15 USC Section 223

(16 January 1996)

§ 223. Units of electrical measure

From and after July 21, 1950, the legal units of electrical and photometric measurement in the United States of America shall be those defined and established as provided in the following paragraphs.


The unit of electrical resistance shall be the ohm, which is equal to one thousand million units of resistance of the centimeter-gram-second system of electromagnetic units.


The unit of electric current shall be the ampere, which is one-tenth of the unit of current of the centimeter-gram-second system of electromagnetic units.

Electromotive force and electric potential–volt

The unit of electromotive force and of electric potential shall be the volt, which is the electromotive force that, steadily applied to a conductor whose resistance is one ohm, will produce a current of one ampere.


The unit of electric quantity shall be the coulomb, which is the quantity of electricity transferred by a current of one ampere in one second.


The unit of electrical capacitance shall be the farad, which is the capacitance of a capacitor that is charged to a potential of one volt by one coulomb of electricity.


The unit of electrical inductance shall be the henry, which is the inductance in a circuit such that an electromotive force of one volt is induced in the circuit by variation of an inducing current at the rate of one ampere per second.


The unit of power shall be the watt, which is equal to ten million units of power in the centimeter-gram-second system, and which is the power required to cause an unvarying current of one ampere to flow between points differing in potential by one volt.

Energy–joule; kilowatt-hour

The units of energy shall be (a) the joule, which is equivalent to the energy supplied by a power of one watt operating for one second, and (b) the kilowatt-hour, which is equivalent to the energy supplied by a power of one thousand watts operating for one hour.

Intensity of light–candela

The unit of intensity of light shall be the candela, which is one-sixtieth of the intensity of one square centimeter of a perfect radiator, known as a “black body”, when operated at the temperature of freezing platinum.

Flux of light–lumen

The unit of flux of light shall be the lumen, which is the flux in a unit of solid angle from a source of which the intensity is one candela.

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