Cine-film projection : a practical manual for users of all types of 16-mm. (1952)

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When this substance is exposed to light (excited) it re-acts by producing a stream of electrons, or rather, it produces an electric current. As the light (from the exciter via the film) is increased, the current thus produced increases correspondingly. When the light varies rapidly, the current likewise varies. The current produced by the photo-electric cell is quite small and to guard against loss by resistance and possible interference, it is placed very close to a special valve which has as its primary function the immediate amplification of the current. In the average amplifier, this valve and its attendant circuit form what is known as the pre-amplifier, or pre-stage. If the photo-electric cell is allowed to become coated with dust it will be prevented from receiving the full strength of the light directed upon it. It will then produce a much smaller current, resulting in loss of volume. So the photo-electric cell must, of course, be included in the weekly clean. Exciter Lamp This little lamp, being in most cases fitted with a bayonet cap, can be inserted either of two ways — the right and the wrong way. You will notice that the filaments are formed by two uprights and a cross-piece, and it is the latter which provides the illumination. If the lamp is wrongly inserted, one end of the top filament will be nearer to the optics than the other, which means that one side of the sound-rack will receive a greater degree of illumination. This in turn will result in unequal volume between bass and treble frequencies of sound heard from the loudspeaker. The exciter lamp shown in Fig. 6 has been drawn specially to show you what the filaments look like, but you know now that in actual practice the lamp must be inserted so that the top filament is parallel with the face of the sound-lens. The Smoothing System — Damping The smoothing system consists of a series of rollers so arranged that the film passing the scanning-beam is completely steadied, both as to lateral movement and the irregularities in its speed. In many projectors a heavy flywheel ensures that the film is carried along at an even rate, pressure rollers keep the 40