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, and this is just what all well-designed protective devices are intended to do.  The general problem which these heating hazards present with relation to telephone apparatus and circuits is:  To cause all parts of the telephone system to be made so as to carry successfully all currents which may flow in them because of any internal or external pressure, or to supplement them by devices which will stop or divert currents which could overheat them.