Unit, Fundamental.
The three units of length, ma.s.s and time, the centimeter, gram and second, are termed fundamental units. On them is based the absolute system of units, and on multiples of them the practical system of units.
Unit Jar.
A Leyden jar which is used as a unit of measure of charge.
It consists of a Leyden jar about 4 inches long and 3/4 inch diameter, with about 6 square inches of its outer and the same of its inner surface coated with tinfoil. It is placed between a source of electricity and a larger jar or battery of jars which is to be charged.
The inner coating connects with the machine; the outer coating with the jars to be charged. Short conductors terminating in k.n.o.bs connect with inner and outer coatings, and the k.n.o.bs are adjusted at any desired distance apart.
By the charging operation the large jar or battery of jars receives a charge by induction, and the charge of the small jar is at first equal to this quant.i.ty. After a while a spark pa.s.ses from k.n.o.b to k.n.o.b, discharging the small jar. This indicates the reception by the large jars of the quant.i.ty of electricity represented by the charge of the small jar. The charging goes on, and for every spark approximately the same quant.i.ty of electricity is received by the larger jars.
The sparking distance m is directly proportional to the quant.i.ty of electricity, and inversely proportional to the area of coated surface, or is proportional to the potential difference of the two coats. This is only true for short sparking distance, hence for accuracy the k.n.o.bs should be adjusted not too far from each other.
555 STANDARD ELECTRICAL DICTIONARY.
Unit of Supply.
A commercial unit for the sale of electric energy, as defined provisionally by the English Board of Trade; 1,000 amperes flowing for one hour under an E. M. F. of 1 volt; 3,600,000 volt-coulombs, or 1,000 watt-hours, are its equivalent. It is equal to 1000/746 = 1.34 electric horse power.
Synonym--Board of Trade Unit.
[Transcriber"s note: Now called a kilowatt-hour.]
Units, Circular.
A system of units of cross-sectional area, designed especially for use in describing wire conductors. The cross-sectional area of such is universally a circle, and the areas of two wires of different sizes vary with the square of their radii or diameters. Hence if the area of a circle of known diameter is determined it may be used as a unit for the dimensions of other circles. Any other circle will have an area proportioned to the area of the unit circle, as the squares of the diameters are to each other.
In practise the commonest circular unit is the circular mil. This is the area of a circle one mil, 1/1000 inch, in diameter and is equal to .0000007854 square inch. A wire two mils in diameter has an area of four circular mils; one ten mils in diameter has an area of one hundred circular mils.
Thus if the resistance of a given length of wire 1 mil in diameter is stated, the corresponding resistance of the same length of wire of the same material, but of other diameter, is given by dividing the first wire"s resistance by the square of the diameter in mils of the wire in question.
As it is a basic unit, most conveniently applied by multiplication, the smaller units are used; these are the circular mil, and circular millimeter.
Units, Derived.
Units derived by compounding or other processes, from the three fundamental units. Such are the units of area, volume, energy and work, momentum and electric units generally. In some cases the dimensions of the derived unit may reduce to those of a simple unit as inductance reduces to length, but the unit, as deduced from the fundamental ones, is still a derived unit.
Units, Practical.
A system of units employed in practical computation. The absolute units, especially in electricity, have been found too large or too small, and the attempt to make them more convenient has resulted in this system. It is based on exactly the same considerations as the absolute system of units, except that multiples of the original fundamental units of length, ma.s.s, and time have been taken as the base of the new system.
These basic units are multiples of the fundamental units. They are the following: The unit of length is 1E9 centimeters; the unit of ma.s.s is 1E-11 gram; the unit of time remains 1 second.
While this has conduced to convenience in giving better sized units, micro- and mega-units and other multiples or fractions have to be used.
The following are the princ.i.p.al practical electric units:
Electrostatic Electromagnetic C. G. S Units. C. G. S. Units.
Intensity-Ampere equal to 3E9 1E-1 Quant.i.ty-Coulomb " 3E9 1E-1 Potential-Volt " (1/3)* E-2 1E8 Resistance-Ohm " (1/9)* E-11 1E9 Capacity-Farad " 9E11 1E-9
556 STANDARD ELECTRICAL DICTIONARY.
Universal Battery System.
A term in telegraphy. If several equal and high resistance telegraphic circuits are connected in parallel with each other from terminal to terminal of a battery of comparatively low resistance each circuit will receive the same current, and of practically the same strength as if only one circuit was connected. This is termed the universal battery system. It is a practical corollary of Ohm"s law. The battery being of very low resistance compared to the lines the joining of several lines in parallel practically diminishes the total resistance of the circuit in proportion to their own number. Thus suppose a battery of ten ohms resistance and ten volts E. M. F. is working a single line of one hundred ohms resistance. The total resistance of the circuit is then one hundred and ten ohms. The total current of the circuit, all of which is received by the one line is 10/110 = .09 ampere, or 90 milliamperes. Now suppose that a second line of identical resistance is connected to the battery in parallel with the first. This reduces the external resistance to fifty ohms, giving a total resistance of the circuit of sixty ohms.
The total current of the circuit, all of which is received by the two lines in equal parts, is 10/60 = .166 amperes. But this is equally divided between two lines, so that each one receives .083 ampere or 83 milliamperes; practically the same current as that given by the same battery to the single line. It will be seen that high line resistance and low battery resistance, relatively speaking, are required for the system. For this reason the storage battery is particularly available.
The rule is that the resistance of the battery shall be less than the combined resistance of all the circuits worked by it.
Unmarked End.
The south-seeking pole of a magnet, so called because the other end, called the marked end, is usually marked with a scratch or notch by the maker, while the south pole is unmarked.
V.
(a) Symbol for velocity.
(b) Symbol or abbreviation for volume.
(c) Symbol or abbreviation for volt.
557 STANDARD ELECTRICAL DICTIONARY.
V. A.
Symbol or abbreviation for voltaic alternatives, q. v.
Vacuum.
A s.p.a.ce dest.i.tute of any substance. The great pervading substance is in general sense the atmosphere. It is the gaseous mixture which surrounds and envelopes the earth and its inhabitants. It consists of a simple mixture of oxygen, 1 part, nitrogen, 4 parts, with 4 to 6 volumes of carbonic acid gas in 10,000 volumes of air, or about one cubic inch to one cubic foot. It presses with a force of about 14.7 lbs. per square inch under the influence of the force of gravity. The term vacuum in practise refers to any s.p.a.ce from which air has been removed. It may be produced chemically. Air may be displaced by carbonic acid gas and the latter may be absorbed by caustic alkali or other chemical. The air may be expelled and the s.p.a.ce may be filled with steam which is condensed to produce the vacuum. Of course in all cases the s.p.a.ce must be included in an hermetically sealed vessel, such as the bulb of an incandescent lamp.
But the universal method of producing a vacuum is by air pumps. An absolute vacuum means the entire absence of gas or air, something almost impossible to produce. A high vacuum is sometimes understood to mean one in which the path of the molecules is equal in length to the diameter of the containing vessels, as in Crookes" Radiometer and other apparatus for ill.u.s.trating the radiant condition of matter. The air left after exhaustion is termed residual air or residual atmosphere.
[Transcriber"s note: Dry air is about .78 nitrogen, .21 oxygen, .01 argon, .00038 carbon dioxide, and trace amounts of other gases. Argon was suspected by Henry Cavendish in 1785. It was discovered in 1894 by Lord Rayleigh and Sir William Ramsay.]
Vacuum, Absolute.
A s.p.a.ce free of all material substance. It is doubtful whether an absolute vacuum has ever been produced.
Vacuum, High.
An approximate vacuum, so nearly perfect that the molecules of the residual gas in their kinetic motions rarely collide, and beat back and forth between the walls of the containing vessel, or between any solid object contained in the vessel and the walls of the vessel. The gas in such a vacuum is in the radiant or ultra-gaseous state. (See Ultra-gaseous Matter.)
Vacuum, Low.
A vacuum inferior to a high vacuum; a vacuum in which the molecules collide with each other and do not move directly from side to side of the containing vessel.