DEFINITIONS OF ELECTRIC AND MAGNETIC UNITS
The ohm is the resistance of a thread of mercury at the temperature of melting ice, 14.4521 grams in mass, of uniform cross-section and a length of 106.300 centimeters.
The ampere is the current which when passed through a solution of nitrate of silver in water according to certain specifications, deposits silver at the rate of 0.00111800 of a gram per second.
The volt is the electromotive force which produces a current of 1 ampere when steadily applied to a conductor the resistance of which is 1 ohm.
The coulomb is the quantity of electricity transferred by a current of 1 ampere in 1 second.
The ampere-hour is the quantity of electricity transferred by a current of 1 ampere in 1 hour and is, therefore, equal to 3600 coulombs.
The farad is the capacitance of a condenser in which a potential difference of 1 volt causes it to have a charge of 1 coulomb of electricity.
The henry is the inductance in a circuit in which the electromotive force induced is 1 volt when the inducing current varies at the rate of 1 ampere per second.
The watt is the power spent by a current of 1 ampere in a resistance of 1 ohm.
The joule is the energy spent in I second by a flow of 1 ampere in 1 ohm.
The horse-power is used in rating steam machinery. It is equal to 746 watts.
The kilowatt is 1,000 watts.
The units of capacitance actually used in wireless work are the microfarad, which is the millionth part of a farad, because the farad is too large a unit; and the C. G. S. electrostatic unit of capacitance, which is often called the centimeter of capacitance, which is about equal to 1.11 microfarads.
The units of inductance commonly used in radio work are the millihenry, which is the thousandth part of a henry; and the centimeter of inductance, which is one one-thousandth part of a microhenry.
Note.--For further information about electric and magnetic units get the Bureau of Standards Circular No. 60, called Electric Units and Standards, the price of which is 15 cents; also get Scientific Paper No. 292, called International System of Electric and Magnetic Units, price 10 cents. These and other informative papers can be had from the Superintendent of Documents, Government Printing Office, Washington, D. C.