200 Prof. E. Rutherford on the Charge 



the numbers obtained are in good agreement, although the 

 magnitude of the current due to the electrons varied consi- 

 derably in the different experiments. The fact that the current 

 either in the positive or negative direction was independent 

 over a wide range of the strength of the magnetic field, when 

 once a certain small value of the magnetic field bad been 

 reached, shows that the strength of the field was sufficient 

 to bend back all the slow-moving electrons emitted by the 

 plates. 



The failure of the earlier experiments to detect the charge 

 carried by the a rays led to the suggestion that the a particle 

 was uncharged at the moment of expulsion, but gained a 

 positive charge in its passage through the gas. It is probable 

 that the a. particle, if initially uncharged, would lose a negative 

 electron by collision with the gas molecules, and so retain a 

 positive charge. This point came up in the discussion at the 

 close of the Bakerian Lecture (Joe. cit.) last year, and was 

 referred to by me in the paper " Present Problems of Radio- 

 activity," read before the Internatianal Congress of Arts and 

 Sciences, St. Louis, 1904. The same suggestion was also 

 made by Bragg*. 



There does not, however, now seem any doubt that the a. 

 particle is charged at the moment of its expulsion. The 

 pressure of the gas in the experiments was so low that only 

 a small fraction of the a particles could come into collision 

 with the gas molecules. This is brought out by the com- 

 parison of the values of the positive and negative currents in 

 a strong magnetic field. The difference between the currents, 

 in the two directions is probably due to the small ionization 

 produced in the residual gas by the passage of the a particles 

 through it. In some cases the value of this ionization current 

 was not more than one-tenth of the current due to the charge 

 on the a. particles alone. This indicates that only a small 

 fraction of the a particles come into collision with gas 

 molecules. On the other hand, the magnitude of the charge 

 carried by the a rays was independent of the state of the 

 vacuum over a considerable range, showing that the residual 

 gas had no effect in altering the number of charged a particles 

 absorbed in the upper plate. 



We may thus conclude that the a particles are positively 

 charged at the moment of their release from the radium 

 plate, and, remembering that the film of radium used in the 

 experiments was very thin, there is no obvious reason for 

 supposing that they are not charged at the moment of their 

 expulsion from the radium atoms. 



* Phil. Mao-. Dec. 1004. 



