QUANt.i.tY--Q.--The Coulomb is the quant.i.ty of electricity conveyed by an ampere in a second. It is equal to one-tenth of a C. G. S.
unit of quant.i.ty.
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 quant.i.ties, 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 pa.s.ses 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.
a.s.suming "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.
HEAT UNITS.
The British Unit is the amount of heat required to raise one pound of water from 60 degrees to 61 degrees Fahrenheit. It is 251.9 times greater than the metric unit, therm or calorie, which is the amount of heat required to raise one gramme of water from 4 degrees to 5 degrees centigrade.
Joule"s Equivalent--J.--is the amount of energy equivalent to a therm or calorie, the metric unit of heat. It is equal to 41,600,000 ergs.
The heat in therms generated in a wire by a current = Volt X ampere X time in seconds X 0.24.
LIGHT UNITS
The British Unit is the light of a spermaceti candle 7/8-inch in diameter, burning 120 grains per hour (six candles to the pound).
They sometimes vary as much as 10 per cent, from the standard. Mr.
Vernon Harcourt"s standard flame is equal to an average standard candle.
The French Unit is the light of a Carcel lamp, and is equivalent to 9 T/Z British units.