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32 MOTION PICTURE HANDBOOK ("neutral") wire. As has been said, the main reason for the use of alternating current lies in the fact that it can be generated at high voltage, carried to the place desired at that pressure and there easily transformed to lower voltage. This cannot be done with direct, or at least it is not practical to do it. Direct current is to be preferred, but for this one objection and the three-wire system was evolved as a means of carrying relatively large quantities of direct current electro- motive force without prohibitive wiring cost. The dia- gram shows two methods of attaching a projection lamp to this system. In one we attach to the two outer wires and will get the full series pressure of the two dyna- mos, usually (practically always) iio-volt machines, which will be 220 volts, and we must provide rheostat resistance for that pressure. The other and usual method is to attach to the center (neutral) and one outer wire, which gives us the pres- sure of one dynamo, usually 110 volts. Incandescent service circuits are run by attaching to one outer and the neutral wires and using 110-volt lamps. Service circuits may be attached to the two outer wires, however, by using 220-volt lamps, but it is not desirable to do so, since the high resistance lamp is not so efficient. In this system the center wire is called the neutral, and is both positive and negative. In practice the service circuits are run from both sides and kept as evenly balanced as is prac- tical. The lamps of the circuits on the two sides burn in series, and the neutral only carries current equal to the difference in the load on the opposite sides. If the load of the opposite sides is evenly balanced there will be no current at all flowing in the neutral. It is possible, therefore, for the fuse of the neutral main to blow without affecting the lights of the system at all. As a matter of fact, however, it is seldom or never that a sys- tem is thus evenly balanced. You may cut your projection lamp in on any service circuit, which is heavy enough to carry the load by simply attaching, as instructed for Fig. 12 system. You may also attach direcly to the mains if desired. Fig. 14 is a diagram of the multiple series system of wiring, not much in use, however. In this system a very considerable range of voltage is possible, the lamps being burned in series. Ascertain the voltage of the lamps by looking at the tag pasted on them and multiply the voltage of the lamp by the number of lamps in the series, and the product will be the total voltage car-