Weid. Ann., 46, p. 281.—On the Behaviour of Alkali Metal Cathodes in Geissler Tubes; On Photo-Electric Discharge in a Magnetic Field; and On the Measure of Photo-Electric Currents in Potassium Cells by means of a Galvanometer.
Results:—The resistance of a Geissler tube provided with a cathode surface of pure alkali metal is diminished by the light from the sparks of an induction coil; especially when the pressure is ·1 to ·01 mm. of mercury. The resistance which rarefied gas opposes to an electric current in a magnetic field is greatest in the direction normal to the magnetic lines. The changes of resistance effected by any kind of light in a vacuum tube with alkali metal cathode can be measured galvanometrically. (A Daniell cell gives 100 divisions on a Rosenthal galvanometer when coupled up through such an illuminated tube, each division meaning about 10⁻¹⁰· ampere.)
Fig. 64.
Explanation of Fig. 64.—A vacuum tube of rarefied hydrogen containing alkali metal as cathode, say the liquid K-Na alloy, or solid K or Na. A spark gap at S serves as alternative path, and a stream of sparks can occur to the plate P in the dark. But when light falls on the surface A this stream of sparks can cease, showing that the resistance of the vacuum tube is diminished.
Fig. 65.
Explanation of Fig. 65.—Showing position of magnetic poles with respect to the vacuum tube discharge. With the poles across the line of discharge, as in Fig. on left, excitation of the magnet opposes the leak from the surface. With the poles as in Fig. on right, the discharge is not much affected, it is even sometimes slightly increased.
Fig. 66.