Richardson's handbook of projection (1930)

1030 HANDBOOK OF PROJECTION FOR the input circuit is variable, or vibratory, which has the effect of reproducing those variations or vibrations in the grid voltage. You will now understand that the flow of electrons from F to P will be in exact proportion to the rise and fall in the negative potential of the grid, and that this rise and fall is controlled by what comes from the photoelectric cell, hence since what comes from the P E cell is an exact electrical reproduction of what the light beam has picked up from the film, it follows that the flow of electrons will be in exact proportion to the sound modulations the P E cell current represents. That may sound a bit complicated, but study it a bit. Once you get the idea the action is clear. And now we arrive at the crux of this whole matter. The action of the grid requires the application of an extremely small amount of power. The current coming from the P E cell is extremely weak, but it nevertheless is ample to operate not only one but several grids. On the other hand, we may apply a relatively very large amount of power to the plate, and since the flow of electrons from F to P through G in Fig. 387 serves to reproduce in this larger power every fluctuation of or modulation in the current coming from the P E cell because of the rise and fall in grid voltage it sets up, we are thus enabled to use a very weak power to set up an exact duplication of itself in a very much greater power and that is what constitutes amplification. The hand of a locomotive engineer is enormously weak as compared with the power of the engine, but nevertheless that weak hand is able to control the action of that great engine and its heavily laden train of cars. The principle is essentially the same. As a matter of