  Accurate measurement of the electric current requires a stable and calculable
resistor for an ideal current to voltage conversion. However, the temporal
resistance drift of a physical resistor is unavoidable, unlike the quantum Hall
resistance directly linked to the Planck constant h and the elementary charge
e. Lack of an invariant high resistance leads to a challenge in making small
current measurements below 1 muA with an uncertainty better than one part in
106. In this work, we demonstrate a current to voltage conversion in the range
from a few nano amps to one microamp with an invariant quantized Hall array
resistance. The converted voltage is directly compared with the Josephson
voltage reference in the framework of Ohm's law. Markedly distinct from the
classical conversion, which relies on an artifact resistance reference, this
current-to-voltage conversion does not demand timely resistance calibrations.
It improves the precision of current measurement down to 8 10 -8 at 1 muA.
