8 THE ROYAL SOCIETY OF CANADA 
The arrangement inside the receiver (Fig. 2) is practically that de- 
vised by Franck and Pohl,!and modified later by Lattey.? It is specially 
suitable for use in measuring mobilities with small volumes of a gas. P 
is a brass plate with a piece of copper covered with polonium fastened 
to the inside. G is a piece of gauze. Fastened to G and around the 
chamber PG is a guard ring R. This makes the field uniform in the 
chamber when G is either positive or negative. $ is a screen of gauze 
about 1 mm. from the electrode E. The distance from P to G and 
from G to 8S was about 1-5 ems. 
If the velocity of the negative ion were being measured P was made 
negative with respect to G, while G was made alternately positive and 
A E : 
MA x S iG P 
Dees. = se 
|| 
negative with respect to S. S was kept negatively charged to 17 volts 
with respect to E by means of a battery of small dry cells B, in the end 
of the cylinder. When G was negative with respect to S, the negative 
ions travelled across towards 8. If they reached S before G was made 
positive, they were forced to the electrode E. When G was made posi- 
tive, the negative ions were prevented from moving across and the 
negative ions in the chamber GS were drawn back. 
The screen S prevented a charge being induced on the electrode E 
when the potential of G was changed. As two or three seconds often 
elapsed between alternations, the electrometer without the screen S 
oscillated so as to render the taking of measurements impossible. 
1 Verh. d. Deutch. Phys. Ges., p. 69, 1907. 
2 Proc. Roy. Soc., July 28, 1910. 
