1, the standard current flows through a precision resistor box, you can get the precision resistor box on the corresponding many files of accurate voltage values.

2, a precision resistor, a potentiometer for adjusting the current is connected in series to the resistor box and the power supply constitutes the circuit, precision resistor → resistor box → potentiometer → power supply → back to the precision resistor,,.

3, Now measure the potential on the precision resistor without affecting the current. The way to do this is to parallel the standard potential to the top (your 200mv), adjust the potentiometer (the voltage drop on the precision resistor will be changed accordingly) until no current is generated in the ** circuit formed by the standard battery and precision resistor (measured by a current detector), at this time, the potential (voltage drop) on the precision resistor is equal to the standard potential, and so you get the standard current value. Assuming that your precision resistor is 2 ohms, the adjustment is 100 mA standard current.

At this point, if your standard resistor box is 1 ohm, the voltage drop across the resistor box is 1 X 100=100 millivolts, and similarly, if the resistor box is 100 ohms, the voltage drop is 10 volts. You now know the voltage drop across the resistor box directly from its reading.

4, to be more accurate multimeter connected to one end of the resistance box and potentiometer connected to the end, the other end through the same current detector access and precision resistors connected to the other end of the resistance box. Multimeter → resistance box → current meter → return to the multimeter, constituting the third circuit.