The Diseases Of Electrical Machinery

ELECTRICAL MACHINERY. il struction may both lead to sparking. In the former case this shows itself only after some weeks’ working, so that it is not found out in the test room. Too Hard Mica Insulation.—The varying de- grees of hardness of mica must be here noticed. Mica which, as is well known, is an exceedingly good in- sulator, is very variable in regard to its hardness. If we are to use mica in building commutators, we must see that it is about as hard as the copper, in order that the brushes should wear away the copper and the mica at the same rate. As it is certainly not easy always to fulfil this condition, we have become accustomed to using mica which is softer than the copper. Since it is impossible to examine each mica slip in the workshop, it may happen that some piece of mica is used which is harder than the copper. It is then clear that the copper will be worn away sooner than the mica, so that, after some weeks’ work- ing, the mica stands out a little above the copper. This cannot be satisfactorily determined either by the eye or by touch, since a piece that may project less than the hundredth of an inch is sufficient to start the brushes chattering, and thus may cause sparking. This sparking is very dangerous, since it damages the copper in a remarkably short time. Grinding with glass-paper is no use, since such hard mica is not touched by glass-paper. The application of glass- paper only increases the fault, since the copper alone is rubbed away. In such cases a light cut must be