expressed in terms of the other units using the relationship , and thus 1 A = 1 W/V. Current can be measured by a multimeter, a device that can measure electrical voltage, current, and resistance. Former definition in the SI Until 2019, the SI defined the ampere as follows: The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between these conductors a force equal to newtons per metre of length. Ampère's force law states that there is an attractive or repulsive force between two parallel wires carrying an electric current. This force is used in the formal definition of the ampere. The SI unit of charge, the coulomb, was then defined as "the quantity of electricity carried in 1 second by a current of 1 ampere". Conversely, a current of one ampere is one coulomb of charge going past a given point per second: In general, charge was determined by steady current flowing for a time as . This definition of the ampere was most accurately realised using a Kibble balance, but in practice the unit was maintained via Ohm's law from the units of electromotive force and resistance, the volt and the ohm, since the latter two could be tied to physical phenomena that are relatively easy to reproduce, the Josephson effect and the quantum Hall effect, respectively. Techniques to establish the realisation of an ampere had a relative uncertainty of approximately a few parts in 10, and involved realisations of the watt, the ohm and the volt. Present definition The 2019 redefinition of the SI base units defined the ampere by taking the fixed numerical value of the elementary charge to be 1.602 176 634 × 10−19 when expressed in the unit C, which is equal to A⋅s, where the second is defined in terms of , the unperturbed ground state hyperfine transition frequency of the caesium-133 atom. The SI unit of charge, the coulomb, "is the quantity of electricity carried in 1 second by a current of 1 ampere". Conversely, a current of one ampere is one coulomb of charge going past a given point per second: In general, charge is determined by steady current flowing for a time as . Constant, instantaneous and average current are expressed in amperes (as in "the charging current is