485 
1908— y.] A Simple Radioscope and a Radiometer. 
follows : — When the catch U is lifted the air in the expansion chamber at 
once begins to expand, but it is slightly retarded by the resistance of the 
plunger ; but by the time the plunger clears the end of the tube it is shot 
out with considerable velocity, being in fact a very quickly opening valve, 
and in this way a very free passage is given for the latter part of the 
expansion. The catch U is in the form of a nut screwed on to the end of 
the lever. The object of this was to enable tests to be made of the effects 
of different lengths of stroke of the plunger. With a long stroke the first 
part of the expansion will be retarded most, but the plunger will acquire a 
greater velocity, opening the pipe quicker, and the latter part of the 
expansion be more rapid. A short stroke of the plunger, while quickening 
the first part, will slightly retard the latter, as it does not open so quickly. 
After a number of tests neither of these additions seemed to improve the 
working of the instrument to an extent sufficient to make up for the 
greater trouble given by their use. 
Both the radioscope and the radiometer have the defect that they only 
test the more penetrating rays— -that is, the / 3 - and y-rays ; while the a-rays, 
which have most of the energy of the radiation, are stopped by the walls 
of the expansion chamber. It was thought, therefore, that if the radio- 
active body could be introduced into the expansion chamber, it would be 
capable of measuring much smaller degrees of radioactivity. When 
working with the apparatus shown in figs. 4 and 5, it was found to be 
possible to open one end of the expansion chamber and allow dusty air to 
enter that end without it in any way interfering with the action of the 
instrument, the reason for this being that the dusty air only penetrates 
a short distance into the tube, and the compressions and expansions make 
the dust particles into cloud particles and deposit them before they 
penetrate into the expansion chamber. Taking advantage of this, it seemed 
quite easy to introduce the radioactive body into the expansion chamber. 
The apparatus shown at R, fig. 3, was therefore constructed. V is the 
connecting tube as in figs. 4 and 5, and C" is the branch pipe to the com- 
pressor. In this case the branch pipe is connected at a small angle to the 
tube Y to allow as free a passage as possible for the expanding air. The 
open end of the tube V is fitted with a union joint U and a stuffing box S, 
through which passes the fine steel wire W. The end of the wire inside 
the tube is provided with a screw on which may be fitted a small pair of 
forceps, or other arrangement that may be suitable for the particular 
experiment. 
When we wish to introduce a small piece of any substance into the 
chamber B the union joint U is unscrewed and the forceps or other 
