Magnetization and Magnetic Change of Length. 551 



The current was measured by a Siemens-Halske ammeter, 

 which was occasionally compared with a Kelvin ampere- 

 balance. 



The experiment was conducted in the following order. 

 The adjustment of the magnetizing and compensating coils 

 having been completed, the specimen-holder containing the 

 ovoid was fixed vertically in the correct position in the mag- 

 netizing coil. The ovoid was then vertically stretched up- 

 wards by means of a copper wire with a spiral spring, special 

 care being taken to stretch the copper wire in the direction 

 of the axis of the ovoid. The magnetic change of length 

 was then measured in the usual way. The magnetization 

 and the magnetic hysteresis were next observed. Liquid 

 air was next gently poured into the Dewar tube in the 

 magnetizing coil, until the tube was nearly filled with the 

 "liquid ; then the exposed parts above the magnetizing coil 

 were carefully protected with cotton-wool. Owing to the 

 boiling of the liquid, a small oscillation of the image in the 

 field of the telescope was at first observed, but after some 

 ten minutes the image became almost steady. The change 

 of length was then taken. Next, adding more liquid air to 

 that in the Dewar tube, when necessary, the magnetization 

 and the magnetic hysteresis were measured. Lastly, when 

 the specimen was heated to the temperature of the room, the 

 change of length and the magnetization were again noted, 

 After the experiments, the compensation of the magnetizing 

 current was always tested, and found perfect, except in a 

 few cases. 



(a) Magnetization of Ferromagnetic Metals. 



In Table I. the magnetization in different fields is given, 

 where I is the intensity of magnetization, and H the internal 

 field (external field — demagnetizing force). The temperature 

 of liquid air has been assumed to be - 186° C, but owing 

 to the fractional evaporation of nitrogen, as the experiment 

 proceeds, the actual temperature may, according to circum- 

 stances, be greater or less by a few degrees than the above 

 value. The figures of the last row in each column are the 

 residual magnetism when the external field vanishes. 



