164 THE ROYAL SOCIETY OF CANADA 
electrodes. In one case he maintained his plates 2 mm. apart and 11 
mm. above a preparation of actinium, and had a potential difference 
of 250 volts between the plates. He measured the distribution of 
the active deposit under different pressures, obtaining results shown 
in the first two columns of Table ITI. 



TABLE III. 
Pressure in Fraction p of Corrected Percentage 
mm. total activity on | Fraction py. fraction on Cathode 
anode. anode. Activity. 
135008 -064 -046 -018 98-2 
192 -043 -032 -011 98-9 
220 -027 -019 -008 99-2 
500 -022 -012 -010 99.0 
750 -021 -008 -013 98-7 



It is evident that with the plates so close together, recoil will 
play an important part, particularly at low pressures. The fraction 
pi of the total activity deposited on the anode by recoil is given by 
R 17 
Pi 4 
distance between the plates. The values of p1 given in column 3 
have been calculated assuming an approximate value of -065 mm. 
for the average range of actinium rest-atoms in air at atmospheric 
pressure. The differences betwee p and p; are given in column 4. 
They represent the fractions of the total activity which settled on 
the anode by diffusion. The values in column 5 are calculated as 
in the case of radium. As the pressure is lowered below one atmos- 
phere, the percentage cathode activity at first increases and then 
decreases. The first rise is due doubtless to the increased effective- 
ness of the potential gradient at lower pressures owing to the smaller | 
density of the ions. At still lower pressures, however, there is more 
opportunity for neutralized rest-atoms to diffuse between the plates 
and be deposited on the anode. To this can probably be ascribed 
the later fall of the percentage cathode activity. That this effect 
may become great can be seen from some experiments of Kennedy’s 
in which the plates 2 mm. apart were placed 1-3 mm above the actin- 
ium preparation. Even with a potential difference of 1,150 volts 
only 60% of the total activity was found on the cathode. 
Turning to Table III, we see that under a potential gradient of 
1,250 volts per cm., 98-7% of the actinium rest-atoms are collected 
where R is the average range of the rest-atoms and d is the 

7 Rutherford, R.A. Substances and their Radiations, p. 404 
