Richardson's handbook of projection (1930)

MANAGERS AND PROJECTIONISTS 1029 by battery B. Moreover, grid G is negatively charged or "biased" as the engineer terms it, with respect to filament F by battery C. Grid G is really the element which controls the action of the electrons and in so doing unlocks the door for sound amplification. The action is as follows : Filament F is heated by current from the A battery, as shown. This causes electrons to be "boiled out" or thrown off, as already described. These electrons from the filament bear a negative charge, hence have an affinity for positively charged Plate P, which of course attracts them, and we thus have a current flow established between Filament F and Plate P, but it is uncontrolled, hence of no practical value in sound amplification. The number or percentage of the electrons thus attracted to Plate P will be dependent upon the strength of the potential, up to the point where all that are liberated from Filament F are attracted. And we now arrive at the crux of the whole matter. Grid G, interposed between Filament F and Plate P, is negatively charged by the "C" battery. The flow of negatively charged electrons is from Filament F, toward positively charged Plate P, and since Grid G is negatively charged, and negative repels negative, it acts as a sort of electrical valve, which opens and closes with every slightest alteration in applied voltage, to permit of more or less electrons from Filament F to Plate P. Don't overlook the vital fact that the voltage of Grid G is controlled in its variations by the current emanating from the photo-electric cell, which latter fact is explained as follows : The E M F supplied by Battery C to the grid is of course steady, but the current coming from the P E cell over