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MANAGERS AND PROJECTIONISTS 969
inch. The back plate is anchored in fixed position. It remains rigid and immovable.
Under this condition it may readily be understood that any back and forth movement (vibration) of the front plate (the "diaphragm") caused by the impact of the sound waves in the air upon it, will result in a change in the distance of separation between the two, the amount of depth of vibration, and therefore the variation in distances of separation, being directly dependent upon how much movement any sound wave causes or sets up in the diaphragm. It will also be understood that the amount of movement any wave will set up in the diaphragm will be in proportion to the force exerted by that wave, which is governed entirely by the acoustic energy of the source originating the wave.
That much seems understandable enough, and when we consider that each variation in air wave frequency, no matter how slight, will cause variation in rapidity of diaphragm vibration, we begin to see a bit further into this mysterious thing.
If you have done as I have advised, and studied the matter under "The Condenser/' page 1017, you will know that each difference in the separation distance of the plates of a condenser, no matter how slight, has the effect of altering the capacity, or in technical terms the "capitance" of the condenser, which is another step in advance in understanding this matter.
We will now proceed to take the final step. If we connect the two condenser plates to a source of E. M. F., such as, for example, a storage battery, one plate to each polarity, and set the condenser plates vibrating by means of sound waves, the condenser will have its capacity altered with every variation of separation dis