If due to this latter cause, it will be accompanied by sparking at the brushes. If a shunt dynamo, insert an ammeter into the shunt circuit, and note if the deflection be steady. If this be not the case, the variation in the current most probably proceeds from imperfect contacts thrown into vibration.
Ques. How is the insulation affected by moisture?
Ans. Moisture tends to decrease the insulation resistance, thus in effect producing a short circuit with its attendant heating.
Ques. How is moisture in the field coils detected?
Ans. It is easily detected by applying the hand to the coils, when they will be found to be damp, and in addition steam or vapor will be given off where the machine is working.
The fault may be remedied by drying and varnishing the coils.
Ques. What is the indication of short circuits in the field coils?
Ans. This fault is characterized by an unequal heating of the field coils. If the coils be connected in series, the faulty coil will be heated to a less extent than the perfect coils; if connected in parallel, the faulty coil will be heated to a greater extent than the perfect coils. The former can thus be easily located.