Disappearance of Gas in the Electric Discharge. 593 



appears to coincide with that obtained when it is increased. 

 But once more there are indications that \f g and Vg 1 are not 

 precisely the same, but that it is merely very difficult to 

 detect the difference between them. As the pressure is 

 decreased, the curves of type IV. shade continuously into 

 those of type III. 



Observations on the Glow Potential. 



6. Many of the facts just described are well known. Several 

 observers have noted that the glow enters suddenly at a 

 definite potential accompanied by a large increase of current, 

 hi particular, Horton and Davies * have described the 

 phenomenon in a recent paper. But it was found that 

 the glow potential was so intimately connected with the 

 disappearance of gas under the discharge that a large 

 number of observations on it were made in various circum- 

 stances. A full account of the results and the conclusions 

 to be based on them is reserved for a later paper; here only 

 the facts that are immediately relevant to our main purpose 

 will be recorded. 



A few remarks may be made on the method of determining 

 the glow potential. It has been said that the appearance of 

 the glow and the sudden increase of the current occur at the 

 same potential, and all experiments indicate that this state- 

 ment is accurately true. But it is usually very much easier 

 to observe the increase of current than the occurrence of 

 luminosity, for (especially in hydrogen) the glow is often so 

 faint that without special optical arrangements it is difficult 

 to see it in the neighbourhood of the incandescent filament ; 

 moreover, there is always the chance that the glow may 

 occur in a region of the spectrum which does not affect the 

 eye. Accordingly in all observations it is the increase of 

 current and not the luminosity that has been observed when 

 accurate measurements are taken. At low pressures, corre- 

 sponding to curve III., there is no difficulty in observing 

 this increase in the form of a "kick" of the microammeter 

 A, but there is an even more convenient method of deter- 

 mining Yff. Since V was supplied through a potential 

 divider of high resistance, the rapid increase of current 

 causes a fall of potential across the electrodes, and, owing to 

 the difference between V g and V^', this fall does not cause 

 the glow to cease. Accordingly the method adopted was 

 to increase V regularly and to watch the voltmeter; the 

 pointer rises to a definite value and then drops back ; the 



* F. Horton and A. C. Davies, Proc. Roy. Soc. A, March 1920. 



