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NOTES ON ELECTRICAL TERMS. 469
Coulomb.
Coulomb is the unit of quantity in measuring electricity. A
coulomb is the amount of electricity conveyed by one ampere in
one second.
The name coulomb is after a French philosopher, Charles
Augustin de Coulomb (1736-1806).
In practical work the coulomb is obtained by multiplying
the ampere by the time in seconds in which the current is acting.
A coulomb is therefore the same as ampere-second. As
this quantity is very small it is customary with electricians to
measure in ampere-hours. One ampere-hour is, of course, 3600
coulombs. The name coulomb is not much used by practical
men but it is very customary to speak of ampere-hours. For
instance, if a storage battery could furnish 5 amperes of current
for a time of 40 hours it would be said to have a capacity of
5 X 40 = 200 ampere-hours. It could, of course, also be said to
have a capacity of 720,000 ampere-seconds or 720,000 coulombs.
Ohm.
Ohm is the practical unit of electrical resistance. The
standard (international ohm) is the resistance at the temperature
of degree centigrade of a column of mercury 106.3 centimeters
long and 1 square millimeter area. Such a column of mercury
will weigh 14.4521 grams.
The name ohm is after a German electrician, Georg Simon
Ohm (1787-1854).
As examples, we may mention that 1000 feet of Xo. 10 cop-
per wire have a resistance of about 1 ohm and, remembering
the resistance of 1000 feet of No. 10 wire, we can almost by
mental calculations get the approximate resistance of 1000 feet
of any other size of wire by the rules given on page 462.
The resistance is different in different materials. Materials
having high resistance are called insulators, such as, for instance,
silk, cotton, paper, fiber, glass, etc.
Among the common metals copper is the best conductor of
electricity; next comes aluminum, having a resistance about twice
that of copper. Iron has from 6 to 7 times the resistance of copper.
The resistance in ohms of a circuit is obtained by dividing
the volts by the amperes, and the quotient is the ohms.
Watt.
Watt is the unit of power, and is the product of the volts
and the amperes.
1000 watts is called a kilowatt.
746 watts = 550 foot-pounds per second = 1 horse power.
736 watts = 75 kilogram-meter per second = 1 metric horse
power.
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