Theory of the Striated Discharge. 997 



proportional to p - *, where p is the density of the gas ; the 

 electric force X has in this case to a first approximation the 

 value it has in the uniform positive column. The potential 

 difference between the bright parts of consecutive striations 

 will be proportional to Xp~*, and since X is approximately 

 proportional to p, the potential difference between consecu- 

 tive striations will, in this case, be approximately pro- 

 portional to the square root of the pressure of the gas. 

 Wehner's experiments (Ann. der Pliys. xxxii. p. 49, 1910) 

 show a very marked diminution of the potential difference 

 between consecutive striations as the pressure diminished : 

 thus at the pressure of 1'48 mm. of Hg the potential differ- 

 ence was about 33 volts, while at *9 mm. of Hg it was only 

 about 9 volts. 



Proportion between the bright and dark parts of the 

 Striations. 



This varies very much with the type of gas, the pressure, 

 and the intensity of the current. In some cases the bright 

 parts are very thin and clear-cut, while in others the 

 appearance is more like a gradual waning and waxing in 

 intensity at regular intervals. An effect of this kind is 

 indicated by the solution of the problem given on page 986 ; 

 in this case there is a continuous background of uniform 

 luminosity. The bright and dark parts of the striations are 

 more definitely separated in the problem discussed on p. 988. 

 The dark parts correspond to the places where the energy 

 of the electrons is less than that corresponding to the 

 ionizing potential, the bright parts to those where it is 

 greater. Under the conditions assumed for this problem, 

 we see from equation (19) that the thickness of the dark part 

 depends upon nothing but the intensity of the current; 

 within the limits of pressure for which this solution is 

 applicable it is independent of the pressure, nor does it 

 depend upon the nature of the gas. The expression for the 

 thickness of the bright place is given by equation (20) ; 

 it depends on p the pressure of the gas and on c and /3, 

 quantities which vary from one gas to another. AVe see 

 from equations (19), (20), and (16*), that the ratio of 

 thicknesses of the bright part of the striation to that of- the 

 dark part is always proportional to (E — E )/E , where E is 

 the maximum value of the energy of the electrons and E 

 the minimum energy which can give rise to ionization. 

 Thus, when the bright parts are very thin, the maximum 

 energy of the electrons should differ but little from that 

 required for ionization. 



