The SI unit of electric current is the ampere, the symbol of which is A. The ampere is defined by taking the fixed numerical value of the elementary charge e to be 1,602176634×10⁻¹⁹ when expressed in the unit C, which is equal to A⋅s, where the second is defined in terms of Δν_Cs. In practice, the ampere can be realized by the units of voltage and resistance, based on the Josephson and quantum Hall effects, respectively.

The Estonian national standard laboratory for electrical quantities realizes the unit of electric current via the 
voltage unit volt (V) and the resistance unit ohm (Ω). The volt is maintained using Zener type voltage references at the nominal values of 1 V and 10 V. The electric resistance unit ohm is maintained with a group of
precision resistors in the range of 1 mΩ to 10 kΩ.

DC voltage measuring instruments are calibrated in the range of 10 mV to 1 kV with the relative expanded measurement uncertainty of (1 to 20)·10-6.

Resistance measuring instruments are calibrated in the range of 1 mΩ to 10 kΩ with the relative expanded measurement uncertainty of (1 to 5)·10-6.

Andrei Pokatilov
andrei.pokatilov@metrosert.ee