532 Dr. Norman Campbell : Further 



in fig. 3 A was o£ aluminium while D was covered with soot; 

 in each case curve 1 represents the readings without a mag- 

 netic field, 2 with a magnetic field, and 3 the difference 

 between 1 and 2. The magnetic field was of such a strength 

 that a further increase in it produced no further effect. A 

 correction is made in every case for the activity of the 

 thorium deposit, so that the readings are comparable. 



7. It will, be noted that in the curves 1 the current for 

 large values of V is not zero, but a finite positive quantity. 

 The existence of a positive current would seem at first sight 

 necessarily to indicate that positive ions were travelling from 

 A to D, for the preliminary experiments with the hole 

 covered with aluminium leaf show that no delta rays are 

 liberated from D. These ions are naturally attributed to the 

 presence of gas remaining in the vessel, but their number 

 is unexpectedly large. Observations with the vessel con- 

 taining gas at measurable pressures showed that the current 

 due to the presence of air at the pressure indicated by the 

 McLeod gauge should be only O'l per cent, of the current 

 carried by the delta rays, whereas it appears to be some 

 15 per cent. It is probable that some of the gas adhering 

 to the surface of A is only liberated when the alpha rays 

 strike it and ionize it ; a similar explanation of some of his 

 observations has been put forward by Pound in a recent 

 paper,* and it is doubtless correct. But if the positive cur- 

 rent is due to this cause, it should beunaffecte 1 by the action 

 of a magnetic field ; for experiments at higher pressures 

 showed that such a fjeld had no influence on the current due 

 to the ionization of the air, at least for potential differences 

 greater than 2 volts. In fig. 3, when D was covered by 

 sqot, the application of a magnetic field does not change the 

 magnitude of the positive current, but when D is of alu- 

 minium the magnetic field decreases the positive a^ well as 

 the negative current. Similar measurements, wdien D was of 

 brass and gold, showed that it w T as only when D was covered 

 with soot that the positive current was unchanged by the 

 magnetic field, and it was also noted that in this case the 

 positive current was somewhat smaller than the others. 



8. There seems only one explanation possible of the change 

 of the positive current with the magnetic field. In his work 

 en the reflexion of slow electrons, v. Baeyer f has shown that 

 .electrons with speeds of the order of 10 volts excite so many 

 secondary rays at the surface of metals on which they are 

 incident, that it is possible for more electrons to leave such 



* V. E. Pound, Phil. Mag. May 1912, p. 813. 

 f 0. v. -Baeyer, Phys. Zeit. x. p. 176 (1909). 



