A Simple Form of Photo-electric Photometer

Abstract

DR. N. R. CAMPBELL (Phil. Mag., 111, pp. 945, 1041 ; 1927) has described a new method of using a gas-filled photo-electric cell by which small illuminations can be measured by the use of a telephone only. This method is, however, only applicable to gas filled, and not to vacuum, cells; and for accurate measurements of strong illumination the latter are much more trustworthy. It is possible by using the well-known phenomenon of the intermittent discharge through a neon lamp to employ a somewhat similar method in the case of a vacuum cell. The circuit necessary is very simple. The photo-electric cell and the neon lamp are connected in series with a high tension battery of suitable voltage, and the neon lamp is shunted with a condenser. Under these conditions intermittent flashing will occur in the lamp when the photo-electric cell is illuminated, and the frequency of the flashes will increase with the illumination. It is accordingly only necessary to time this frequency to obtain a measure of the illumination. To do this it has been found more convenient to insert a telephone in series with the shunting condenser, and to count the clicks heard in it.