SHEILA TERRY / SCIENCE PHOTO LIBRARY SHEILA TERRY / SCIENCE PHOTO LIBRARY
Early photoelectric cell. Photoelectric cells are used to convert light falling on them into electricity. This type of photoelectric cell was developed specifically for early television transmitters. Photoelectric cells make use of an effect whereby metals emit electrons when exposed to light (photo-emissivity). A metal plate of either rubidium, potassium or sodium (the cathode) is enclosed in a vacuum and connected to the negative pole of a battery. A loop of wire (the anode) faces the plate. When exposed to light, electricity crosses the gap between the cathode and anode more easily, enabling the cell to produce impulses of electricity dependent on the intensity of the light falling on it.
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