362 Prof. Owens on Thorium Radiation. 



3. Comparison of radiations from different salts. 



4. Types of radiation. 



5. Selective absorption. 



6. Effect of suspended particles in the path of the conduc- 

 tion-current. 



7. Variation of conduction-current with pressure of gas. 



8. Absorption of radiations in air. 



The particular thorium salts employed as a source of radia- 

 tion were the oxide, sulphate, and nitrate. The method used 

 for comparing the intensities of the radiations under different 

 conditions was similar to that of Rutherford*. 



A layer of the active material was spread uniformly over a 

 small platinum plate resting on a larger brass one, the whole 

 being covered, except the surface of the active material, with 

 a heavy lead sheet to cut off stray radiations. An insulated 

 parallel brass plate held at a suitable distance was connected 

 to one pair of quadrants of a sensitive electrometer, the other 

 pair of quadrants being permanently to earth. By means of 

 a battery of small lead cells the lower plate could be maintained 

 at different constant potentials. 



When the quadrants of the electrometer were separated 

 the top plate gradually acquired the potential of the lower 

 one, and the rate of movement of the electrometer needle was 

 taken as a measure of the current through the gas. The 

 gradual charging of the top plate was due to the movement 

 under the action of the applied electromotive force of the 

 ions produced throughout the volume of the gas by the 

 radiation absorbed by it. 



1. Conditions affecting the Constancy of the Radiation. — The 

 apparatus used in obtaining the relations of current and 

 electromotive force, in comparing and analysing radiations, 

 and in investigating the effect of suspended particles in the 

 path of the conduction current, is shown in fig. 1. B x is an 

 insulated brass plate on which rests the platinum plate P. 

 On the platinum plate is spread the active material. Over 

 the whole is the lead sheet L, with its centre cut away, ex- 

 posing an area of salt of 25 sq. cm. B 2 is an insulated plate 

 with a vertical movement and connected to the electrometer 

 E. Both plates are enclosed in a metal box A, fitted with a 

 door on one side to admit the plates. Placing a thick layer 

 of thorium salt (about 1 mm.) on P, charging the lower plate 

 to 95 volts and separating the quadrants very shortly after 

 closing the door of the surrounding box, the rate of leak was 

 found to gradually increase but finally to reach a constant 



* Phil. Mag., Jan. 1899. 



