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NOTES ON ELECTRICAL TERMS. 471
1 joule = 0.1019 kilogram-meter mechanical work.
1 joule = 0.000948 B. T. U. of heat *
1 joule == 0.7373 foot-pounds of mechanical work.
The heat produced by an electric current will be:
B. T. U. = 0.000948 X C2 X R X t.
B. T. U. = British Thermal Units.
C = Current in amperes.
R = Resistance in ohms.
t = Time in seconds.
Farad.
Farad is the unit of electrical capacity. A conductor or a
condenser whose capacity is one farad will hold a quantity of
electricity equal to one coulomb, when the electromotive force
is one volt.
The name farad is after an English electrician, Michael
Faraday (1791-1867).
Henry.
Henry is the unit of inductance and is the inductance of a
circuit in which the variation of a current at the rate of one
ampere per second induces an electromotive force of one volt.
The name henry is from an American physicist, Joseph
Henry (1797-1878).
Derived Units.
1 megohm = 1 million ohms.
1 micro-ohm = 1 millionth of an ohm.
1 milliampere = 1 thousandth of an ampere.
1 microfarad = 1 millionth of a farad.
Of all these units the volt, ampere, ohm, watt and kilowatt
are the units most used in practical work.
Example.
How many ohms of resistance are there in 6 pounds of
copper wire No. 12 ?
Solution
:
In table No. 85 the resistance of one pound of copper wire
No. 12 is given as 0.07993. Therefore, the resistance in 6
pounds of copper wire will be 6 X 0.07993 = 0.48 ohms.
* B. T. U. is British Thermal Unit and is the heat required to increase the
temperature of one pound of water one degree Fahrenheit at a temperature
between 39 and 40 degrees.
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