on Ionization Currents. 123 



that the a-rays from the emanation pass through them both. 

 After passing through the window the rays enter the ionization 

 chamber through a narrow slit in the lead block B. The vertical 

 dimension of this slit is everywhere l mm , and its horizontal 

 dimension (i. e., normal to the plane of the diagram) increases 

 from l mm at the outer to 10 mm at the inner surface of the block. 

 The lead stops all but the very penetrating 7-rays ; and thus the 

 ionizing rays are confined practically to a narrow beam midway 

 between the plates C, D, and E. A battery of small accumu- 

 lators keeps the plate C charged to any desired potential, plate 

 J) serves as the electrode, and plate E, which is put to earth, 

 acts as a sort of guard to the electrode against the effects of 

 secondary rays produced by the a-rays striking the end of the 

 box. The a-rays do not touch the plates, and therefore the 

 possible effects of secondary rays at the surfaces are eliminated. 

 In order to protect the wires connecting the electrode D with 

 the Wilson electroscope F from ionization currents due to the 

 penetrating 7-rays, they are completely embedded in pure 

 paraffin e except for a small space (x where a contact make and 

 break allows the electrode to be put to earth or charged to any 

 desired potential. To shield the electroscope itself from the 

 7-rays, a lead block H about 4 cm thick is interposed between it 

 and the source of rays, and further the electroscope is not 

 placed in the direct line of the beam of raj^s passing through 

 the slit, but a little to one side, and in such a position that to 

 reach the electroscope the 7-rays from the source have to pass 

 through not only the two lead screens B and H but also one of 

 the pole pieces of the electromagnet used to produce the mag- 

 netic field. These precautions taken, the direct influence of 

 penetrating 7-rays is scarcely perceptible and quite negligible. 

 To protect against electrostatic influence the paraffine covering 

 the lead wires is incased in metal tubes and these together 

 with the metal box L containing the plates and that containing 

 the electroscope are put to earth. The entire ionization cham- 

 ber with its plates and mica window, etc. lies between the poles 

 of an electromagnet in such a position that the lines of mag- 

 netic force are normal to the plane of the figure. The two 

 plates D and E are each 5 cm long and 3 c,n broad, and the 

 distance between them and C is 2 cm . 



I have found the following method of using a Wilson elec- 

 troscope accurate and reliable. The gold leaf is connected 

 through the contact make and break G, not directly to earth, 

 but to a reversing key I, which in turn is connected as shown 

 to earth and to two points on an electric circuit K containing 

 the resistances R x and K 2 . After choosing suitable values for 

 Rj and R 2 , one can change quickly the potential of the gold 

 leaf from any desired positive to an equal negative value (or 



