496 
size as that of (A). By turning the screw (T) the position of (B) can be 
made to change through a range of about 8.5 cm. Its distance in cm. or 
mm. from (A) can be determined by reading the position of the disk (Q) 
with reference to the scale (Z). 
To the bottom of the ionisation chamber four brass rods 1.2 cm. in 
diameter and 15 cm. in length are securely fastened by means of screws 
for legs. Leveling screws are adjusted to the lower ends of these legs. 
The electroscope can be converted into an emanation electroscope by 
placing stopcocks (M) and (N) in the sides of the ionisation chamber. The 
active gas can be admitted to the electroscope by exhaustion through one 
stopcock and attaching the source of gas to the other stopcock. Or the 
active gas can be forced into the ionisation chamber by means of a pres- 
sure bulb. In testing active gases it is usually necessary to reduce the 
capacity to the lowest amount possible. This can be done by removing 
the cylinder (H) and the condenser plate (A). A small rod should be 
screwed onto the end of (1D) in place of the disk (A). Figure four shows 
the instrument mounted ready for use. 
The instrument can be changed into one for measuring the “Variations 
of the ionisation produced by an alpha particle along its path,” by en- 
closing the condenser plate (A) in a metal box which has a number of 
short, small, brass tubes passing through the bottom. (See Makower and 
Geiger’s Practical Measurements in Radio-activity, article 32, page 46.) 
A drawing of the box is shown in Fig. 3 (B) and a cross section in Fig. 
3 (©). 
After a thorough test it has been shown that this one instrument with 
its attachments can be used for a wide range of radio-active measure- 
ments, and that it is well adopted to general laboratory work. 
Physical Laboratory, 
Barlham College. 
