Coulomb
The coulomb (symbol: C) is the SI derived unit of electric charge. Historically, it was defined as the charge delivered by an electric current of one ampere in one second, with the ampere being defined as the current needed to generate a magnetic force of 0.2 micronewtons per metre between 2 parallel wires 1 metre apart. Under the 2019 redefinition of the SI base units, which took effect on 20 May 2019, the elementary charge was assigned the exact value of , making the coulomb exactly 1/() = elementary charges. The same number of electrons has the same magnitude but  opposite sign of charge, that is, a charge of −1 C. This makes the coulomb one of the few SI units to depend on the value of only one of the SI defining constants. Despite this, the coulomb is, somewhat incongruously, still classified as a derived unit, and the ampere is still classified as an SI base unit. Definition. The SI defines the coulomb in terms of the ampere and second: 1 C = 1 A × 1 s. The 2019 redefinition of the ampere and other SI base units fixed the numerical value of the elementary charge when expressed  in coulombs, and therefore fixed the value of the coulomb when expressed as a multiple of the fundamental charge (the numerical values of those quantities are the multiplicative inverses of each other). The ampere is defined by taking the fixed numerical value of the elementary charge "e" to be coulombs. The ampere was previously defined in terms of two wires of infinite extent. Thus 1 coulomb is exactly formula_1 elementary charges where formula_2 and the numerator formula_3 is a prime number. Thus one coulomb is the charge of approximately , where the number is the reciprocal of It is impossible to  realize exactly 1 C of charge, since the number of elementary charges is not an integer. By 1878, the British Association for the Advancement of Science had defined the volt, ohm, and farad, but not the coulomb. In 1881, the International Electrical Congress, now the International Electrotechnical Commission (IEC), approved the volt as the unit for electromotive force, the ampere as the unit for electric current, and the coulomb as the unit of electric charge. At that time, the volt was defined as the potential difference [i.e., what is nowadays called the "voltage (difference)"] across a conductor when a current  of one ampere dissipates one watt of power. The coulomb (later "absolute coulomb" or "abcoulomb" for disambiguation) was part of the EMU system of units. The "international coulomb" based on laboratory specifications for its measurement was introduced by the IEC in 1908. The entire set of "reproducible units" was abandoned in 1948 and the "international coulomb" became the modern coulomb. SI prefixes. Like other SI units, the coulomb can modified by adding a prefix that multiplies it by a power of 10.