ampere

(A or amp)

   the SI base unit of electric current, named for the French physicist André-Marie Ampère (1775-1836), one of the pioneers in studying electricity. The current official definition of the ampere goes like this: suppose we have two parallel conductors, infinitely long and having negligible cross section. Place these conductors one meter apart in a perfect vacuum. One ampere is the current which, if it's flowing in these conductors, creates between them a force of 0.2 micronewtons per meter of length. (You're welcome to object that no one can make an infinitely long conductor, nor a perfect vacuum. But scientists can use the idealized definition to construct appropriate real-world equipment in their laboratories.) It is likely that new definitions will be adopted at the CGPM meeting in 2011; the proposed definition is that the ampere represents a flow of exactly 6 241 509 479 607 717 888 elementary charge units (such as electrons) per second. The other electrical units are all defined in terms of the ampere. For example, one ampere represents a current flow of one coulomb of charge per second. One ampere of current results from a potential distribution of one volt per ohm of resistance, or from a power production rate of one watt per volt of potential. The unit is known informally as the amp, but A is its official symbol.