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.

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.)

A space free of all material substance. It is doubtful whether an
absolute vacuum has ever been produced.

A space destitute 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 space 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 space may be filled with steam which is condensed to
produce the vacuum. Of course in all cases the space 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 illustrating 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.]

Symbol or abbreviation for voltaic alternatives, q. v.

(a) Symbol for velocity.

(b) Symbol or abbreviation for volume.

(c) Symbol or abbreviation for volt.

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.

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.

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, mass, 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 mass 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 principal practical electric units:

Electrostatic Electromagnetic
C. G. S Units. C. G. S. Units.
Intensity-Ampere equal to 3E9 1E-1
Quantity-Coulomb ” 3E9 1E-1
Potential-Volt ” (1/3)* E-2 1E8
Resistance-Ohm ” (1/9)* E-11 1E9
Capacity-Farad ” 9E11 1E-9

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.