6 
PROF. 0. W. RICHARDSON ON THE EMISSION OF ELECTRONS 
at a low pressure, is shown in fig. 2. The three curves correspond to units of current 
diminishing approximately by successive factors of 10, according to the different 
capacities, given on the respective curves, added to the electrometer system. 
At the beginning of this experiment the pressure was less than 0 -001 mm. and the 
admission of the COCl 2 did not show any ascertainable increase. It was therefore at 
most 0-001 mm. After 20 minutes the pressure had risen to 0-0015 mm., after 70 
minutes it was 0-003 mm., and after 100 minutes 0-004 mm. These data strongly 
suggested that the admitted CO CL was mainly adsorbed and did not make itself felt 
on the gauge until the chlorine had been fixed and the carbon monoxide, a much less 
adsorbent gas, liberated. It will be seen that the chemical emission is most vigorous 
at first and rapidly falls off. The rate of decay, whether considered absolutely or in 
proportion to the amount of emission, steadily diminishes with lapse of time. This 
probably means that the active gas initially is that in the immediate neighbourhood 
of the drops, whereas later on it diffuses from more remote parts of the apparatus. It 
is probable that the rate of emission is proportional to the chemical action occurring, 
and that this is likewise proportional to the partial pressure of the active gas at the 
surface of the drop. 
Whilst fig. 2 exhibits the decay of the effect in a typical way under the conditions 
referred to, it is not typical of the conditions holding during the majority of the measure¬ 
ments, when a much higher degree of constancy of the saturation current was attained. 
For example, in one set the saturation current only varied between the limits 26 -5 and 
31-5 during the whole experiment, which lasted over two hours. However, this change 
with time was always present and its effect had to be eliminated. To accomplish this, 
determinations of the current under varied conditions, as, for example, under different 
voltages, were alternated with measurements of the saturation current under some 
