622 Prof. Skinner on the Drop of Potential 



into the cathode dark space where its drop rose to 61 volts,, 

 the drop between the cathode and the adjacent gas increased 

 from 425 to 485 volts. As the anode was brought nearer,, 

 the drops increased until the battery of 1200 volts could not 

 maintain a current. These values were obtained with a gas- 

 pressure of 0*5 mm. and with a zero gradient in the cathode 

 dark space. Jn Table III., observations of the same kind 

 with a gas-pressure of 2 mm. are given. For the same effect 

 on the drop the electrodes must be brought closer than at 

 the lower pressure, but in quality the results are the same : 

 there is a simultaneous increase in the drop of potential at 

 the two electrodes when they are brought near together. 

 That it is solely a drop at the electrodes that makes the 

 increase in the discharge-potential is proved in Table III.,. 

 where the total potential-difference observed between th( 

 electrodes is just equal to the sum of the drops. This also 

 shows that the gradient in the gas is an inappreciable quantity. 

 Such, however, is not always the case, as shown by the follow- 

 ing observations taken at another time : — 



Drops in Volts between 

 Anode and Gas. Cathode and Gas. Anode and Cathode. 



65 755 835 



where the drop in the gas is found to be about 15 volts,, 

 which value, however, is small compared with the total differ- 

 ence of potential. 



It seems difficult to conceive the forces required to produce 

 ionization at the electrodes as increased by decreasing the 

 space between them. On the other hand, attributing the drop 

 at the electrode to the velocity of impact of the discharging 

 ions, it is obvious that, when the velocity with which the ion 

 reaches the second electrode is increased by increasing the 

 velocity with which it leaves the vicinity of the first, the 

 variations at the two electrodes will be simultaneous and 

 mutual. 



Furthermore, a drop of potential at the electrodes is ob- 

 served in conducting gases in general, even when ionization 

 only takes place some distance from them. In general, the 

 drop is reduced by producing the ionization at the surface of 

 the electrodes. In a vacuum-tube Hittorf found that when 

 the cathode was raised to a temperature of incandescence (so 

 that the surrounding gas w r as ionized) a current could be 

 passed by a comparatively small difference of potential between 

 the electrodes'*. Child has observed that in gases conductive 



* W. Hittorf, Wied. Ann. xxi. p. 133 (1884). 



