114 Prof. E. Rutherford on Uranium Radiation and 



homogeneous. Rontgen and others have observed that the 

 tf-rays are in general of a complex nature, including rays of 

 wide differences in their power of penetrating solid bodies. 

 The penetrating power is also dependent to a large extent on 

 the stage of exhaustion of the Crookes tube. 



In order to test the complexity of the radiation, an electrical 

 method was employed. The general arrangement is shown 

 in fig. 1. 



Ffc. 1. 



The metallic uranium or compound of uranium to be 

 employed was pow T dered and spread uniformly over the centre 

 of a horizontal zinc plate A, 20 cm. square. A zinc plate B, 

 20 cm. square, w y as fixed parallel to A and 4 cm. from it. 

 Both plates were insulated. A was connected to one pole of 

 a battery of 50 volts, the other pole of which was to earth ; 

 B was connected to one pair of quadrants of an electrometer, 

 the other pair of which was connected to earth. 



Under the influence of the uranium radiation there was a 

 rate of leak between the two plates A and B. The rate of 

 movement of the electrometer-needle, when the motion was 

 steady, was taken as a measure of the current through the 

 gas. 



Successive layers of thin metal foil were then placed over 

 the uranium compound and the rate of leak determined for 

 each additional sheet. The table (p. 115) shows the results 

 obtained for thin Dutch metal. 



In the third column the ratio of the rates of leak for each 

 additional thickness of metal leaf is given. Where two 

 thicknesses were added at once, the square root of the observed 

 ratio is taken, for three thicknesses the cube root. The 

 table shows that for the first ten thicknesses of metal the rate 

 of leak diminished approximately in a geometrical progression 

 as the thickness of the metal increased in arithmetical pro- 

 gression. 



