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128 a conductor becoming red-hot may set fire to a building.

To guard against this danger a short piece of the conductor is made very thin, so that in the event of the current becoming so strong that the rest of the conductor becomes sensibly hot, this little piece shall become so hot that it fuses, and thus interrupts the continuity of the conductor, that is, causes an interruption of the current. This is the principle of protecting electric circuits against overheating. The short bit of the conductor, intended by its destruction to save the rest, is called the fuse, and this is so placed that by its melting it cannot cause a fire. Such fuses are found in every domestic installation for lighting. The fuse wire is enclosed in a tube or plug of porcelain, and sometimes the cavity is filled in with carborundum powder to act as an absorbent of the heat momentarily generated by the explosive fusion of the wire.

In this connection it is interesting to note that the seemingly obvious is not always the best. At first fuse wires were made of tin or lead, simply because these metals fuse at a low temperature, and it seemed obvious that the lower the temperature of fusion the quicker would the device act. This is a fallacy. If lead or