﻿200 Ionization produced by Carbon at High Temperatures. 



The lowest value found at 1300°, with a potential difference 

 of 330 volts, was 1*5 X 10~ 9 ampere per square cm. surface 

 of carbon, whereas Richardson found at this temperature 

 1-3 xlO- 4 . At 2000° with a potential difference of 200 

 volts, the lowest value found was 4xl0 -7 ampere, while 

 Richardson's formula gives, at this temperature, about 

 100,000 amperes. 



Above 1800° the ionization only increased very slightly 

 with the temperature. 



These results clearly show that the large currents hitherto 

 obtained with heated carbon cannot be ascribed to the 

 emission of electrons from carbon itself, but that they are 

 probably due to some reaction at high temperatures between 

 the carbon or contained impurities and the surrounding 

 gas Q s which involves the emission of electrons. 



We are deeply indebted to Prof. Rutherford and Dr. H. 

 (reio-er for much valuable advice during i^ course of this 

 work. 



Physical Laboratories, 



Victoria University, 

 Manchester. 



Addendum. 



Effects similar to the above results obtained with carbon 

 have been noticed by Soddy * in the case of the Wehnelt 

 lime cathode. It was found that, at very low pressures, this 

 cathode ceased to produce ions, whereas, according to 

 previous experimenters, greater saturation currents were 

 given the lower the pressure. Soddy concludes that 

 electronic emission can only account for a negligible 

 fraction of the total current, the remainder being carried by 

 the residual gas. 



Richardson f reaffirms the opinion that the negative leak 

 from hot platinum is independent of the pressure at low 

 pressures, and that these gases et only appear to increase the 

 fundamental electronic discharge from the hot cathode by a 

 process involving the occurrence of ionization by collisions.'' 



Wehnelt i also contradicts the above inference of Soddy, 

 and states that in the best possible vacuum the total current 

 is sustained by the electrons emitted from the glowing- 

 oxides, and indeed at all pressures below about 0"1 mm. 



* ' Nature,' lxxvii. p. 53 (1907). 

 t ' Nature/ lxxvii. p. 197 (1907). 

 X Phys. Zeitschr. ix. p. 134 (1908). 



