The Standard Electrical Dictionary Part 165

Vacuum, Partial.

A s.p.a.ce partially exhausted of air so as to contain less than an equal volume of the surrounding atmosphere. It really should come below a low vacuum, but is often treated as synonymous therewith.

Vacuum, Torricellian.

The vacuum existing above the mercurial column in a barometer tube. The principle of this vacuum is applied in the Geissler and other air pumps.

(See Pump, Geissler--Pump, Sprengel--Pump, Swinburne.)

558 STANDARD ELECTRICAL DICTIONARY.

Valency.

The relative power of replacing hydrogen or combining therewith possessed by different elements; the number of atomic bonds belonging to any element. Thus oxygen has a twofold valency, is bivalent or is a dyad, and combines with two atoms of hydrogen because the latter has a unitary atomicity, is monovalent or is a monad.

Valve, Electrically Controlled.

A valve which is moved by or whose movements are regulated by electricity.

In the block system of railroad signaling the semaph.o.r.es are worked by weights and pneumatic cylinders and pistons. The valves for admitting or releasing the compressed air are operated by coil and plunger mechanism.

There are many other instances of the control of valves by the electric current.

Vapor Globe.

A protecting gla.s.s globe surrounding an incandescent lamp, when the lamp is to be used in an atmosphere of explosive vapor, as in mines or similar places; or when in a place where it is exposed to dripping water which would break the hot lamp bulb if it fell upon it.

Variable Period.

The period of adjustment when a current is started through a conductor of some capacity. It is the period of duration of the variable state, q.

v., in a conductor. As indicated in the next definition in a cable of high electrostatic capacity a variable period of nearly two minutes may exist. This indicates the r.e.t.a.r.dation in signaling to be antic.i.p.ated in cables and other lines of high capacity.

Variable State.

When an electric circuit is closed the current starts through the conductor with its full strength from the point of closure, and advances with a species of wave front so that some time elapses before it attains its full strength in the most distant parts of the conductor, owing to its having to charge the conductor to its full capacity at the given potential. The state of the line while the current thus varies is called the variable state.

A long telegraph line when a message is being transmitted may be always in the variable state. The current at the receiving end may never attain its full strength.

In the case of such a conductor as the Atlantic cable, 108 seconds would be required for a current to attain 9/10 of its full strength at the distant end, and but 1/5 second to attain 1/100 of its final value.

During the period of increase of current the variable state exists.

Variation of the Compa.s.s.

The declination of the magnetic needle. (See Elements, Magnetic.) As the declination is subject to daily, annual and secular variations, it is unfortunate that this term is synonymous with declination. Thus the variation of the compa.s.s means its declination, while there is also the variation of the declination and of other elements. The term variation of the compa.s.s is more colloquial than the more definite expression "declination," or "magnetic declination."

559 STANDARD ELECTRICAL DICTIONARY.

Variometer.

An apparatus used in determining the relative values of the horizontal component of the earth's magnetic field in different places.

Varley's Condenser.

A static condenser whose conducting surfaces are platinum electrodes immersed in dilute sulphuric acid. When the potential difference is 1/50th that of a Daniell's cell, two square inches of platinum have a capacity equal to that of an air condenser whose plates have an area of 80,000,000 square inches, and separated 1/8th of an inch from each other. As the E. M. F. increases the capacity also increases.

Varley's Resistances.

Variable resistances formed of discs of carbonized cloth, q. v., piled up, and pressed together more or less firmly to vary the resistance as desired.

Varnish.

A glossy transparent coating of the nature of paint, applied as a protective, or ornamental coating to objects.

Varnish, Electric.

Alcoholic or etherial varnishes are the best for electrical apparatus.

They dry quickly and perfectly, and tend to form surfaces unfavorable to the hygroscopic collection of water. Sealing wax dissolved in alcohol, or sh.e.l.lac dissolved in the same solvent are used for electrical apparatus, although the first is rather a lacquer than a varnish.

Etherial solution of gum-copal is used to agglomerate coils of wire. It is well to bake varnished objects to harden the coating.

Varnish, Red.

A solution of sealing wax in 90 per cent. alcohol. It is best made thin and applied in several coats, each coat being allowed to dry perfectly before the next is applied. It is often seen on Leyden jars. It is a protector from surface leakage.

Vat.

A vessel for chemical or other solutions. A depositing vat is one in which a plating solution is worked, for the deposition of electroplate upon articles immersed in the liquid, and electrolyzed by an electric current.

Velocity.

The rate of motion of a body. It is usually expressed in distance traversed per second of time. The absolute unit is one centimeter per second or kine. The foot per second is very largely used also.

The dimensions of velocity are length (L) divided by time (T) or L/T.

Velocity, Angular.

Velocity in a circle defined by the unit angle, or the angle which subtends a circular arc equal in length to itself. The radius of the circle traversed by the moving body does not enter into this definition, as the real velocity of the object is not stated. If its angular velocity and the radius of the path it travels are given its actual velocity can be deduced.

560 STANDARD ELECTRICAL DICTIONARY.

Velocity of Signaling.

The speed of transmission of electric signals is affected by the nature of the line, as regards its static capacity, and by the delicacy of the receiving instruments, which may need a more or less strong current to be affected. Thus of an original current one per cent. may suffice to operate a sensitive instrument. This might give almost the velocity of light, while if the instrument would only respond to the full current nearly two minutes (see Variable State) might be required for the production of a signal.

Velocity Ratio.

A term applied to the ratios existing between the electrostatic and electro-magnetic units. If we take as numerators the dimensions of the different qualities in the electrostatic system, and their dimensions in the electro-magnetic system as denominators, the fractions thus obtained reduce to expressions containing only velocity or V in some form. Thus if we divide the dimensions of the electrostatic quant.i.ty by the dimensions of electro-magnetic quant.i.ty the quotient is simply V or velocity. A like division for potential, electrostatic and electro-magnetic gives (1/V), and so on.

The value of the velocity ratio is very nearly 3E10 (sometimes given as 2.98E10) centimeters per second. This is almost exactly that of light (2.9992E10 centimeters per second.) This is one of the proofs of Clerk Maxwell's magnetic theory of light. (See Maxwell's Theory of Light.)