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, 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