180 Prof. E. Rutherford on the Magnetic and 



plate P so that the rays entered the vessel V only through 

 the aluminium foil. 



It is necessary in these experiments to have a steady 

 stream of gas passing downwards between the plates in order 

 to prevent the diffusion of the emanation from the radium 

 upwards into the testing vessel. The presence in the testing 

 vessel of a small amount of this emanation, which is always 

 given out by radium, would produce large ionization effects 

 and completely mask the effect to be observed. 



For this purpose a steady current of dry electrolytic 

 hydrogen of 2 c.c. per second was passed into the testing 

 vessel, streamed through the porous aluminium foil, and 

 passed between the plates, carrying with it the emanation 

 from the apparatus. 



The use of a stream of hydrogen instead of air greatly 

 simplifies the experiment, for it increases at once the ioni- 

 zation current due to the a rays in the testing vessel, and (at 

 the same time) greatly diminishes that due to the j3 and 

 7 rays. 



This follows at once from the fact that the a rays are much 

 more readily absorbed in air than in hydrogen, while the rate 

 of production of ions due to the /S and 7 rays is much less in 

 hydrogen than in air. The intensity of the u rays after 

 passing between the plates is consequently greater when 

 hydrogen is used ; and since the rays pass through a sufficient 

 distance of hydrogen in the testing vessel to be largely 

 absorbed, the total amount of ionization produced by them 

 in hydrogen is greater than in air. 



With the largest electromagnet in the laboratory I was only 

 able to deviate about 30 per cent, of the a rays. Through the 

 kindness of Professor Owens, of the Electrical Engineering 

 Department, J was, however, enabled to make use of the 

 upper part of the field-magnet of a 30 kilowatt Elison 

 dynamo. Suitable pole-pieces are at present being made for 

 the purpose of obtaining a strong magnetic field over a con- 

 siderable area; but with rough pole-pieces I have been enabled 

 to obtain a sufficiently strong field to completely deviate the 

 a rays. 



The following is an example of an observation on the 

 magnetic deviation : — 



Pole-pieces 1*90 x 2*50 cms. 



Strength of field between pole-pieces 8370 units. 



Apparatus of 25 parallel plates of length 3*70 cms., width 

 '70 cm., with an average air=space between plates of "042 cm. 



Distance of radium below plates 1*4 cm. 



