The Standard Electrical Dictionary Part 24

Bridge.

(a) A special bar of copper connecting the dynamos to the bus wire, q. v., in electric lighting or power stations.

(b) Wheatstone's bridge, q. v., and its many modifications, all of which may be consulted throughout these pages.

British a.s.sociation Bridge.

The type of Wheatstone bridge used by the committee of the a.s.sociation in determining the B. A. ohm; the meter bridge, q. v.

Broadside Method.

A method of determining the magnetic moment of a magnet. The magnet, n, s, under examination is fixed so that it is at right angles to the magnetic meridian, M, R, which pa.s.ses through its own center and that of a compa.s.s needle. From the deflection of the latter the moment is calculated.

FIG 67. BROADSIDE METHOD.

Bronzing.

In electro-plating the deposition of a mixture or virtual alloy of copper and tin. In general manipulation it resembles the operation of depositing gold and silver alloy, or of bra.s.sing.

For bronzing the following bath is recommended:

Prepare each by itself (a) a solution of copper phosphate and (b) a solution of stannous chloride in a solution of sodium pyrophosphate. For a, dissolve recently precipitated copper phosphate in concentrated solution of sodium pyrophosphate. For b, add to a saturated solution of sodium pyrophosphate solution of stannous chloride as long as the precipitate which is formed dissolves. Of these two solutions add to a solution of sodium pyrophosphate which contains about 1.75 oz. of the salt to the quart, until the precipitate appears quickly and of the desired color. For anodes use cast bronze plates. Sodium phosphate must be added from time to time; if the deposit is too light add copper solution, if too dark add tin solution. (W. T. Brannt.)

90 STANDARD ELECTRICAL DICTIONARY.

Brush.

In electric current generators and motors, the pieces of copper or other material that bear against the cylindrical surface of the commutator are thus termed. Many different constructions have been employed. Some have employed little wheels or discs bearing against and rotating on the surface of the commutator. A bundle of copper strips is often employed, placed flatwise. Sometimes the same are used, but are placed edgewise.

Wire in bundles, soldered together at their distant ends have been employed. Carbon brushes, which are simply rods or slabs of carbon, are used with much success.

Synonym--Collecting Brush.

Brush, Carbon.

A brush for a dynamo or motor, which consists of a plate or rod of carbon, held in a brush holder and pressed against the commutator surface.

Brushes, Adjustment of.

In electric current generators and motors, the brushes which bear upon the commutator when the machine is in action need occasional adjustment.

This is effected by shifting them until sparking between them and the commutator is nearly or quite suppressed.

Fig. 68. BRUSH HOLDER.

Brushes, Lead of.

In a dynamo electric generator, the lead or displacement in advance of or beyond the position at right angles to the line connecting the poles of the field magnet, which is given the brushes. In a motor the brushes are set back of the right angle position, or are given a negative lead.

(See Lag.)

91 STANDARD ELECTRICAL DICTIONARY.

Brush Holders.

The adjustable (generally) clutch or clamps for holding the commutator brushes of a dynamo, which keep them in contact with the commutator, and admit of adjustment by shifting backward and forward of the brushes to compensate for wear. They are connected to and form part of the rocker, q. v. By rotating the latter the brush-holders and brushes are carried in one direction or other around the commutator, so as to vary the lead as required.

Brush, Pilot.

A third brush, used for application to different parts of a revolving armature commutator to determine the distribution of potential difference between its different members. (See Curve of Distribution of Potential in Armature.) One terminal of a volt-meter is connected to one of the regular brushes, A, of a dynamo; the other to a third brush, p, which is pressed against different portions of the commutator of the dynamo. The readings of the volt-meter are plotted in a curve of distribution of potential.

Fig. 69. PILOT BRUSH.

Brush, Rotating.

Brushes for taking off the current from dynamo commutators, or giving current connection to motors, whose ends are in the form of rollers which rotate like little wheels, and press against the commutator surface.

Brush, Third.

A third brush is sometimes provided in a dynamo for regulating purposes.

Applied to a series machine it adjoins one of the regular brushes and delivers its current to a resistance, to whose further end the regular circuit is connected. By a sliding connection the resistance is divided between the third brush circuit and the regular circuit, and by varying the position of this contact regulation is obtained.

It is to be distinguished from the pilot brush used for determining the characteristic of the commutator, although based on the same general principles.

Fig. 70. THIRD BRUSH REGULATION.

92 STANDARD ELECTRICAL DICTIONARY.

Brush, Wire Gauze.

A collecting or commutator brush for a dynamo or motor, which brush is made of wire gauze rolled up and compressed into shape.

Buckling.

The bending up and distortion of secondary battery plates. It is largely due to over-exhausting the batteries. Where the E. M. F. is never allowed to fall below 1.90 volt it is far less liable to occur.

Bug.

Any fault or trouble in the connections or working of electric apparatus.

Bug Trap.

A connection or arrangement for overcoming a "bug." It is said that the terms "bug" and "bug trap" originated in quadruplex telegraphy.

Bunsen Disc.