Genesis of Ions by the Motion of Positive Ions. ia9 
The first set of experiments were made at pressures 
between °5 and 2 mms. with air and from 1 to 8 mms. with 
hydrogen. The experiments which we have recently made 
include pressures up to 6 mms. with air and 20 mms. with 
hydrogen. 
The principles which underlie the experimental investi- 
gations have already been explained. It was shown that 
when a number mp of ions are set free by the action of ultra- 
violet light from the negative electrode in a gas and travel 
in a uniform field to the positive electrode, the total number 
(n) of ions which arrive at the positive electrode is given by 
the formula 
(a—f) ee 
0 — Bele Bd? 
where « is the number of molecules ionized by a negative 
ion in -going through a -centimetre of the gas, the 
corresponding number for a positive ion, and d the distance 
between the parallel plates which form the electrodes. 
The values of a and 8 corresponding to given values of 
the pressure p and electric force X may be deduced from 
determinations of the currents between the plates at different 
distances apart when the force and pressure are kept constant. 
The above equation shows that when the distance d is 
increased to an amount which is determined by the condition 
a— Be(@—8)4—(), the quantity n becomes infinite, and a current 
should continue to pass through the gas after the ultra-violet 
light has ceased to act on the electrode. Let a be the value 
of d which satisfies this condition so that the potential 
X xa should suffice to produce a spark in the gas when 
the distance between the electrodes a is given by the 
equation 
7 __loga— log 8. 
[= a 
Having determined the quantities 2 and 8 for various 
forces and pressures, the corresponding quantities a and X x a 
were calculated and may be compared with the sparking 
potentials V determined experimentally for the distance a 
between the plates. , 
The following tables give the results of the investigations. 
The distance d between the plates is given in centimetres, 
and the currents g, determined experimentally for the different 
distances, with constant pressure and electric force are given 
in arbitrary units. The order of the currents generally used 
may be found in amperes by multiplying the values of ¢ by 
the factor 10-”. 
