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
Aluminium-conductor steel-reinforced cable nearby conductors the distribution of current within each conductor will be constrained to smaller regions. The resulting current crowding is termed as the proximity effect. This crowding gives an increase in the effective AC resistance of the circuit, with the effect at 60 Hertz being greater than at 50 Hertz. Geometry, conductivity, and frequency are factors in determining the amount of proximity effect. The proximity effect is result of a changing magnetic field which influences the distribution of an electric current flowing within an electrical conductor due to electromagnetic induction. When an alternating current (AC) flows through an isolated conductor,