Ques. What should be done if the brushes begin to spark excessively? Ans. First, look at the ammeter to see if an excessive amount of current is being delivered; second, see if the brushes make good contact with the commutator, and if the latter have a bar too high, or too low, and an open circuit. [Illustration: Figs. 681 and 682.--Remedies for leakage of oil from self-oiling bearings. If there be sufficient space, a metal ring may be attached to the shaft as in fig. 681. With this arrangement the high speed of the shaft will carry the oil outside of the ring and throw it off in the oil reservoir. Another way is to insert a tin apron, as shown in fig. 682 at T, which will serve to drain the oil which may creep along the shaft, and also cut off the draft from the pulley which may suck the oil out of the bearing. Sometimes a tin fan is attached to the pulley, which tends to drive the oil back into the bearing, and which also assists in keeping the box cool.] Ques. What should be done if the current be excessive? Ans. If the current exceed the rated capacity by more than 50 per cent., and continue for more than a few minutes, the main switch should be opened, otherwise the machine may be seriously injured. Ques. How does an excessive current injure a dynamo? Ans. By causing it to overheat which destroys the insulation of the armature, commutator, etc. Lubrication.--The shaft bearings of dynamos may be lubricated by sight feed oilers or oil rings. The latter method is almost universally used. An oil well is provided in the hollow casting of the pedestals as shown in fig. 728. Oil rings revolve with the shaft and feed the latter with oil, which is continuously brought up from the reservoir below. The dirt settles to the bottom and the upper portion of the oil remains clear for a long period, after which it is drawn off through the spigot and a fresh supply poured in through openings provided in the top. The latter are often located directly over the slots in which the rings are placed, so that the bearings can be lubricated directly by means of an oil cup, if the rings fail to act or the reservoir become exhausted. [Illustration: Fig. 683.--Imaginative view of a shaft showing its rough granular structure. In operation these minute irregularities interlock and act as a retarding force, or frictional resistance. Hence, the necessity for lubrication--a lubricant presents a thin intervening film against which the surfaces rub.] Ques. What kind of oil can should be used in filling the reservoir, or oil cups? Ans. One made of some non-magnetic material such as copper, brass, or zinc. If iron cans be used, they are liable to be attracted by the field magnets, and thus possibly catch in the armature. Ques. What is the indication of insufficient lubrication? Ans. The bearings become unduly heated. Ques. What precaution should be taken with new dynamos?