How can an analog multimeter have a combined mV and µA scale?

Question

Looking at analog multimeters on Amazon I came across several that have one switch position that appears combined for the lowest V and µA settings. I've used various analog multimeters over many years and never ran across one with such a combined setting. Then I found that even current Triplett Model 630 being sold has it, however, older 630s on eBay don't seem to. For this particular meter, the lowest DC voltage range is 300mV and the lowest current range is 60µA. There are only two jacks for the two probes, so I don't understand how such a combined switch setting could measure either current or voltage (or both). I included a picture of a Hioki 3030-10 made in Japan with an identical setting, and many very cheap ones from China also have a similar setting.

I haven't found any manuals that explain how to use that type of setting (although a manual is not something I've ever needed before to operate a multimeter).

Answer 1

Because that's the position that connects the movement's coil directly to the meter's terminals, but also with a shunt resistor in parallel. The coil has a resistance of 5 kΩ and the movement deflects to full scale when 50 µA flows through it. Because of the shunt resistor in parallel, the current at the terminals is 60 µA (50 µA through the movement and 10 µA through the shunt resistor), at which point the voltage across it is 60 µA x 5 kΩ = 300 mV. Therefore, full scale is both 300 mV and 60 µA.

The 20 kΩ / V rating is only true for the 3 V and above ranges. For the 0.3 V, the rating is actually 16.7 kΩ / V.