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.

313.  How Current is Measured.  The preceding Section has shown clearly the danger of too strong a current, and the necessity for limiting the current to that which the wire can safely carry.  There are times when it is desirable to know accurately the strength of a current, not only in order to guard against an overload, but also in order to determine in advance the mechanical and chemical effects which will be produced by the current.  For example, the strength of the current determines the thickness of the coating of silver which forms in a given time on a spoon placed in an electrolytic bath; if the current is weak, a thin plating is made on the spoon; if the current is strong, a thick plating is made.  If, therefore, the exact value of the current is known, the exact amount of silver which will be deposited on the spoon in a given time can be definitely calculated.

[Illustration:  FIG. 233.—­The principle of the galvanometer.]

Current-measuring instruments, or galvanometers, depend for their action on the magnetic properties of current electricity.  The principle of practically all galvanometers is as follows:—­

A closely wound coil of fine wire free to rotate is suspended as in Figure 233 between the poles of a strong magnet.  When a current is sent through the coil, the coil becomes a magnet and turns so that its faces will be towards the poles of the permanent magnet.  But as the coil turns, the suspending wire becomes twisted and hinders the turning.  For this reason, the coil can turn only until the motion caused by the current is balanced by the twist of the suspending wire.  But the stronger the current through the coil, the stronger will be the force tending to rotate the coil, and hence the less effective will