Danger of an Oversupply of Current

How Electricity may be Measured:
If a small toy motor is connected with one cell, it rotates slowly; if connected with two cells, it rotates more rapidly, and in general, the greater the number of cells used, the stronger will be the action of the motor. But it is possible to send too strong a current through our wire, thereby interfering with all motion and destroying the motor. We have seen in Section 288 that the amount of current which can safely flow through a wire depends upon the thickness of the wire. A strong current sent through a fine wire has its electrical energy transformed largely into heat; and if the current is very strong, the heat developed may be sufficient to burn off the insulation and melt the wire itself. This is true not only of motors, but of all electric machinery in which there are current-bearing wires. The current should not be greater than the wires can carry, otherwise too much heat will be developed and damage will be done to instruments and surroundings.

The current sent through our electric stoves and irons should be strong enough to heat the coils, but not strong enough to melt them. If the current sent through our electric light wires is too great for the capacity of the wires, the heat developed will injure the wires and may cause disastrous results. The overloading of wires is responsible for many disastrous fires.

The danger of overloading may be eliminated by inserting in the circuit a fuse or other safety device. A fuse is made by combining a number of metals in such a way that the resulting substance has a low melting point and a high electrical resistance. A fuse is inserted in the circuit, and the instant the current increases beyond its normal amount the fuse melts, breaks the circuit, and thus protects the remaining part of the circuit from the danger of an overload. In this way, a circuit designed to carry a certain current is protected from the danger of an accidental overload. The noise made by the burning out of a fuse in a trolley car frequently alarms passengers, but it is really a sign that the system is in good working order and that there is no danger of accident from too strong a current.