238 Mr. P. J. Edmunds on the 



This is in general agreement with experience. In the ex- 

 periments carried out with cylinders lag was practically 

 unnoticed. 



5. The theory shows that, assuming some initial ionization, 

 the number of ions produced by collision becomes infinite if 

 a certain distribution of electric force is maintained in the 

 g - as. But as soon as the current has commenced two in- 

 fluences come into play which tend to alter the electric field. 

 In the first place, the current tends to discharge the system 

 and so lower the potential. In the second place, the current 

 constitutes a charge in the gas which changes the electric 

 force. When one of the electrodes is sharply curved and 

 the other nearly plane, the effective ionization all occurs 

 near the sharply curved electrode. The result is that the 

 current is carried through the greater part of the gas by ions 

 of one sign only, namely that of the sharp electrode. This 

 charge lowers the force near this electrode and diminishes 

 the ionization taking place near it without raising the force 

 in other parts of the field sufficiently to cause extra ionization. 



The first of the two counteracting effects depends on the 

 capacity of the system and the rate of supply of electricity. 

 The second depends chiefly on the velocity of the ions. This 

 velocity becomes large at low pressures, and is greater for the 

 negative ions than for the positive. With cylinders it was 

 often found that with the lower pressures the nature of the 

 discharge could be changed from a violent spark, discharging 

 the system almost completely, to a rapid succession of small 

 sparks by reducing the capacity of the system as much as 

 possible. 



Now a discharge will continue even after the potential 

 has dropped considerably below that necessary to start a 

 discharge, provided there are a sufficient number of ions in 

 the gas. This number of ions depends on the current that 

 has passed through the gas immediately before. Thus with 

 a small capacity the potential drops before enough ions have 

 been produced to carry on the discharge, and the current 

 ceases at a fairly high potential ; whereas with a large 

 capacity in the same case there might be a spark nearly 

 completely discharging the system. 



In the case of point discharges it was found that the 

 external capacity did not have much effect on the nature of 

 the discharge. In this case the current is much more con- 

 centrated than with cylinders, and it is probably necessary 

 fo make the capacity very small indeed to bring its effect 

 Into prominence. 



In order that a truly continuous or glow-discharge should 



