he 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.

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 th