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ii4 SOUND MOTION PICTURES
les to a greater or lesser extent. The earliest equipments employed both high and low voltage batteries. The highvoltage set furnished 360 volts for the plate circuits of the amplifiers, and were known as the B batteries. The lowvoltage batteries furnished a 12-volt supply for heating the amplifier filaments (A batteries), for lighting the sound lamp filaments (F batteries), and for magnetizing the receivers at the horns (H batteries). Since the plate voltage for the present types of main amplifiers is obtained by rectification, the B batteries are no longer being installed. The larger equipments still retain F batteries, to supply some of the amplifier tube filaments and the filaments of the sound lamps and indicating lamps and H batteries. The smaller equipments use the same 12-volt battery for both filaments and horns. It is worthy of note that in Western Electric installations the low-voltage batteries have been usually supplied in duplicate with the object that one set shall always be kept charged while the other is being used. This provision for alternate use obviates any danger of battery exhaustion, even in houses where the equipment operates almost continuously. The two sets are distinguished by numbers, thus: F-i, F-2, H-i, H-2, A-i, A-2. Duplication of the B batteries was not necessary, as the amount of current they had to supply was small compared with their charging rate. The batteries are put on charge, or connected to the operating circuit, as the case may be, by means of suitable switches mounted on panels enclosed in metal cabinets, various types having been used with the different kinds and sizes of equipments. On these panels each battery is usually connected to a switch having two positions, "Charge" and "Operate"; the position in which this switch is set then determines whether the battery receives current or gives it out. When the theatre power supply is direct current, rheostats are used in the battery charging