ELECTRON EMISSION FROM GLOWING SOLIDS. 
279 
heating is evidence that the emission depends either upon the evolution of gas, or 
upon the gas remaining in the hot wire. P»jchardson’s experiments to test the 
former hypothesis may be taken as showing that the emission does not depend upon 
the evolution of gas, but there remains the possibility that it depends upon the gas 
which is still in the wire. This latter view was put forward by H. A. Wilson 4 " to 
explain the results of some experiments with a platinum wire heated in hydrogen 
gas. A series of observations were made at different pressures and it was found that 
on changing the gas pressure the resulting change in the thermionic current took 
some time to become established. A similar effect was obtained when the tempera¬ 
ture of the wire was altered, keeping the gas pressure constant, in which case the 
current variation lagged behind the alteration of temperature. These experiments 
indicate that the increase in the thermionic emission produced by hydrogen is due to 
the gas inside the platinum, and that it takes time for equilibrium between the gas 
in the wire and that surrounding it to become established after any change in the 
conditions has been made. 
It appeared probable that information as to the origin of the electron emission from 
glowing solids could be obtained by experimenting with those substances which give 
rise to exceptionally large emissions, namely, the oxides of the alkaline earth metals. 
These oxides are all stable substances and ones which would not be expected readily 
to enter into reaction with the residual gases left in the apparatus. The possibility 
of chemical action in these cases would appear to depend on the oxide being 
electrolysed by the passage of the discharge, in which case there might be some 
re-combination of the constituents. The view that the electron emission from 
glowing lime occurs as a result of the re-combination of calcium and oxygen which 
have been liberated electrolytically has been put forward by Fredenhagen.! The 
author does not think that there is very strong experimental evidence that such 
chemical changes do give rise to the emission of electrons, but this point will be 
referred to later. If, for the present, we admit the possibility of the chemical union 
of calcium and oxygen giving rise to an emission such as is observed in the case of 
the Wehnelt cathode, the plausibility of Fredenhagen’s theory depends upon 
whether the conductivity of these oxides can be shown to be accompanied by 
electrolysis. 
In 1905 the writer investigated the electric conductivity of lime at high tempera¬ 
tures and came to the conclusion that the conductivity is mainly carried on by 
electrons set free in the interior of the oxide, but that some electrolysis also occurs. 
Thus there is the necessary condition for the hypothesis as to the origin of the 
activity of a lime cathode put forward by Fredenhagen. The following experi¬ 
mental results are given by Fredenhagen as supporting this hypothesis:—- 
* H. A. Wilson, ‘Phil. Trans.,’ A, vol. 202, p. 243, 1903. 
t K. Fredenhagen, ‘Ber. K. Sachs. Ges. Wiss.,’ Leipzig, vol. 65, p. 42, 1913. 
