[DEMPSTER] MOBILITIES OF IONS IN GASES i 
A summary of the measurements made on the mobility of the 
negative ion in air is given in Table II. 
The first nine readings were taken before the measurements with 
the positive ion were made and the last five afterwards. The lower 
value obtained for the product pk corresponding to the last five pres- 
sures cited can very probably be attributed to a change in the state 
of the air supplied by the liquid air machine, as the mobility of the 
negative ion in air at atmospheric pressure is known from Lattey’s 
investigation to be greatly affected by changes in the amount of mois- 
ture present. 
It is evident, however, from a survey of all the results given in 
Table II, that there is a gradual increase in the value of pk as the 
pressure rises, since the ratio of the mobility of the negative ion to that 
of the positive ion at high pressures is about 1.7, while at the lowest 
pressures investigated this ratio was only about 1.26. 
It might be thought, perhaps, that ions of the same size but possess- 
ing different mobilities might be present. The method used above 
would evidently give the velocity of the fastest ions present. But 
another method was used which showed that the ions of one size all 
had the same mobility. When we measure the charge c,, with the com- 
mutator running at a certain speed and a voltage applied more than 
sufficient to bring the ions across, and then measure the charge €, with 
the voltage kept on steady, we see that if “f” is the fraction of a revolu- 
tion and T the actual time the ions are travelling in the first measure- 
C 
Co— C 
moving ions were present, this would give a smaller velocity than the 
first method. But in the case of both the positive and the negative 
ion the velocity was the same. 
In conclusion, I wish to express my gratitude to Professor McLennan 
for his interest and valuable advice throughout the investigation. 
The Physical Laboratory, 
May 138th, 1911. 
Note added June 23rd by Professor McLennan: 
In some measurements which Mr. Dempster has made since the results given 
above were obtained he has found that the mobilities of the negative ions in air 
when measured with the arrangement of apparatus indicated above were somewhat 
greater when high fields were applied to the space PG than when low ones were used. 
One interpretation of this result would be that with low fields in P.G the ions had 
sufficient time to become loaded before they were introduced into the space between 
S and G while with high fields they were introduced before some of them at least 
were loaded. This would account for the higher mobilities observed. The com- 
plete measurements, however, will be communicated in a later paper. 
June 26th, 1911. J. C. McLennan. 
Sec. III., 1911. 2. 
ment, then in time T they travel - x1-5 ems. If more slowly 
