Application of the Photoelectric Element as a Receiving Device for Wireless Telegraphy

(Die Verwendung der lichtelectrischen Zellen als Empfangsinstrument für drahtlose Telegraphie. Verh. d. D. Phys. Ges. 16, 668, 1914).

The intensity of radiotelegraphic signals is a quantity very important for practical radiotelegraphy, but one that is rather difficult to measure. The difficulties lie both in the weakness of the received current and in the instability of the crystal detectors which at present serve as the principal instruments for reception in radiotelegraphy, and, consequently, in the impossibility of comparing the results of measurements made at different times and under different conditions. H. Behnken made use, for measuring the intensity of reception, of the circumstance that electric oscillations exert an influence on the magnitude of the photoelectric effect; in this connection it turned out that the change in the magnitude of the effect is proportional to the effective current intensity in the antenna and, moreover, under rigorously equal conditions, remains quite constant.

The photoelectric cell used by Behnken has as its light-sensitive electrode one made of potassium; the other electrode is made of platinum and is connected to the heated electrode of the self-inductance \(S_1\), connected to a long self-inductance \(S_1\); the antenna and the earth are wound, where possible, with a large number of turns in order to increase the potential imposed upon the cell under the influence of the oscillations. An electrometer, Zutz-Edelmann’s \((E)\), serves for measuring the current. In order to prepare the apparatus for receiving radio signals, the cell is illuminated with a Nernst lamp \((L)\), whereby a certain photo-effect arises, and the electrometer gives a deflection; this deflection is compensated by a certain additional voltage, applied by means of an accumulator \((Ak)\) with a potentiometer \((R)\), and, in this way, the electrometer is again set to zero; under these conditions the connection of the electrometer leaf with the earth by means of the key \((K)\) should not cause a deflection. In the presence of oscillations in the antenna, the potential applied to the cell will undergo periodic changes and will cause a deflection of the electrometer, which will be proportional, owing to the amplitude of the voltages arising in the coil \(S_2\), i.e. proportional to the magnitude of the current in the antenna. Such an arrangement gave, in Behnken’s experiments, a deflection of the electrometer of 4–5 scale divisions during operation of the radio station of the Eiffel Tower (distance Paris—Berlin about 900 km), which in sensitivity is fully comparable with reception by detector.

S. Rzhevkin.