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612 RICHARDSON'S BLUEBOOK OF PROJECTION
ture therefore operates through the circuits of this cabinet to adjust the amount of direct current delivered to the control coil by the full-wave rectifier, V-l and V-2. The next step in seeing how this is done is to study that rectifier circuit. It differs in important details from rectifiers we have seen before.
The Circuits of V-l and V-2
The filaments of V-l and V-2 are heated b>the second secondary from the top, in power transformer,T-l. The plates are powered by the uppermost secondary of that transformer. The positive leg of the output connects to the filaments, the negative side of the circuit returning to the center-tap of the plate transformer. The output of this rectifier flows through the control coil just above L-2. There is a branch circuit through resistors R-l and R-2, just above the control coil. Condenser C-l (below L-2) is a filter that keeps the current through the control coil steady.
The unusual feature of this circuit lies in the grids of the two tubes. They control the current flowing through the vacuum, and therefore govern the saturation of the core of L-2 and hence the speed of the motor. Grid-controlled rectifiers of immense size are sometimes used in power houses, but are strange to projection rooms aside from these control cabinets. The grids of V-l and V-2 carry a positive bias. Negatrons are attracted to them, and there is a flow of current through the external circuits connecting grid and filament. However, the positive charge of the grid is much less than that of the plate, and only a comparatively small portion of all the negatrons emitted go to the grid. The rest respond to the stronger attraction of the plate. The function of the grid is to deprive the plate of current by attracting some of the negatrons. The reading of the milliammeter M-l depends upon the positive charge of the grid, that is, upon how many negatrons the grids of V-l and V-2 are able to attract from the plates of those tubes.