309 
ment. The size of the chamber will be limited only by the potential of the 
central rod. The potential must be at least the saturation potential, that is the 
potential must be great enough to pull out the ions as fast as they are formed. 
With the usual potential, about 300 volts, the diameter may be made 15 or 
20 centimeters. The height may be made as great as is convenient to use. 
The general plan of the instrument is shown in the figure. A, is the 
ionization chamber, B, is the chamber containing the gold leaf. LL, is the leaf, 
W, is the window through which the leaf is read on the scale. C, is the charg- 
ing system. S, is the sulphur plug and R, is the central rod. For a more 
detailed deseription of the method of making and reading an electroscope I 
will refer to my paper on The Radioactivity of Spring Water. (Ind. Acad. 
Proc. 1914.) 
The top of the chamber, B, has a dise with a flange fastened to it. The 
diameter of this dise is such as to fit the ionization chamber. The lower end 
of the chamber, A, is closed and a hole is cut large enough to let the sulphur 
plug, S, pass. The gause cylinder, G, is soldered to a dise which will fit the 
inside of the large cylinder and pass the plug, S. A dise of diameter of the 
gause cylinder is soldered in the top. A lid fits over the top of the large 
eylinder. 
To fill in the material to be tested the chamber A, is removed from off 
the chamber B, the gause cylinder is placed inside and the material is packed 
lightly between the two walls. The lid is placed on and the chamber A, is 
placed on the chamber B. 
Correction must be made for the absorption of a@ rays by the gause. 
This can be determined by getting the ionization current of uranium nitrate 
when free and when covered with a sample of the gause, using an ordinary @ 
ray electroscope. 
Or the electroscope may be calibrated by filling in a material of known 
activity between the gause and the outside cylinder. Or uranium nitrate may 
be mixed with an inactive substance in known proportions and placed in the 
electroscope. 
In testing soils the sample should be allowed to dry for a few days as 
fresh damp soil contains a large amount of radium emanation which has 
come up from the lower material. 
Indiana University, December 1, 1915. 
