An LF356 op amp is used as a gain amplifier with the output taken across R5. When a full-scale current of 1 mA is flowing through the meter, exactly 1 V appears across R5 (should be 1% tolerance or better).

The Circuit Diagram of DC Millivoltmeter

The Tabel for R1

This is fed back to R2 to the summing junction of IC1 (a full-scale produces 1uA). This offsets the current through R1. R1 has a value of 1 M ohm/V which is used to zero the meter. R4 provides some overcurrent protection for the meter.