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496 RICHARDSON'S BLUEBOOK OF PROJECTION
studying them. The electrical relationships of the different parts are shown as simply as the draftsman could draw them. But the actual physical apparatus must be arranged with respect to convenience in wiring and repair, and with regard to economy of space inside the cabinet that contains them.
Figure 112 shows the same equipment and wiring as Figure 113, drawn exactly as they are placed and wired in the physical cabinet. Every part shown in Figure 113 can be located in Figure 112 by reference to the part numbers. For example, the filter retard coil L-l is shown at the lower left of Figure 112, just above the field rheostat terminal F.
Just above L-l, in Figure 112, is a complicated-seeming arrangement of resistors and a tap-switch, which would be troublesome to trace in detail if Figure 113 did not explain to us exactly what that resistance is, and precisely how it is wired into the circuit. It is, of course, D-4, the generator field control, or voltage regulator. There will never be the least occasion to trace its wiring in detail except in case of trouble within that resistor itself, or in its tap-switch contacts. Everything else the projectionist will ever need to know about it is most conveniently shown in Figure 113.
Storage Battery Charger
Another method of converting line power to direct current of suitable voltage is to use the commerciallysupplied electricity to charge storage batteries. In a few systems, in the past, as much as 350 volts at low amperage was drawn from storage batteries for amplifier "B" supply. This practice never became widespread because of the inconvenience involved in cleaning and watering the large number of cells necessary. But storage batteries are quite commonly used for /ow-voltage d. c. since only a few cells are required.
(10) Where the line power is direct current, the charger is merely a rheostat that serves to reduce the line voltage to that necessary for charging. (11) A. c,