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Applications of Photo-electric Control

    Naturevolume 135page1068 (1935) | Download Citation



    THE G.E.C. Journal (General Electric Company) of February gives an interesting review of electrical progress and development in 1934. Many useful devices are described. Aerodrome obstruction and boundary lights must be switched on when daylight is poor and when darkness approaches in the evening. It is essential that the pilots see the boundary of the aerodrome and any obstructions in the vicinity. The photo-cell has been successfully applied to the control of these lights. At Croydon Airport the switching on of the obstruction light is controlled by a photocell amplifier. Another useful application of photocells is to control the speed of escalators. The wear and tear of escalators like those in the underground railways of London which are in continuous use is extremely heavy; and renewals and repairs are expensive and, owing to the restricted space, are difficult to carry out. It is desirable to keep the speed low during slack periods at the less-frequented stations when no one is on the escalator. At the entrance to the stairway, a suitable lamp is installed to shine across the footway on to a photo-cell similarly mounted on the other side, just below the handrail. When this beam is interrupted by the entrance of a passenger, the stairway is speeded up in several stages so that the passenger feels no shock, and the escalator continues to run at a high speed until the passenger has time to reach the top. This is attained by a time delay device using a radio valve. If other passengers come on to the stairway before the last one reaches the top, the time delay device resets, so that the high speed continues until the last passenger gets to the top, after which the low speed comes into operation.

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