PRACTICAL UNITS OF ELECTRICITY.
RESISTANCE-R.—The Ohm is the resistance of a column of mercury 106.3 centimetres long, 1 square millimetre in cross-section, weighing 14.4521 grammes, and at a temperature of 0 degrees centigrade. Standards of wire are used for practical purposes. The ohm is equal to a thousand million, 10^9, electromagnetic or Centimetre-Gramme-Second ("C. G. S.") units of resistance.
The megohm is one million ohms.
The microhm is one millionth of an ohm.
ELECTROMOTIVE FORCE—E.—The Volt is that electromotive force which maintains a current of one ampere in a conductor having a resistance of one ohm. The electromotive force of a Clark standard cell at a temperature of 15 degrees centigrade is 1.434 volts. The volt is equal to a hundred million, 10^8, C. G. S. units of electromotive force.
CURRENT—C.—The Ampere is that current which will decompose 0.09324 milligramme of water (H2O) per second or deposit 1.118 milligrammes of silver per second. It is equal to one-tenth of a C. G. S. unit of current.
The milliampere is one thousandth of an ampere.
QUANTITY—Q.—The Coulomb is the quantity of electricity conveyed by an ampere in a second. It is equal to one-tenth of a C. G. S. unit of quantity.
The micro-coulomb is one millionth of a coulomb.
CAPACITY—K.—The farad is that capacity of a body, say a Leyden jar or condenser, which a coulomb of electricity will charge to the potential of a volt. It is equal to one thousand-millionth of a C. G. S. unit of capacity.
The micro-farad is one millionth of a Farad.
By Ohm's Law, Current = Electromotive Force/ Resistance,
or C = E/R
Ampere = Volt/Ohm
Hence when we know any two of these quantities, we can find the third. For example, if we know the electromotive force or difference of potential in volts and the resistance in ohms of an electric circuit, we can easily find the current in amperes.
POWER—P.—The Watt is the power conveyed by a current of one ampere through a conductor whose ends differ in potential by one volt, or, in other words, the rate of doing work when an ampere passes through an ohm. It is equal to ten million, 10^7, C. G. S. units of power or ergs per second, that is to say, to a Joule per second, or 1/746 of a horse-power.
A Watt = volt X ampere, and a Horse-power = Watts/746.
HEAT OR WORK—W.—The Joule is the work done or heat generated by a Watt in a second, that is, the work done or heat generated in a second by an ampere flowing through the resistance of an ohm. It is equal to ten million, 10^7, C. G. S. units of work or ergs. Assuming "Joule's equivalent" of heat and mechanical energy to be 41,600,000, it is the heat required to raise .24 gramme of water 1 degrees centigrade. A Joule = Volt x ampere x second. Since 1 horse-power = 550 foot pounds of work per second,
W = 550/746 E. Q. = .7373 E. Q. foot pounds.