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36 MOTION PICTURE HANDBOOK Resistance Devices. Resistance is perhaps the one most important thing to the operator, and many have been the heated arguments as to the relative merits of various types of machine made for this pur- pose. Generally speaking, resistance devices may be divided into four classes, viz.: rheostats, transformers, choke coils and arc rectifiers. The rheostat is the oldest form of projection resistance, and for direct current is the only one available. Resistance is neces- sary from the fact that the carbons of a projection arc lamp form a dead short circuit when brought together. Means must therefore be provided to allow of but a certain limited quantity of current passing through the short thus made or the wires would burn up instantly were it not for the fuses, since the lamp would take far more current than the fuses and wires would carry. In fact, could such a condition be maintained, the only limit to current flow would be the capacity of the dynamo feeding the system. To prevent this, resistance is inserted in the circuit, and we will hrst consider that form known as the rheostat. Difil'erent metals possess different degrees of con- ductivity (current-carrying power), copper wire being the best of any metal commercially available. That is to say, a copper wire of a given size will carry a larger amount of current with- out heating than a wire made from any other metal combining the toughness and ductility necessary and at the same time not too costly. On the other hand, an alloy of certain other metals possesses high resistance to current, and wire made from this alloy is used in rheostats. The resistance device is to the elec- tric circuit exactly what the valve is to the water pipe. If you wish to get a certain quantity of water from a water pipe you don't take the cap off its end—you install a valve and open it just enough to let through the desired quantity. If you wish a certain quantity of current—say forty amperes—from a wire charged at 110 volts, you cut the wire and connect in a certain length of resistance wire calculated to allow 40 amperes to pass at a pressure of 110 volts. If the pressure were suddenly raised