Short Circuit in Individual Coils.--This is a common fault, which makes its presence known by a violent heating of the armature, flashing at the commutator, flickering of the light on lighting circuits, and by a smell of burning varnish or overheated insulation. When these indications are present, the machine should be stopped at once, otherwise the armature is liable to be burnt out. The fault is due either to metallic dust lodging in the insulation between adjacent bars of the commutator, or to one or more convolutions of the coils coming into contact with each other, either through a metallic filing becoming embedded in the insulation or damage to the insulation. [Illustration: Fig. 700.--Method of locating short circuited armature coil. Disconnect the external and field circuits from the armature, and pass a large current--say from 20 to 100 amperes--from a battery (B) or another dynamo through the whole armature by means of the brushes. Then, having previously well cleaned the commutator, measure the difference of potential between adjacent segments all round the commutator (C), by means of a voltmeter or galvanometer (G), the terminals of which are connected to adjacent segments, as shown. The short circuited coil or coils will be located by the difference of potential between the corresponding segments being little or nothing. It may be remarked, however, that this is not always a decisive test. In some cases the short circuit may be intermittent, or may disappear as soon as the armature ceases to rotate. In such cases, the short circuit is caused by the wire coming into contact through the action of the centrifugal forces developed by the rotation of the armature.] Ques. How is the faulty coil located? Ans. When the machine is stopped, the faulty coil, if not burnt out, can generally be located by the baked appearance of the varnish or insulation, and by its excessive temperature over the rest of the coils, being detected also by the baked appearance of the varnish or insulation. Ques. What should be done if the machine do not build, and it be suspected that the fault is due to short circuited armature coils? Ans. The field magnets should be excited by the current from a storage battery or another dynamo, and, having raised the brushes from contact with the commutator, the armature should be run for a short time. In stopping, the faulty coil or coils may be located by the heat generated by the short circuit. When the dynamo is started for the purpose of localizing a short circuit, precautions should be taken, and the machine only run for a few minutes at a time until the faulty coil is detected.