318 Prof. Rutherford and Mr. Grier on Deviable 



deviable rays. With the bare radioactive substances the ioni- 

 zation in the testing-vessel is chiefly due to the non-deviable 

 rays. In consequence of this a strong magnetic field does 

 not much alter the ionization-current observed in the electro- 

 meter. The non-deviable rays can, however, be completely 

 absorbed by two or more layers of paper, while the deviable 

 rays pass through with very little absorption. 



The deviable rays consist of rapidly moving electrons, and 

 ionize the air in their passage through it by collision with 

 the molecules. The average deviable rays are so penetrating 

 that they will probably pass through more than a metre of 

 air before the ionizing action is cut down to one half. 



When a strong magnetic field is applied, the paths of the 

 rays are curved, so that only a small fraction of the rays 

 enter the testing- vessel. 



In the experiment a magnetic field of 2200 c.G.s. units 

 was generally employed. This usually reduced the ionization 

 current in the testing- vessel to about 20 per cent, of its pre- 

 vious value. By increasing the strength of field the current 

 steadily diminished, showing that the effect in testing was 

 principally due to deviable rays. A small percentage of the 

 amount of ionization in the testing-vessel was due, in the 

 case of radium, to some extremely penetrating non-deviable 

 rays. These rays have been examined by the photographic 

 method by Villard and Becquerel. The difference between 

 the current in testing-vessel with magnet on and off was 

 taken as a measure of the amount of deviable rays. 



§ 3. Variation of Amount of Deviable Rays with Thickness 

 of Radioactive Layer. 



Different weights of the radioactive substance to be tested 

 were spread over an area of about 9 sq. cms. Four layers of 

 paper over this completely absorbed the a-radiation. 



The following numbers show the result from uranium and 

 radium. The amount of deviable rays is expressed in divisions 

 per sec. of the electrometer-scale, and represents the differ- 

 ence between ionization-current in the testing-vessel with 

 the magnet off and on. 



Uranium Oxide. Radium Chloride. 



Wt. Divns. per sec. Wt. Divns. per. sec. 



•25 gr. "47 "2d 1*5 



•50 „ -90 '5 2-9 



1 „ 1-26 1 5-5 



2 „ 1'70 I'od 6-7 

 5 „ 196 



