Second, it has a low melting point. It melts immediately if you hold a match to it; try this and see. Consequently, long before the fuse wire becomes red hot, it melts in two. It has enough resistance to make it hot as soon as too many amperes flow through; and it has such a low melting point that as soon as it gets hot it melts in two, or blows out. This breaks the circuit, of course, so that no more electricity can flow. In this way the fuse protects houses from catching fire through short circuits. [Illustration: FIG. 137. What will happen when the pin is thrust through the cords and the electricity turned on?] Unfortunately, however, the fuse is almost no protection against an electric arc. The copper vapor through which the electricity passes in an arc has enough resistance to keep the amperage (current) low; so the arc may not blow out the fuse at all. But if it were not for fuses, there would be about as much danger of houses being set on fire by short circuits as by arcs. Perhaps there would be more danger, because short circuits are the more common. EXPERIMENT 73. Put a new piece of fuse wire across the fuse gap. Leave the "nail plug" screwed in the socket. Use a piece of flexible lamp cord--the kind that is made of two strands of wire twisted together (see Fig. 137). Fasten one bared end of each wire around each nail of the "nail plug." See that the other ends of the lamp cord are not touching each other. Turn on the electricity. Does anything happen? Turn off the electricity. Now put a pin straight through the middle of the two wires. Turn on the electricity again. What happens? There is not much resistance in the pin, and so it allows the electricity to rush through it. People sometimes cause fuses to blow out by pinning pictures to electric lamp wires or by pinning the wires up out of the way. A SHORT CIRCUIT AN "EASY CIRCUIT." You always get a short circuit when you give electricity an easy way to get from one wire to the other. But you get no current unless you give it some way to pass from one wire to the other, thus completing the circuit. Therefore you should always complete the circuit through something which resists the flow of electricity, like an electric lamp, a heater, or an iron. Remember this and you will have the key to an understanding of the practical use of electricity.