Explanation of Fig. 66.—Potassium vacuum bulbs containing ⅓ millimetre of hydrogen mounted and connected to battery and galvanometer, and arranged as a photo-electric photometer.
Wied. Ann., 48, p. 625. On the Photo-Electric
Comparison of Sources of Light.
Attempts to make such a potassium cell into a photometer.
Wied. Ann. 52, p. 433. Further Photo-Electric Experiments.
Plates of platinum, silver, copper need exceedingly ultra-violet light before they show any photo-electric power; zinc, aluminium, magnesium show it for visible violet and blue light; the alkali metals, in an atmosphere of rarefied hydrogen, advance their range of sensibility into the spectral red; while under the most favourable conditions they show a sensibility only inferior to that of the eye itself. The authors now use galvanometric methods of measuring the effect, instead of only electrometers, and they arrive at the following results:—
(1) The three alkali metals Na, K, Rb, have different sensibility for differently-coloured lights. For long waves their order of sensibility is Rb, Na, K; though rhubidium is far exceeded by the other two metals in white light.
(2) Illumination of a plane alkali metal cathode surface with polarised light causes greatest discharge if the plane of polarisation is normal to plane of incidence; and least, if the two coincide.
(This is a most remarkable observation. Its probable meaning is that the electric oscillations of light are photo-electrically effective in so far as they are normal to the surface on which they act; while electric oscillations tangential to the surface are scarcely operative. Different angles of incidence must be tried before the proof is complete.—O. J. L.)
(3) Electric oscillations of very short period, such as are given by a Hertz oscillator, are commutated by illumination in the presence of alkali metals in rarefied gas, so as to be able to set up a constant electric tension in the gas.
(A Zehnder tube[39] was used, and the momentary phases of the oscillation during which the metal is negatively charged are apparently taken advantage of by the illumination.)