All parts of the machine should be tested to ascertain which is the hottest, since heat generated in one part is rapidly diffused. This is best done by starting with the machine cold; any serious trouble from heating is usually perceptible after a run of a few minutes at full speed with the field magnets excited. Heating may be due to various electrical or mechanical causes, and it may occur in the different parts of the machine, as in: 1. The connections; 2. The brushes and commutator; 3. The armature; 4. The field magnet; 5. The bearing. Ques. How is heating detected? Ans. By applying the hand to the different parts of the machine if low tension, or a thermometer if high tension, and also by a smell of overheated insulation, paint, or varnish. Ques. What should be done if the odor of overheated insulation, paint or varnish be noticeable? Ans. It is advisable to stop the machine at once, otherwise the insulation is liable to be destroyed. Ques. What is the allowable rise of temperature in a well designed machine? Ans. It should not exceed 80° Fahr., above the surrounding air, and in the case of the bearings, this temperature ought not to be reached under normal conditions of working. If this limit be exceeded after a run of six hours or less, it indicates a machine either badly designed and probably with the material cut down to the lowest possible limit with a view to cheapness, or some fault or other which should be searched for and remedied as early as possible, otherwise the machine will probably be destroyed. Ques. How should the rise of temperature be measured? Ans. It is not sufficient to feel the machine with the hand, but special thermometers must be placed on the armature winding, immediately on stopping the machine, covering them with cotton or wool to prevent cooling. Readings must be taken at short intervals, and continued till no further rise of temperature is indicated. Heating of Connections.--A rise of temperature of the connections may be due to either excessive current, or bad contacts, or both. The terminals and connections will be excessively heated if a larger current pass through them than they are designed to carry. This nearly always proceeds from an overload of the dynamo, and if this be rectified, the heating will disappear. If the contacts of the different connections of the dynamo be not kept thoroughly clean and free from all grit, oil, etc., and the connections themselves be not tightly screwed up, heating will result, and the connections may even become unsoldered. Heating of Brushes, Commutator and Armature.--When heating occurs in these parts, it may be due to any of the following causes: 1, excessive current; 2, hot bearings; 3, short circuits in armature or commutator; 4, moisture in armature coils; 5, breaks in armature coils; 6, eddy currents in armature core or conductor.