594 Research Staff of the G. E. C, London, on the 



maximum value attained is V *. Similarly, V/ can be 

 determined by decreasing the voltage regularly and noting 

 the minimum reading recorded. It' the pressure o£ the gas 

 remained constant during the observations (a condition by 

 no means always fulfilled), Yg could be determined as 

 accurately as the voltmeter could be rend. The deter- 

 mination of "Vy was somewhat less accurate and consistent. 



When the pressure is higher and the condition is that 

 represented by curve IV., the determination of V^ is more 

 difficult ; the increase of current associated with the glow is 

 much less. If the glow is visible it is best to watch tor it ; 

 if it is not, rough measurements of i have to be taken in the 

 neighbourhood of Yg. It is not possible in these circum- 

 stances to determine Yg to less than \ volt, and it is rather 

 surprising that the consistency of the result shows that 

 even this accuracy can actually be obtained. 



It should be pointed out that the values of Yg given are 

 always those between the anode and the negative end of the 

 filament when the glow appears. The remainder of the fila- 

 ment differs less in potential from the anode ; and the true 

 value of Yg, or that which would be found if the filament 

 were all at the same potential, maybe slightly less. But the 

 difference must be a small traction of a volt ; for the true 

 Yg will be the potential difference between the anode and 

 that part of the filament, hot enough to give a thermionic 

 emission which falls within the range where Yg is indepen- 

 dent of that emission, which is nearest to the negative end. 

 Accordingly the observed values have only to be corrected 

 for the potential drop along the portion of the filament which 

 is cooled by the leads. The whole drop along the filament 

 was usually 8*4 volts, and since it never varied by more 

 than J volt, the values obtained must be accurate within 

 the error of observation, which was never less than O'l volt. 



The first observations proved that the glow potential is 

 independent of the temperature of the filament and of the 

 thermionic emission from it within wide limits. If there is 

 no thermionic emission, the glow does not appear, of course, 

 until the spark potential of the gas is reached, but the change 

 from this condition to that in which the glow occurs at the 

 very much lower potential obtained with a hot cathode is so 

 rapid that it could not be determined certainly whether the 

 change was continuous or discontinuous. Once the lower 



* V^ determined in this manner is doubtless not exactly the same as 

 that determined from the " kick " of the current, unless the resistance in 

 series is practically infinite. But the resistance was sufficiently peat to 

 make any difference between the two values inappreciable to experiment. 



