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ELECTRICAL ACTION 19
through which they can continue to move without encountering apparatus in which they meet appreciable resistance. But the less the resistance of a circuit, the greater the current which a given volt will be able to drive through that circuit. The current in the short-circuit of Fig. 3, which encounters no resistance at all except that of the copper wires, would almost certainly be too strung for the wires or tor the generator, and would burn out one or the other, except for protective devices as explained hereafter.
(41) The words short-circuit and ground are often used interchangeably, and a short-circuit such as that of Fig. 3 is often described as a "ground" even when no earth connection is involved. It is obvious, of course, that the condition in that diagram would be most likely to occur in practice through a mistake in making earth connections. If by error the positive instead of the negative side of the motor were grounded to earth, while either the lamp circuit or the generator were grounded at its negative terminal, the conductivity of the earth would supply the place of the dotted line of Fig. 3, with more or less disastrous results. The protection given by fuses in such cases is discussed elsewhere.
(42) A short-circuit is always a relative condition. In the case of Fig. 3 both the lamps and the motor have somewhat low resistance, no more than a few ohms. They can be short-circuited effectively only by some conductor of still lower resistance, namely, a copper wire or a good connection through earth. But many circuits used in the projection room have very high resistance; in some cases resistances as high as millions of ohms. Such a circuit could be "shorted" very effectively by a resistance of 10,000 ohms; where in the case of Fig. 3, if the dotted line represented 10,000 ohms it would have no more practical effect than if it were an open switch. A short circuit is always a connection of less resistance than the normal circuit. A high resistance connection can