The Diseases Of Electrical Machinery 1904

i8 THE DISEASES OF somewhat greasy. This has the advantage of causing a very small wear of the commutator. In addition, owing to the exceeding fineness of the metal leaves, the brush sticks very close to the commutator and produces a good contact surface. The single dis- advantage of this brush is that it wears away very quickly. This could, however, only be helped if one gave up some of the previously mentioned advantages. This brush cannot be used for low-voltage machines, such as electroplating machines. It is therefore to be remarked by the way that low-voltage machines of less than 10 volts do not work well with Boudreaux brushes, since the lubricating action of the brushes causes a small resistance under the brushes. This resistance is of no importance in the case of machines of higher voltage, where it may be entirely neglected, since owing to the current being comparatively small there is no noticeable drop of voltage. On the other hånd, in the case of low-voltage machines the current is comparatively large, and as the heating at the brushes may consequently be considerable, an ex- cessive heating of the commutator may result; and the voltage-drop, due to the contact-resistance, would be too great. Uncertain Contact between the Armature Wires and the Commutator.—A by no means un- usual fault, which causes considerable commutator sparking, consists in uncertainty of contact between the armature conductors and the commutator lugs