Abstract
The magnitude of the purely thermal emission of electrons from the alkali metals has not been fully determined. The first investigation was by Prof. O. W. Richardson on the emission from sodium. This was contained in a horizontal steel tube in which a wire stretched parallel to the axis served as collecting electrode. The conditions, however, were such that there was a considerable evolution of gas in the apparatus during the heating, and the currents obtained showed no approach to saturation. The negative currents at constant potential difference agreed with the emission formula
i
= A
1
ЄT
½
e
-
b
/t in which A
1
= 10
31
6 = 3·16 X 10
4
. K. Fredenhagen also investigated the emission from sodium and potassium. He worked with a glass apparatus and purified the metal by a two or threefold distillation. In his final experiments the tube was evacuated as far as possible and then sealed off. Observations were then taken while the alkali metal was heated continuously for many hours. The currents obtained were found to depend largely on the treatment of the tube, and showed a tendency to decrease with the duration of the experiments. Fredenhagen thus concluded that the observed currents were not due to the metal itself, but to some gas which could be partly removed by repeated distillation of the metal, and by long heating in an apparatus in which the metal was continually vaporising, condensing, and flowing back to its original position. He also concluded that the gas was probably hydrogen, although he had no direct proof. The currents were usually greatly reduced by the action of an induction coil discharge, and he found that a connection existed between the magnitudes of the thermal currents and the photo-electric currents in so far as they were found to decrease together.
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