MAGNETIZATrON AND MAGNETIC CHANGE OF LENGTH. 31 



II. SECOND EXPERIMENT. 



To obtain a constant low temperature lying between the 

 ordinary and the liquid air temperatures, a method of slow cooling 

 was applied. The specimen-holder in the former apparatus was 

 water-tightly covered with a brass cylinder, and a suitable amount 

 of liquid air was poured into the interspace between the cylinder 

 and the Dewar tube. The temperatures above — 15°C were, 

 however, obtained by dipping the specimen directly into a freezing 

 mixture (snow and common salt) contained in the Dewar tube. 

 The experiment was commenced with the specimen in the annealed 

 state, and the measurements at successively decreasing temperatures 

 were made. During one set of observations, which usually required 

 7 or 8 minutes, the temperature was fairly constant and its change 

 did not exceed one degree in the most unfavorable case. Since 

 the cooling was very slow and the specimen was doubly enclosed 

 in copper and brass tubes, the temperature of the specimen may 

 be regarded as constant throughout its entire length. 



The temperature of the specimen was measured by a thermo- 

 electric couple of platinum and german silver. The wires were 

 insulated with a thin caoutchouc tube. One of the junctions was 

 brought in contact with the specimen at its middle, while the 

 other was insulated with asbestos papers and inserted in a copper 

 tube. This tube was dipped into the water bath, and its tem- 

 perature was observed with a thermometer placed in the bath. 

 The thermoelectric current w^as measured with a low resistance 

 galvanometer. The calibration of the galvanometer was made 

 by using a mercury thermometer and a petroleum-ether ther- 

 mometer. 



