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The Three-Electrode Vacuum Tube — Kellogg 951
between high pitch and low pitch sounds is preserved only when the reproduced sound is of practically the same loudness as the original sound.
I referred a few minutes ago to the use of a sound pickup for recording, in which all other factors were subordinated to quality. I had in mind what is known as the condenser transmitter. In this a metal diaphragm about 0.0001 inch thick and stretched practically up to the elastic limit of its material is moved by the sound waves so that it approaches and recedes from a stationary electrode. A practically constant charge is maintained on the condenser formed by the diaphragm and the stationary electrode. The voltage across a condenser is equal to the charge divided by the capacity. Hence the voltage changes with the distance between the diaphragm and the stationary electrode, and this variable voltage is impressed on the grid of an amplifier tube. The power output obtainable directly from a condenser transmitter is practically zero, but fortunately it requires zero power, under suitable conditions, to produce voltage changes between the grid and filament of an amplifier tube. Although the condenser transmitter is rated as very insensitive (as compared, for example, with the ordinary carbon transmitter), it is of interest to note that with the amplification which we customarily associate with it, we obtain a usable signal from it when the diaphragm excursion is only of the order of the diameter of a single atom, or about one hundred millionth part of an inch.
Other factors essential to a successful talking motion picture owe their existence largely to the vacuum tube amplifier. I have in mind in particular high quality loud speakers. These owe their existence to the amplifier tube; first, because without the ample power and high quality voice currents for their operation which amplifiers can furnish, the loud speaker development would have been extremely difficult; and secondly, because the vacuum tube is the foundation of popular radio reception, and without the stimulus of this field of application loud speaker development would not be near where it is today.
I have thought it worth while to dwell thus far on the part played by vacuum tubes in the talking motion picture, for such a survey is helpful to our perspective. The remainder of the paper will be devoted to an outline of the conditions necessary for the proper functioning of the amplifier tubes.