Here is the changing ammeter scale 1mA to 100uA. Suppose that, you want to measure a current for about 10 uA(microamps).
How do you do? In when you have an ammeter scale 1 mA only.
If you will take it to measure directly, it can not read correctly. Because the meter needle will slightly bounce up only.
How it works
We need the circuit that converts a small current (from input) into a larger current(to ammeter). The amplifier is the answer.
I like a transistor it may be a good amplifier for this. Amplifiers require transistors that are partially ON.
I test this circuit first. It is a simple amplifier using 2N3904. It works. You can use another transistor (or BC337 or 2N2222 or BC148) is only one.
As you know about the transistors.
A very small base-emitter current will cause a much larger collector-emitter current to flow.
When the input current (100uA) flows into the base lead.
It makes many currents(1mA) from the battery flows through R1, the ammeter, and collector-emitter to ground. It will show a full-scale deflection on the 1mA meter.
The R1-resistor protects the meter and transistor from too much current (which can kill them).
The VR1-1K potentiometer—adjusts the circuit to measure low current as you want.
You may try this circuit instead of the old ammeter. Then adjust VR1 to read the correct currents.
Just this is completed the setting of this circuit.
I use a power supply from the 9V battery 9V. It is available a long time until we forget it.
Also you will use 3V battery, so good. Just adjusts a value of the R1 from 4.7K to 1.5K only.
Up to $20 shipping discount on first order now: https://jlcpcb.com/quote