This form of commutator is used on some machines designed especially for arc lighting, such as the Brush and Thomson-Houston machines. They consist of a comparatively small number of segments each of which covers a wide angle, and are separated from each other by air gaps.
3. The segments should be of considerable depth to permit returning occasionally so that their circular form may be preserved;
4. The insulating material must be such that it will not absorb oil or moisture;
Mica is best adapted for insulation, but as there are a great many varieties, differing greatly in hardness and other equalities, it is important to select the kind that wears at the same rate as the segments. If the mica be too hard, the wearing of the segments will leave it projecting and prevent proper contact with the brushes; again, if the mica be too soft, it will result in furrows or depressions between the segments into which copper dust will collect, causing short circuits.
CHAPTER XXI
BRUSHES AND THE BRUSH GEAR
With respect to construction, brushes may be broadly classified as: 1, those made of metal, and; 2, those made of carbon. There are several varieties of metal brush, such as:
1. Gauze brushes;
2. Wire brushes;
3. Laminated or strip brushes.
Gauze Brushes.—These are very flexible and yielding, their use being attended with little wear of the commutator.
Ques. What is the construction of a gauze brush?