18 



MR. O. W. KICHAKPSON ON THE IONISATTON 



This method was rather rough and very tedious to use. Great care was necessary 

 to get reliable results with it, and the strain this involved, added to the natural 

 difficulties of the experiments, rendered the method almost impracticable. However, 

 a number of series of observations were taken by this method of direct comparison. 

 The results of one of them are given in the following table : 



The temperature was about 800 C. and the voltage + 200. Roughly speaking, 

 these numbers serve to confirm those which were obtained by the resistance method ; 

 they show that the leak varies very little with the pressure at high pressures. On 

 the other hand, there is a more rapid variation than that previously found, indicating 

 that the resistance method did make the temperatures too low at the higher 

 pressures. 



It was found that a better way to make use of this optical method of obtaining a 

 constant temperature was to determine the change of resistance required to keep the 

 filaments equally bright when the pressure was varied, and to use the results thus 

 got to correct the readings for the leak at constant resistance to what they would be 

 at constant temperature according to the optical criterion. This procedure may 

 appear pointless at first sight, but it is not. The advantage of it lies in the fact that 

 it separates the difficulties of the optical regulation process from those which are due 

 to the vagaries of the leak itself. The leak then was measured with the wire heated 

 so that its resistance remained constant ; this is done by the purely mechanical 

 process of keeping a galvanometer spot at the middle of a scale, so that all the 

 attention of the observer could be devoted to the actual measurement of the leak 

 itself. Similarly, in finding the way the resistance changed for the same brightness, 

 all the attention could be devoted to seeing when the two wires were equally bright. 

 It was far easier to carry out both these operations separately and combine the 

 results than to do both things at once, and the results obtained were far more 

 consistent. 



Working in this way a curve was obtained giving the resistance at various 

 pressures corresponding to a constant temperature. A curve was also plotted, from 

 experiments which will be described later, showing the relation between the leak at 

 constant pressure and the resistance. On combining these two curves so as to 

 eliminate the resistance, a third curve was obtained which gave the factor by which 

 the leak at any pressure had to be multiplied to bring it to the value it would have 

 at the temperature which the wire was at when the pressure was 1 millim. 



