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110 The Audio-Visual Handbook
jector is extremely light in weight and may be carried from one room or building to another even by a pupil in the intermediate grades. Another advantage is that it may be attached to any standard houselighting circuit without causing undue strain upon the wiring. The average circuit protected with a 10 ampere fuse will be sufficient to operate any of the 16 mm. silent projectors.
For some years, all educational sound pictures were either 35 mm. sound on film or 16 mm. film with sound recorded on a disc. There was a feeling for a few years that it would be impractical to record sound on 16 mm. film. The chief difficulty was that of finding a place on the film for the sound track. This problem was solved by omitting the sprocket holes on one side of the film. It has been found entirely satisfactory to use film which has the sprockets on one side only, as the better makes of 16 mm. sound motion-picture projectors are built with such accuracy that very little pull is required at the aperture gate to move the film from one frame to the next.
Another problem confronting those who developed the 16 mm. sound film was that of reducing the sound track from its original length of 1000 feet on 35 mm. negative to the new and shorter length of 400 feet on 16 mm. positive. This reduction has been accomplished optically with marked success and the recording and reproducing range on 16 mm. film now approximates that of the ordinary 35 mm. sound films. Some of the new types of sound recording such as the mirrophonic process used by Western Electric and the ultra-violet higher fidelity process used by RCA actually increase the range far beyond that which has been accomplished by reduction to 16 mm. film. On the other hand, first-class 16 mm. reductions cover the range of normal sounds in a thoroughly satisfactory manner. In many cases the person with an ear untrained to notice minute sound variations will be unable to detect whether the sound comes from 16 mm. or 35 mm. film.
As a result of perfecting 16 mm. sound films and projection apparatus, the 16 mm. sound film is taking the place of the 35 mm. film for educational purposes, except in those situations where it is desirable to use theatrical films for combined educational and recreational purposes. This situation is changing inasmuch as some of the theatrical producers are now reducing their best films to 16 mm. for use among schools. It is expected this change will become more general, thus gradually eliminating the use of 35 mm. equipment among schools except where it is necessary to project pictures longer distances and to greater size than can be accomplished satisfactorily with 16 mm. film and projection equipment. Many schools are using both types of