The Diseases Of Electrical Machinery

ELECTRICAL MACHINERY. 13 with the glass-paper may be recommended. In no form whatever may emery be nsed. From the above it can be seen that the turning down of a commutator should be entrusted to an experienced hånd, since an inexperienced man could totally destroy the com- mutator. . There is a great difference between turning iron and turning other metals, and a man skilied only in turning iron might do much harm. Artificial Mica Segments.—It may be men- tioned that recently a kind of artificial mica—called megohmite—has been used as a substitute for natural flaked mica. Megohmite has the advantage of a smaller degree of hardness, while at the same time it does not suffer from the fault of other artificial kinds of mica—namely, the melting out, when heated, of the lac used to cement the mica flakes together. Press-spahn and vulcanised fibre are very unsuit- able materials for the insulation of commutators, above all, when carbon brushes are used. These materials shrink witli heating and swell when damp. Machines with such commutators cannot be described as of modern design. With old machines having metal brushes, press-spahn was permissible. Loosening of the Commutator Segments.—An error in design is shown by the not uncommon loosen- ing of the commutator. This fault can be easily tested by striking with a hammer (with a piece of wood in between) upon the individual segments, and seeing whether they move back. The loosening of com-