F. H. Richardson's bluebook of projection (1942)

444 RICHARDSON'S BLUEBOOK OF PROJECTION beside its tubes, but all other parts are merely assistant pieces, put there to help the valves carry out their function. All the parts that go to make up an amplifier, with their separate purposes and their interconnections, will be described in full detail a little later on. Here we are primarily interested in dividing up a complete sound system into its constituent units, of which the amplifier is only one. Speakers (d) The relatively powerful sound current drawn from the output circuit of an amplifier cannot be detected by human ears until it is converted into air vibrations of corresponding strength and frequency. (15) The apparatus that performs this conversion is the speaker, which is most easily understood by remembering that it is really an electric motor, driven by electrical power and producing mechanical power in the form of air waves. Most electric motors create mechanical power in the form of rotary motion. They turn a wheel. The speaker is a motor not designed to turn a wheel, but to actuate a piston. The piston, of course, is the diaphragm of the speaker, which operates upon the surrounding air at the frequency, and in accordance with the strength, of the current driving it. Speakers also will be described, in detail, in their proper place. Power Sources (e) Every sound system requires power to operate it, (16) particularly direct current at both low and high voltages. Modern amplifiers often operate by simple connection to an a. c. power line, but a. c. at commercial line voltage does not enable them to perform their work, therefore (17) within the amplifier itself that power is converted to d. c. of suitable voltage and amperage. Now, this work of power conversion is no part of the function of an amplifier. It is performed inside the amplifier casing only as a matter of convenience in con