MAGNETIZATION AND MAGNETIC CHANGE OF LENGTH. 



was then vertically stretched upwards by means of a copper wire 

 with a spiral spring, special care being taken to stretch the copper 

 wire in tlie 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 oscilla- 

 tion 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 magnetiza- 

 tion 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) IVIagnetization of Ferromagnetic IVietals. 



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 circumstances, be greater 

 or less by few degrees than the above value. The figures of the 

 last row in each column are the residual magnetism, when the 

 external field vanishes. 



