The Diseases Of Electrical Machinery

ELECTRICAL MACHINERY. 9 it is best to excite the field magnets of the booster either from the main machine, or from the battery itself. Commutator Lubrication.—To the numerous causes of sparking, although there is really no fault in the armature, belongthe materials so much abhorred by every dynamo maker which are sold for lubricating commutators. Whatever they may be called, it is only in the rarest cases that they are of any use, and in an exceedingly large number of cases they do con- siderable damage. One may take it for granted that the maker has calculated the machine so that it works sparklessly with the brushes supplied. If however in spite of this, the machine sparks, the use of some graphitic compound will not alter the State of affairs. The undesirable increase in the brush resistance may cause an otherwise good machine to spark. If one, however, thinks it necessary to give the collector a polish, which is in general only possible with carbon brushes, it should be done with a drop of pure mineral oil, which must be carefully rubbed over the whole surface. [The editor prefers a touch of vaseline, applied on a bit of clean cotton cloth; cotton waste should not be used.] Spare Brushes.—When inserting new brushes, it is to be very carefully seen that the surface of the new brushes fits exactly to the surface of the commutator. It is scarcely to be feared that any one will imitate the