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AMATEUR MOVIE MAKING 115
to a certain limit. Beyond this limit, the force of the current will not carry the flow across the gap and the arc is broken. This simple form of lamp is not practical, because without a resistance to act as a gate, such a tremendous amount of current would be drawn across the arc that the connecting wires would be fused. When we add the resistance, this acts as a gate allowing only a certain amount of current to flow. This protects the line. In addition, fuse plugs are used of a sufficient capacity to take care of the rated current of the lamp. Arcs as used for motion photography by the commercial and amateur worker usually have a maximum pull of twenty or twenty-five amperes. They should only be used on lines where wiring has been installed which will carry such a load. A twenty ampere lamp will often blow twenty ampere fuses, so twentyfive or thirty ampere fuses should be used. For use on ordinary lines a ten ampere draw is provided by a switching arrangement. This cuts down the light out of all proportion to the current, ten amperes usually giving about one-fourth the amount of light furnished by the same lamp on twenty amperes. For this reason manufacturers have devoted their attention to producing a lamp which would give adequate light from a ten ampere current.
It has been found that the material used in making the arc rods has much to do with the photographic power of the light. This has led to experimentation with the result that these rods are now made of a special carbon. Through the center of this carbon rod is a hole. This hole is filled with a compound which gives an intensely white flame. In this manner, a small ten ampere lamp which may be plugged into any ordinary house socket will actually give a more powerful light than was obtained from the old time, heavy current lamps using solid carbons. So we find this type of lamp divided into two large classes, the double range arc which gives roughly 5,000 candlepower on ten amperes and 18,000 to 20,000 candlepower on the twenty ampere pull; and the amateur lamps which give from 10,000 to 12,000 candlepower on a ten ampere pull. The latter type of lamps are made so that even at the instant of making the arc the current consumption will not rise