will gain speed quickly enough so that
its armature winding will not be overheated. A larger motor of that
kind can not be started safely without introducing resistance into the
armature circuit on starting, and cutting it out gradually as the
armature gains speed. Such a motor could be made self-protecting by
having the armature winding of much larger wire than really is
required for mere running, choosing its size great enough to carry the
large starting current without overheating itself and its insulation.
It is better, and for long has been standard practice, to use starting
boxes, frankly admitting that such motors are not self-protecting
until started, though they are self-protecting while running at normal
speeds. Such a motor, once started, may be overloaded so as to be
slowed down. So much more current now can pass through the armature
that its winding is again in danger. Overload circuit-breakers are
provided for the very purpose of taking motors out of circuit in cases
where, once up to speed, they are mechanically brought down again and
into danger. Such a circuit-breaker is a device for protecting against
an _internal_ hazard; that is, internal to the power system of which
the motor is a part.

Another example: In certain situations, apparatus intended to operate
under impulses of large current may be capable of carrying its normal
impulses successfully but incapable of carrying currents from the same
pressure continuously. Protective means may be provided for detaching
such apparatus from the circuit whenever the period in which the
current acts is not short enough to insure safety. This is cited as a
case wherein a current, normal in amount but abnormal in duration,
becomes a hazard.

The last mentioned example of damage from internal hazards brings us
to the law of the electrical generation of heat. _The greater the
current or the greater the resistance of the conductor heated or the
longer the time, the greater will he the heat generated in that
conductor._ But this generated heat varies directly as the resistance
and as the time and as the square of the current, that is, the law is

Heat generated = _C^{2}Rt_

in which _C_ = the current; _R_=the resistance of the conductor; and
_t_ = the time.

It is obvious that a protective device, such as an overload
circuit-breaker for a motor, or a protector for telephone apparatus,
needs to operate more quickly for a large current than for a small
one, a