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). 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.
