conduct a long series of 

 experiments on this subject, the results of which we propose here 

 briefly to summarise, leaving for a future communication fuller 

 details and discussion of the results. The experimental work has 

 consisted in making measurements, chiefly by ballistic galvanometer 

 methods, of the permeability and hysteresis loss in certain samples of 

 iron and steel, taken in the form of rings or cylinders. The first 

 experiments were concerned with the variation of the magnetic 

 permeability of .soft iron under varying magnetic forces, the iron 

 being kept at a constant low temperature, obtained by placing it in 

 liquid air in a state of very quiet ebullition in a vacuum vessel. 



* This is in close agreement with the values obtained by Guillaurae, Mascart, 

 and Strecker for temperatures between 0C. and +30 C. 



