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284
RICHARDSON'S BLUEBOOK OF PROJECTION
(33) While the tube plate is positively charged the flow of electrons from filament to plate is as free as though these two elements were connected by wire, making due allowance for differences in resistance. So long as the plate remains positive the arc circuit may be traced by following the current direction from negative to positive: (34) from the lower or negative end of the filament secondary to its center tap, thence to the filament and thence around its secondary coil circuit. From this current come the emited electrons which flow through the plate and arc, and thence back to the upper transformer secondary.
(35) Passing on to the consideration of the next succeeding alternation of the mainline current, we discover that the secondary has reversed its polarity, which means that the plate is now also negatively charged. Since the electrons emitted by the filament are always negative, and like does not attract like, no electrons will flow to the plate, and therefore none to the arc. (36) Hence, as regards a single plate, only one "side" of the a. c. alternations can be used. Current flows through one-half of each alternation, but none flows during the succeeding half. Therefore, with one tube, we will have direct but very highly pulsating d. c. at the light source. All this is illustrated in Fig. 74A but it is a very undesirable type of rectifier for projection. Let us consider one more suited to the work.
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TUBS
Figure 74B
(37) Fig. 74B shows a modification of Fig. 74 A. The secondaries of the transformer are center-tapped,