564 Prof. Rutherford and Mr. Soddy on 



sliding-resistance. By means of the latter the current was 

 kept always constant at O900 amp. The potential-leads 

 were connected with a Weston millivoltmeter which registered 

 at ordinary temperature a deflexion of about 6 millivolts, on 

 60 divisions on the scale, at the temperature of liquid air 

 rather less than 2 millivolts. This corresponds to a resistance 

 at ordinary temperature of about *01 ohm., and it can readily 

 be shown that the heating effect of the current in the spiral 

 is negligible. No substance seems to be known which is 

 liquid at the ordinary temperature and remains liquid at the 

 temperature of condensation of the radium emanation (about 

 — 150°). Ethyl chloride most nearly fulfils this condition, 

 but solidifies in the neighbourhood of —140° C. A bath of 

 this substance, however, proved useful in one series of mea- 

 surements with the thorium emanation. The rest of the 

 determinations were carried out in a bath of liquefied ethylene. 

 This boils at — 103 o, 5 and freezes at —169°, and gave just 

 the range of temperature desired in these experiments. About 

 70 litres of the gas, purified by fractional distillation, was ordi- 

 narily used. This was sent into the apparatus by the tube A^ 

 VI. XIV. fig. 1, escaping by the tube carrying the stirring- 

 rod. The liquefied ethylene always covered the top of the 

 spiral to the depth of several centimetres. The apparatus 

 was surrounded by a tall copper cylinder well covered with 

 lagging which contained the liquid air. Copper was prefer- 

 able to glass for the purpose, for it ensured in the actual deter- 

 minations a more uniform supply of heat to the apparatus. 



Calibration of the Copier Thermometer. 



The readings of the voltmeter described were determined 

 at the following temperatures : 100°, 0°, the boiling-point of 

 ethylene — 103 o, 5, the freezing-point —169°, and the tem- 

 perature of liquid air. The ethylene employed was carefully 

 fractionated for this purpose. 120 litres were condensed and 

 the first and last 20 litres rejected, the determinations being- 

 made with the middle fraction. The temperature of liquid 

 air is a variable, depending on the composition of the liquid, 

 but if the latter is known the temperature can be fixed with 

 great accuracy from the tables given by Baly. A sample 

 was therefore drawn off into a gas-holder from beneath the 

 surface of the liquid, at the time the temperature was read, 

 and its composition determined by analysis. These constants 

 were frequently redetermined throughout the course of the 

 experiments and found to remain unaltered. 



