1912] Secondary Rays Excited by the Alpha Rays. 153 



ON THE SECONDARY RAYS EXCITED BY THE ALPHA RAYS 

 FROM POLONIUM.— I. 



By V. E. Pound, M.A.* 



{Read ijih January, IQ12). 



I. Introduction. 



Recent researches have shewn that radioactive materials from which 

 a rays are sent off also emit a radiation of negatively charged particles, 

 which will not ionise a gas, and which have been called 8 rays. The 

 other characteristics of this radiation are that it is easily absorbed, it is 

 easily deflected by a magnetic field and it is stopped by a small positive 

 charge placed upon the radioactive substance emitting it. Still later 

 researches by Logeman*^ have shewn that when a rays fall on a polished 

 piece of metal such as copper, this metal emits a secondary radiation 

 with characteristics similar to those of the S rays. Further, it has been 

 found by Duane*^ that the a rays lose their power to produce secondary 

 rays at the same time that they lose their charge and their power to ion- 

 ise a gas. 



The present paper describes some experiments on these secondary 

 rays produced by the a rays of polonium. The apparatus employed at 

 first was somewhat similar to that used by Logeman when he proved the 

 existence of this secondary radiation from metals bombarded by « rays. 

 In the experiments to be described exhaustions were made with a Gaede 

 pump and the pressures were measured with a McLeod gauge. 



II. — Description of Apparatus. 

 The apparatus used in the initial experiments is shewn in Fig. i. 

 It consisted of a brass cylinder about 4 cm. in diameter and 9 cm. in 

 length which had an ebonite plug fitting into each end. Through one 

 of these plugs was led a brass rod which carried a round brass electrode 

 B, about 15 mm. in diameter. The electrode B was surrounded by a 

 circular guard ring C. Through the other plug was led another brass rod 

 and it carried the second electrode, A. A circular surface, ab,oi this elec- 

 trode A, about 15mm. in diameter was coated with a deposit of polonium. 



•Presented by Prof. J. C. McLennan. 



** Logeman, Proc. Roy. Soc. Series A, Vol. 78, Sept. 6, 1907. E. Aschkinass, Ann. 

 der Phys. No. 12, 1908. 



*2 Duane, Comptes Rendus, May 25, 1908. 



