Signalling through space without wires : being a description of the work of Hertz & his successors / by Oliver J. Lodge.

  • Oliver Lodge
Date:
[1900]
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Licence: Public Domain Mark

Credit: Signalling through space without wires : being a description of the work of Hertz & his successors / by Oliver J. Lodge. Source: Wellcome Collection.

Provider: This material has been provided by the Francis A. Countway Library of Medicine, through the Medical Heritage Library. The original may be consulted at the Francis A. Countway Library of Medicine, Harvard Medical School.

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    Wied. Ann.j 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 sensi- bility 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 polari- sation is normal to plane of incidence ; and least, if the two coincide. [This is a most remarkable observation. Its probable mean- ing 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.—0. 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 Zelmder tube* was used, and the momentary phases of the oscillation during which the metal is negatively charged are apparently taken advantage of by the illumination.] (4) The photo-electric dissipation showed by powdered fluorspar is dependent on the colour of the mineral, in such a way that the deepest blue, violet or green specimens are the most sensitive. * See Fig. 13, p. 15.