Metrication in the United States There are no U.S. customary units for electric current, potential difference, or charge since these concepts were developed after the international adoption of metric in science. The metric units ampere, volt, ohm and coulomb are the only units used. The SI term hertz has replaced the term "cycles per second" as a unit of frequency. Energy is often measured in watt hours, BTUs, therms or calories rather than the SI joule. Residential and commercial electrical energy use is metered and billed in kilowatt hours, kW·h, as in most metric countries. Force measurements regularly use the pound instead of the SI
Rayleigh (unit) Rayleigh (unit) The rayleigh is a unit of photon flux, used to measure airglow (auroras, for example). It was first proposed in 1956 by Donald M. Hunten, Franklin E. Roach, and Joseph W. Chamberlain. It is named for John William Strutt, 3rd Baron Rayleigh (1842–1919). Its symbol is R (also used for the röntgen, an unrelated unit). SI prefixes are used with the rayleigh. One rayleigh (1 R) is defined as a column emission rate of 10 photons per square metre per column per second. Note that rayleigh is an apparent emission rate, as no allowances have been made for