repeatedly, it gradually loses its lubricating properties, becoming thick and gritty, and should be occasionally run through a filter and mixed with new oil. The frequency of this change depends on the oil, as well as the number of hours the engine is in operation, and can easily be determined by observation. The oil in the reservoir should stand about 2 inches over the suction and discharge valves, and no water should be allowed to mix with it. Should any water accumulate in the base, it should be drawn off by the cock provided for the purpose before starting the engine.] Ques. How may the field magnets become heated? Ans. By excessive field current; eddy current in pole pieces; moisture; short circuits. Ques. What may be said with respect to excessive field current? Ans. When heating results from this cause, all the exciting coils will be heated equally. It may be due to excessive voltage, in the case of shunt dynamos; or to an overload in the case of compound and series dynamos. In either case it may be remedied by reducing the voltage or overload. If due to the coils being incorrectly coupled up, that is, coupled up in parallel instead of in series, it will be necessary to rectify the connections or insert a resistance in series. Ques. State the causes of eddy currents in the pole pieces? Ans. This fault may be due to defective design or construction of the armature. Slotted armatures are particularly liable to cause this fault, if the teeth and air gap be not properly proportioned. The defect may also be occasioned by variation in the strength of the exciting current. 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. CHAPTER XXXV OPERATION OF MOTORS