The Standard Electrical Dictionary Part 166

[Transcriber's note: The SI metre was defined in 1983 such that the speed of light in a vacuum is exactly 299,792,458 metres per second or about 186,282.397 miles per second.]

Ventilation of Armature.

In a dynamo or motor ventilation of the armature is often provided for by apertures through it in order to prevent heating. This heating is caused by Foucault currents. By proper disposition of the interior of the armature with properly disposed vanes and orifices an action like that of a fan blower can be produced, which by creating a current of air cools the machine very efficiently.

Verticity, Poles of.

Points upon the earth's surface where the horizontal component of magnetic force disappears, leaving only the vertical component active.

The term is derived from the verticity of the dipping needle when over either of them.

561 STANDARD ELECTRICAL DICTIONARY.

Vibration Period.

In electrical resonance the period of a vibration in an electrical resonator. The length of this period indicates the quality of the resonator in responding to electrical oscillations by sympathetic vibration. For conductors of small resistance the period is thus calculated. Let T be the period of one-half a full vibration; L the absolute coefficient of self-induction expressed in centimeters or in henries X 10-9; C the electrostatic capacity of the terminals, also expressed in the same unit; v the velocity of light in centimeters per second. Then we have the formula

T = PI * SquareRoot( L * C ) / v

[Transcriber's note: If the inductance is in henries and the capacitance in farads, frequency in hertz = 1/(2 * PI * squareRoot( L * C ) )]

Vibration, Sympathetic.

A vibration in a cord or other body susceptible of elastic vibration produced by the vibrations of exactly the same period in a neighboring vibrating body. Thus if two tuning forks are tuned to precisely the same pitch, and are placed near each other, if one is sounded it will start the other into vibration by sympathy.

In electricity its application is found in electric resonance experiments. The resonator has a definite period of electric resonance, and is made to give a spark by the exciter of identical period. This is by what may be called electric sympathetic vibration, and is exactly a.n.a.logous to the action of the tuning forks upon each other.

Vibrator, Electro-magnetic.

The make and break mechanism used on induction coils, or other similar apparatus in which by alternate attractions by and releases from an electro-magnet an arm or spring is kept in motion. In most cases the work is done by a single magnet, whose armature is attracted to the magnet, when the latter is excited, but against the action of a spring which tends to pull it away from the magnet. In its motions a make and break action is produced, to give the requisite alternations of attraction and release. Two electro-magnets may be connected so as alternately to be excited and keep an arm carrying a mutual armature in vibration, or the same result may be attained by a polarized relay. The make and break is ill.u.s.trated under Bell, Electric--Coil, Induction-- Anvil.

Villari's Critical Value.

Magnetization induced or residual in a wire is diminished on stretching, provided that the magnetization corresponds to an inducing force above a certain critical value, known as above; this being (Sir Wm. Thomson) about 24 times the terrestrial intensity. Below that critical value tension increases the magnetization of a magnetized wire. The effects of transverse expansive stress are opposed to those of longitudinal stretching. (Daniell.)

Viole's Standard of Illuminating Power.

A standard authorized by the International Congress of 1881. It is the light given by one square centimeter of platinum, melted, but just at the point of solidification. It is equal to 20 English standard candles almost exactly.

It has not been very widely accepted, the tendency among photometrists being to adhere to the old standards, carcel or candle. It is obvious that actual use of the Viole would be very inconvenient and would involve expensive apparatus, difficult to work with.

Synonym--Viole.

562 STANDARD ELECTRICAL DICTIONARY.

Vis Viva.

The kinetic energy of a body in motion; "mechanical energy."

Vitreous Electricity.

Positive electricity; the electricity produced on the surface of gla.s.s by rubbing it with silk and other substances. (See Electrostatic Series.)

The term "positive electricity" should be allowed to supplant it. It is the a.n.a.logue and opposite of resinous electricity.

Vitriol, Blue.

A colloquial or trade name for copper sulphate (Cu SO4).

Vitriol, Green.

A colloquial or trade name for ferrous sulphate (Fe SO4).

Vitriol, White.

A colloquial or trade name for zinc sulphate (Zn SO4).

Volt.

The practical unit of electro-motive force or potential difference. It may be referred to various data.

An electro-motive force of one volt will cause a current of one ampere to flow through a resistance of one ohm.

A condenser of one farad capacity charged with one coulomb will have a rise of potential of one volt.

The cutting of 100,000,000 lines of force per second by a conductor induces one volt E. M. F.

A Daniell's battery gives an E. M. F. of 1.07 volts; about the most familiar approximate standard that can be cited.

It is equal to 1/300 absolute electrostatic unit.

It is equal to 1E8 absolute electro-magnetic units.

[Transcriber's note: The SI definition of a volt: The potential difference across a conductor when a current of one ampere dissipates one watt of power.]

Voltage.

Potential difference or electro-motive force expressed in volts; as a voltage of 100 volts. Thus voltage may express the electro-motive force absorbed in a conductor, while electro-motive force is a term generally applied where it is produced, evolved or present in the object. The term voltage of a lamp expresses simply the volts required, but does not suggest the possession of electromotive force.

563 STANDARD ELECTRICAL DICTIONARY.

Voltage, Terminal.

The voltage or potential difference at the terminals of an electric current generator, such as a dynamo, as distinguished from the total electro-motive force of the dynamo or generator.

In batteries the distinction is not generally made in practice; the total electro-motive force of the battery is made the basis of calculations.