﻿372 Dr. Norman Campbell on the 



is assumed that none would have been found with copper or 

 aluminium. 



The changes produced by making the metal the cathode 

 of an electric discharge were of precisely the same nature 

 as those produced by heating in both the metals on which 

 the effect of both procedures was tried (platinum and 

 nickel). But the changes could be effected much more 

 rapidly by means of the discharge. The lowest ionization 

 that has been observed (in copper) was produced from state 

 A by the passage of a discbarge for only 5 minutes ; to 

 produce state B from state A by heat always required about 

 24 hours, and even after heating for 300 hours (when an 

 ionization as low as that mentioned had not yet been 

 attained) decrease was still in progress. But whereas 

 further heating always produced a decrease, the passage of 

 a discharge sometimes produced a slight increase. The 

 extent to which the ionization could be reduced appeared to 

 depend mainly on the pressure of the gas through which the 

 discharge passed ; the discharge appeared to be most 

 efficient at a pressure of about 2 mm. of mercury, and its 

 efficienc]^ appeared to be closely connected with the amount 

 of " sputtering " which occurred. Moreover, it did not seem 

 to matter whether the plate was made the anode or the 

 cathode of the discharge, but on this point I cannot be 

 certain ; for though a valve-tube was included in the circuit, 

 the form of the electrodes between which the discharge 

 passed (B and P) was such that, at the pressure at which 

 the discharge was most efficient,, P acted as the cathode 

 much more readily than B. It may be that P was acting 

 sometimes as cathode even when the valve-tube was 

 arranged so that it should act as anode. The discharge 

 acted in the same manner whether the gas through which it 

 passed was air, oxygen, hydrogen, or petrol vapour. 



To illustrate the nature of the changes one series of 

 observations may be given in some detail. The metal was 

 copper and the value of R, taken as a measure of the 

 ionization, was that corresponding to incident rays with a 

 speed of 280 volts. When the metal was first inserted, 

 after the surface had been polished by emery and oil, and 

 heated to drive off the oil, R was 2*44. The passage of a 

 discharge in 2 mm. of air for 5 minutes changed R to 0*89. 

 Bombardment with cathode rays from W in a very high 

 vacuum (current carried by rays about 10 ~ 6 amp. ; speed 

 150 volts) for 2 hours increased R to 1*42 ; further treat- 

 ment of the same kind for 22 hours increased R to 1*623 ; 

 further treatment for 52 hours left R unchanged at T621. 



