Box Bridge.

A constriction of Wheatstone's Bridge in which the necessary resistance coils are contained in a single box with plugs for throwing the coils in and out of circuit, and connections to bring the coils into the different arms of the system. The cut shows a box bridge. Connections for the galvanometer, battery wires, and terminals of the unknown resistance are provided, by which its resistances and the connections are brought into the exact relations indicated in the conventional diagram of Wheatstone's bridge. (See Wheatstone's Bridge.)

Referring to the cut, the battery wire, say from the zinc plate, connects at A1, thereby reaching A, its true connecting point. To B1 one end of the galvanometer circuit or lead is attached, thereby reaching B, its true connecting point. To C are connected the other end from the galvanometer and one end of the unknown resistance. The other end of the unknown resistance, and the other end of the battery wire, in this case from the carbon plate, connect to D. At G is an infinity plug, as it is called. When out it breaks the circuit.

In use after the connections are made the key is depressed and the galvanometer observed. The resistance is changed until no action of the galvanometer is produced by closing the circuit when the ratio of the resistances of the arms gives the proportion for calculating the unknown resistances.

Synonym--Commercial Wheatstone Bridge, or commercial form of same.

Fig. 65. TOP OF BOX BRIDGE.

86 STANDARD ELECTRICAL DICTIONARY.

Boxing the Compa.s.s.

Naming the thirty-two points of the compa.s.s in order, and in sequence to any point called out at random. There are many exercises in the relative sailing points and bearings that come under the same head. Thus the direction of two given points being given by names of the compa.s.s points, it may be required to state the number of points intervening.

Brake, Electro-magnetic.

A brake to stop a wheel from rotating. It comprises a shoe, or sometimes a ring, which by electro-magnetic attraction is drawn against the rotating wheel, thus preventing it from turning, or tending to bring it to rest. (See Electro-magnet, Annular.)

Fig. 66. ELECTRIC BRAKE.

87 STANDARD ELECTRICAL DICTIONARY.

Branch.

A conductor branching from a main line. Sometimes the term is restricted to a princ.i.p.al conductor, from which current is distributed.

Branch Block.

In electric wiring of buildings, a block of porcelain or other material with grooves, holes and screws for the connection of branch wires to a main wire. Its functions are not only to afford a basis for connecting the wires, but also to contain safety fuses. As when a branch wire is taken off, fuses have to be put in its line, the branch block carries these also. One end of each fuse connects with a main wire, the other end connects with one of the wires of the branch leader or wire.

Porcelain is a favorite material for them, as the fusing or "blowing out" of the safety fuses cannot set it on fire.

Branch Conductor.

A parallel or shunt conductor.

Brazing, Electric.

Brazing in which the spelter is melted by means of electricity; either current incandescence or the voltaic arc may be used. It is identical in general with electric welding. (See Welding, Electric.)

Branding, Electric.

A system of branding in which the heat of electrically ignited or incandescent conductors is used to produce or burn in the marks upon the surface. For the alternating current a small transformer is connected to or forms part of the tool.

88 STANDARD ELECTRICAL DICTIONARY.

Bra.s.sing.

The deposition of a coating of bra.s.s by electrolysis. The plating bath contains both copper and zinc. As anode a plate of bra.s.s is used. The operation must be constantly watched. The deposition of both metals goes on simultaneously, so that a virtual alloy is deposited. By changing the depth of immersion of the anode the color of the deposit is varied.

As a formula for a bra.s.sing bath the following are typical. They are expressed in parts by weight.

(a) For iron and steel.

I.

Sodium Bisulphate, 200 Pota.s.sium Cyanide, 70 per cent., 500 Sodium Carbonate, 1,000 Water, 8,000 II.

Copper Acetate, 125 Zinc Chloride, 100 Water, 2,000 Add the second solution to the first.

(b) For zinc.

I.

Sodium Bisulphate, 700 Pota.s.sium Cyanide, 70 per cent., 1,000 Water, 20,000 II.

Copper Acetate, 350 Zinc Chloride, 350 Aqua Ammoniae, 400 Water, 5,000 Add the second solution to the first.

Use a bra.s.s anode; add more zinc to produce a greenish color; more copper for a red color. A weak current gives a red color; a strong current lightens the color. The battery power can be altered, a larger or smaller anode can be used, or a copper or zinc anode can be used to change the color of the deposit. The bath may vary from 1.036 to 1.100 sp. gr., without harm.

Break.

A point where an electric conductor is cut, broken, or opened by a switch or other device, or simply by discontinuity of the wires.

Break-down Switch.

A switch used in the three-wire system to provide for the discontinuance of the running of one of the dynamos.

By connecting the positive and negative bus wires to one terminal of the active dynamo, and the neutral bus wire to the other terminal, one dynamo will supply the current and the system operates like a two-wire system, but can only be used for half its normal capacity.

Breaking Weight.

The weight which, applied in tension, will break a prism or cylinder, as an electric current conductor.

89 STANDARD ELECTRICAL DICTIONARY.

Breath Figures, Electric.

If a conductor is electrified and placed upon a piece of gla.s.s, it will electrify the gla.s.s in contact with it by conduction or discharge. On removing the conductor the gla.s.s remains electrified. The localized electrification is shown by breathing gently on the gla.s.s, when a species of image of the conductor is produced by the condensed moisture.

A coin is often used for conductor.

Breeze, Electric.

A term in medical electricity, used to designate the silent or brush discharge of high tension electricity. As an instance of its employment, the electric head bath (see Bath, Electric Head,) may be cited. The patient forming one electrode, being insulated and connected to one of the conductors, the other conductor, on being brought near his person, discharges into his body.