THE BRUSHES.

The brushes bear upon the commutator, and make sliding contact with the armature and working circuits. It is needful that they should have a certain amount of flexibility, in order that they may accommodate themselves to any little inequality which may occur upon the surface of the commutator,

and also to avoid cutting or scoring the latter; with these objects, they are usually made of copper or brass gauze, wire, or flexible strip.

Gauze Brushes.-This type of brush is now very extensively used, owing to its great flexibility and soft and yielding nature, resulting in decreased wear of the commutator. It is

THE BRUSHES.

made up of a sheet of copper gauze, folded round several times, with the wires running in an oblique direction, so as to form a solid flat strip of from 4 inch to 1⁄2 inch in thickness, as shown at A, Fig. 87, the thickness increasing with the volume of the current to be collected. The object of folding the gauze up with the wires running in an oblique direction is to prevent the ends of the brushes fraying or threading out, which would be the case if the gauze was folded up in any other manner.

Wire Brushes.-This brush (B. Fig. 87) which was much used previous to the invention of the gauze brush, is made up of a bundle of brass or copper wires, laid side by side and soldered together at one end, being harder than the gauze brush, it is more liable to cut or score the commutator, and it is also more troublesome to trim.

Strip Brush.-This is probably the simplest form of brush, but is not very extensively used owing to its lack of flexibility. It consists of a number of strips of copper or brass, laid one upon the other and soldered at one end, as in C, Fig. 87.

Carbon Brushes.-When metallic brushes are used upon the commutators of high tension machines, they frequently give rise to excessive sparking, and also heating of the armature, the metallic dust given off appearing to lodge between the segments of the commutator, thus partially short circuiting the armature. To obviate this, carbon brushes are frequently used on such dynamos, this substance being found very effectual in the prevention of sparking. The brushes

THE BRUSHES.

are usually in the form of oblong blocks placed "butt" end on the commutator, and fed forward as they wear away by means of a spring holder.

Wheel Brushes have been employed among the many devices for collecting the current. These consist of small wheels, or disks, bearing against and rotating on the surface of the commutator.

Lead of the Brushes is the dispacement or lead in advance of, or beyond the position at right angles to the line connecting the poles of the field magnet. In a motor the brushes are set back of the right angle or are given a negative lead.

The necessity for the lead arises from the counter magnetism or the magnetic reaction of the armature. In magnetism the tendency of hard iron, or steel especially, is the cause of the "lag" or magnetic retardation. It is to accommodate this variation that brush holders are provided with devices for moving them backward and forward.

NOTE.-Flexible Carbon Brushes.-Carbon commutator brushes, as commonly used, consist of one or two blocks of carbon, pressed against the commutator by a spring to maintain the contact. In the case of street railway motors, subject to violent oscillations, the carbons are often jolted away from the commutator, thus causing sparks and burning of the commutator. In order to dispense altogether with springs and at the same time to maintain the pressure constant, Professor Geo. Forbes of London, has devised a brush of flexible carbon, carbonized cloth, compressed into a metal case open at the side, facing the commutator, the case serving both as a holder for the flexible carbon, and also as a terminal for the circuit; owing to its flexibility, it maintains contact with the commutator at an infinite number of points. By this means it is asserted a much better contact is maintained, which is not broken, or even varied, by the jolting on a railroad car.

THE BRUSHES.

In all well-designed dynamos not less than two brushes are used on each side of the commutator; this allows of either brush being removed, and examined and trimmed, while the machine is running. It also allows of the brushes being ad

FIG. 88.

VARIOUS KINDS OF BRUSHES.

justed upon the commutator independently of each other, any uneven wear of the commutator being thus prevented, and better contact made.

Brush Holders.-In order to secure sparkless collection of the current, and to prevent undue wear of the commutator, it is needful that the pressure of the brushes upon the latter