Electric dc current is an area of expertise at the National Measurement Laboratory for Electrical Quantities. We are realising the national standard for the current unit ampere (A). Ampere is defined as the charge unit coulomb per second and is thus a measure of how many charges that pass through a conductor per time unit. An electron is carrier of the charge -1.602 176 634 ∙ 10-19 coulomb.
When the SI system was redefined on May 20, 2019, the definition of the electrical base unit ampere was changed. Instead of being based on the force between two infinitely long electric conductors the ampere is now defined by the value of the elementary charge e. Simultaneously to the redefinition of the SI system, the elementary charge was given the exact value e=1.602 176 634 ∙ 10-19 C, where the unit coulomb, C, can be expressed as ampere times second, or A·s. It means that one ampere corresponds to a flow of charge of 1 coulomb, i.e. around 6.241 509 074 ·1018 electrons, per second.
Counting electrons is possible, but the technique is still in its infancy. At RISE we therefore realize the current unit by utilizing Ohm's law wich says that the current equals voltage divided by resistance, I = U/R. It follows that the relation for the units is A=V/Ω, i.e. ampere is equal to volt divided by ohm. Both the voltage unit volt (V) and resistance unit ohm (Ω) can be realized very accurately using the Josephson effect and the quantum Hall effect respectively. The measurement uncertainty in the realization if we use Ohm's law will be at best 25 nA/A at 100 μA.
An alternative realisation of the dc current unit is sometimes used for small currents below 100 pA, where the above method gives too large measurement uncertainty. A known and constant voltage ramp is used to charge a capacitor. The traceability in this case comes from the units for direct voltage, frequency and capacitance. The measurement uncertainty in this realisation is at best 30 μA/A.
During the past decade, measuring instruments which can measure smaller currents have been developed. As a consequence, requirements for accurate calibrations in that measurement range has increased. Therefore, our research and development has primarily focused on developing calibration methods with higher accuracy for small currents.
Contact us if you have any questions about our work on electric current. We offer calibration of current measuring and current generating instruments and can also assist in the development of new calibration methods. More information about our calibration can be found on our service pages.