Exciter.

A generator used for exciting the field magnet of a dynamo. In alternating current dynamos, e. g., of the Westinghouse type, a special dynamo is used simply to excite the field magnet. In central station distribution the same is often done for direct current dynamos.

Exosmose, Electric.

The outflowing current of electric osmose. (See Osmose, Electric.)

Expansion, Coefficient of.

The number expressing the proportional increase in size, either length, area or volume, of a substance under the influence generally of heat.

There are three sets of coefficients, (1) of linear expansion, (2) of superficial expansion, (3) of cubic expansion or expansion of volume.

The first and third are the only ones much used. They vary for different substances, and for the same substance at different temperatures. They are usually expressed as decimals indicating the mixed number referred to the length or volume of the body at the freezing point as unity.

Expansion, Electric.

(a) The increase in volume of a condenser, when charged electrostatically. A Leyden jar expands when charged, and contracts when discharged.

(b) The increase in length of a bar of iron when magnetized.

This is more properly called magnetic expansion or magnetic elongation.

Exploder.

(a) A small magneto-generator for producing a current for heating the wire in an electric fuse of the Abel type (see Fuse, Electric), and thereby determining an explosion.

(b) The term may also be applied to a small frictional or influence machine for producing a spark for exploding a spark fuse.

Explorer.

A coil, similar to a magnetizing coil (see Coil, Magnetizing), used for investigating the electro-magnetic circuit and for similar purposes. If placed around an electro-magnet and connected with a galvanometer, it will produce a deflection, owing to a momentary induced current, upon any change in the magnet, such as removing or replacing the armature. It is useful in determining the leakage of lines of force and for general investigations of that nature. It is often called an exploring coil.

Hughes' Induction Balance (see Induction Balance, Hughes') is sometimes called a Magnetic Explorer. The exploring coil may be put in circuit with a galvanometer for quant.i.tative measurements or with a telephone for qualitative ones.

248 STANDARD ELECTRICAL DICTIONARY.

Extension Bell Call.

A system of relay connection, q. v., by which a bell is made to continue ringing after the current has ceased coming over the main line. It is designed to prolong the alarm given by a magneto call bell, q. v., which latter only rings as long as the magneto handle is turned. A vibrating electric bell (see Bell, Electric,) is connected in circuit with a local battery and a switch normally open, but so constructed as to close the circuit when a current is pa.s.sed and continue to do so indefinitely. The distant circuit is connected to this switch. When the magneto is worked it acts upon the switch, closes the local battery circuit and leaves it closed, while the bell goes on ringing until the battery is exhausted or the switch is opened by hand.

Eye, Electro-magnetic.

An apparatus used in exploring a field of electro-magnetic radiations.

It is a piece of copper wire 2 millimeters (.08 inch) in diameter, bent into an almost complete circle 70 millimeters (.28 inch) in diameter, with terminals separated by an air gap. This is moved about in the region under examination, and by the production of a spark indicates the locality of the loops or venters in systems of stationary waves.

F.

Abbreviation for Fahrenheit, as 10? F., meaning 10? Fahrenheit. (See Fahrenheit Scale.)

Fahrenheit Scale.

A thermometer scale in use in the United States and England. On this scale the temperature of melting ice is 32?; that of condensing steam is 212?; the degrees are all of equal length. Its use is indicated by the letter F., as 180? F. To convert its readings into centigrade, subtract 32 and multiply by 5/9. (b) To convert centigrade into F. multiply by 9/5 and add 32. Thus 180? F. = ((180-32) * 5/9)? C. = 82.2? C. Again 180? C. = (180 * 9/5) + 32 = 324? F.

[Transcribers note: 180? C. = (180 * 9/5) + 32 = 356? F. ]

The additions and subtractions must be algebraic in all cases. Thus when the degrees are minus or below zero the rules for conversion might be put thus: To convert degrees F. below zero into centigrade to the number of degrees F. add 32, multiply by 5/9 and place a minus sign (-) before it. (b) To convert degrees centigrade below zero into Fahrenheit, multiply the number of degrees by 9/5, subtract from 32 if smaller; if greater than 32 subtract 32 therefrom, and prefix a minus sign, thus: -10? C. = 32 - (10 * 9/5) = 14?. Again, -30?C. = (30 * 9/5) - 32 = 22 = -22? F.

249 STANDARD ELECTRICAL DICTIONARY.

Farad.

The practical unit of electric capacity; the capacity of a conductor which can retain one coulomb of electricity at a potential of one volt.

The quant.i.ty of electricity charged upon a conducting surface raises its potential; therefore a conductor of one farad capacity can hold two coulombs at two volts potential, and three coulombs at three volts, and so on. The electric capacity of a conductor, therefore, is relative compared to others as regards its charge, for the latter may be as great as compatible with absence of sparking and disruptive discharge. In other words, a one farad or two farad conductor may hold a great many coulombs. Charging a conductor with electricity is comparable to pumping air into a receiver. Such a vessel may hold one cubic foot of air at atmospheric pressure and two at two atmospheres, and yet be of one cubic foot capacity however much air is pumped into it.

The farad is equal to one fundamental electrostatic unit of capacity multiplied by 9E11 and to one electro-magnetic unit multiplied by 1E-9.

The farad although one of the practical units is far too large, so the micro-farad is used in its place. The capacity of a sphere the size of the earth is only .000636 of a farad.

[Transcriber's note: Contemporary calculations give about .000720 farad.]

Faraday, Effect.

The effect of rotation of its plane produced upon a polarized beam of light by pa.s.sage through a magnetic field. (See Magnetic Rotary Polarization.)

Faraday's Cube.

To determine the surface action of a charge, Faraday constructed a room, twelve feet cube, insulated, and lined with tinfoil. This room he charged to a high potential, but within it he could detect no excitement whatever. The reason was because the electricity induced in the bodies within the room was exactly equal to the charge of the room-surface, and was bound exactly by it. The room is termed Faraday's cube.

Faraday's Dark s.p.a.ce.

A non-luminous s.p.a.ce between the negative and positive glows, produced in an incompletely exhausted tube through which a static discharge, as from an induction coil, is produced. It is perceptible in a rarefaction of 6 millimeters (.24 inch) and upwards. If the exhaustion is very high a dark s.p.a.ce appears between the negative electrode and its discharge.

This is known as Crookes' dark s.p.a.ce.

Faraday's Disc.

A disc of any metal, mounted so as to be susceptible of rotation in a magnetic field of force, with its axis parallel to the general direction of the lines of force. A spring bears against its periphery and another spring against its axle. When rotated, if the springs are connected by a conductor, a current is established through the circuit including the disc and conductor. The radius of the disc between the spring contacts represents a conductor cutting lines of force and generating a potential difference, producing a current. If a current is sent through the motionless wheel from centre to periphery it rotates, ill.u.s.trating the doctrine of reversibility. As a motor it is called Barlow's or Sturgeon's Wheel. If the disc without connections is rapidly rotated it produces Foucault currents, q. v., within its ma.s.s, which resist its rotation and heat the disc.

250 STANDARD ELECTRICAL DICTIONARY.

Fig. 168. "FARADAY'S NET."

Faraday's Net.

An apparatus for showing that the electric charge resides on the surface. It consists of a net, conical in shape and rather deep, to whose apex two threads, one on each side, are attached. Its mouth is fastened to a vertical ring and the whole is mounted on an insulating support.

It is pulled out to its full extent and is electrified. No charge can be detected inside it. By pulling one of the threads it is turned with the other side out. Now all the charge is found on the outside just as before, except that it is of course on the former inside surface of the bag. The interior shows no charge.