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merit. The size of the chamber "will he 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. L, 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 description of the method of making and reading an electroscope I 

 will refer to my paper on The Radioactivity of Spring Water, (ind. Acad. 

 Proe. 1914.) 



The top of the chamber, B. has a disc -with a flange fastened to it. The 

 diameter of this disc is such as to fit the ionization chamber. The louver 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 disc which will fit the 

 inside of the large cylinder and pass the plug, S. A disc of diameter of the 

 gause cylinder is soldered in the top. A lid fits over the top of the large 

 cylinder. 



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 a. 

 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. 



