The Diseases Of Electrical Machinery

ELECTRIC AL MACHINERY. 17 copper is to be deprecated. As insulation between the segments, mica alone comes into consideration. Metal Brushes.—The subject of metal brushes is an endless one. For while the fabrication of carbon brushes lies in the hands of a few good firms, the pro- duction of metal brushes has been taken up by a number of makers whose ignorance of the subject is only exceeded by the pushing way in which they advertise their wares. The requirements of a good metal brush are the following : It must possess a low resistance, must be very flexible, must in a short time accommodate itself to the surface of the commutator; and, finally, it should be made of some material that does not wear away the commutator, since it is better that the brushes should be worn away rather than the commutator. A brush which is at present much used, and has been found by the author, by long experiments and in the last few years by almost exclusive use, to be suit- able for all purposes, is the laminated brush made under the patents of Boudreaux.* This brush con- sists of extraordinarily fine leaves of metal, the composition of which is kept secret. It is, however, certain that additions are made to the metal which have the effect that the brush keeps the commutator * These Boudreaux brushes are made by several firms, including the following : L. Boudreaux, 8 Rue Hautefeuille, Paris; Louis Patz, in Dresden; Le Carbone, 36 Lime Street, London, E.C. Brushes of a somewhat similar sort, but not lubricating, are made by several makers, including the firm of P. Ringdorff, of Essen. C