472 Prof. F. Horton on the Application of Thermionic 



In order to make this correction, it is necessary to know 

 the temperature of the gas subjected to ionization by collision. 

 I had at first hoped to be able to ascertain this by means 

 of the thermojunction attached to the platinum strip, by 

 cutting off the heating current and observing the dying 

 away of the thermoelectric current. The thin platinum 

 strip cools very rapidly to the temperature of its sur- 

 roundings, and the thermoelectric current falls rapidly at 

 first 'and then more slowly ; but the method (which gave 

 results in rough agreement with those finally obtained) 

 was discarded for the more accurate method of measuring 

 the temperature with a mercury thermometer. For this 

 purpose the wide glass tube seen in fig. 1 was replaced 

 by another of the same diameter, and having a horizontal 

 side tube at such a level that a thermometer sliding in it 

 could have its bulb placed just over the central hole in 

 the plate B and about *5 mm. above it. The thermometer 

 used had a small cylindrical bulb 7 mm. long and 2*5 mm. 

 wide. Its stem passed through a hole (which it just fitted) 

 along the axis of a small brass cylinder fixed in the side 

 tube. The further end of the thermometer was fixed by 

 sealing-wax into a small cylinder of soft iron which could 

 be moved along the side tube by means of an electromagnet. 

 In this way the bulb of the thermometer could be placed in 

 position for temperature observations. An upper aluminium 

 plate similar to A (fig. 1) was suspended above the lower 

 plate, the distance apart of the plates being about 4*5 mm. 

 The thermometer could be moved between them without 

 touching either plate. Observations of the temperature 

 were taken when the middle of the bulb of the thermo- 

 meter was vertically above the centre of the exposed square 

 of glowing platinum, and also when the bulb was drawn to 

 one side so that no part of it was over the hole. The 

 temperatures registered in these two positions usually 

 differed by about 8° C. 



With this apparatus, observations of the temperature of 

 the air were taken at various gas-pressures, the temperature 

 of the platinum strip being adjusted in each case to 1000° C, 

 the value it had when the results recorded in the tables were 

 obtained. The gas-pressures at which these investigations 

 were made were adjusted so as to be near to the pressures used 

 in the ionization experiments. A curve was drawn connecting 

 these pressures with the observed temperatures of the air, 

 the temperatures plotted being those registered by the 

 thermometer when its bulb was immediately over the 

 exposed square of glowing platinum. From this curve 



