Ampere W/V. Current can be measured by a multimeter, a device that can measure electrical voltage, current, and resistance. The standard ampere is most accurately realized using a Kibble balance, but is in practice maintained via Ohm's law from the units of electromotive force and resistance, the volt and the ohm, since the latter two can be tied to physical phenomena that are relatively easy to reproduce, the Josephson junction and the quantum Hall effect, respectively. At present, techniques to establish the realization of an ampere have a relative uncertainty of approximately a few parts in 10, and involve realizations of
Output impedance order of 1 ohm. When there is a current through a cell, the measured e.m.f. is lower than when there is no current delivered by the cell. The reason for this is that part of the available energy of the cell is used up to drive charges through the cell. This energy wasted by the so-called "internal resistance" of that cell. This wasted energy shows up as lost voltage. Internal resistance is r = (E − V)/I . Output impedance The output impedance of an electrical network is the measure of the opposition to current flow (impedance), both static (resistance)