What would give me better accuracy when measuring really small currents.

measuring with a multimeter on the amps range or measuring the voltage across a resistor and applying ohms law?

 

What would give me better accuracy when measuring really small currents.

measuring with a multimeter on the amps range or measuring the voltage across a resistor and applying ohms law?

Depends.

What do you consider a "really small current"? Huge difference between trying to measure 1 mA vs 1 µA vs 1 nA vs 1 pA.

When you say "multimeter on the amps range" do you really mean on the amps, versus one of the millamp, ranges?

But the real determining factor is how much either measurement will disturb what is being measured.

If I have a circuit that is using feedback to control the current through the load, then I can put a large resistor in series with it and get a high resolution measurement while having very little impact by measuring the voltage across it. But, at some point, the presence of the meter will change the measurement because the current flowing through the voltmeter becomes a non-negligible portion of the total current. But if the load resistance is itself very large, then putting a large resistor in series with it can disturb the measurement too much.

At the end of the day, nearly all ammeters are actually measuring the voltage across a resistor that is placed in series with the load. The problem is that the circuit with and without the ammeter inserted can be significantly different and what you really want to know is how much current is flowing when the ammeter is NOT in the circuit, but all you do know if how much current is flowing when it IS. The advantage of measuring the voltage across a current sense resistor is that you can leave the current sense resistor in the circuit whether you are making the measurement or not. As a result, the current you measure with the voltmeter in place is nearly identical to what is flowing when it is not, provided the meter's resistance is significantly greater than the current sense resistor. Most modern DMMs have a voltmeter input resistance of 10 MΩ. As long as the current sense resistor is not more than about 100 kΩ, your reading is disturbed by less than 1%, which is on par with the intrinsic accuracy of most such meters. A 100 kΩ resistor gives you 1 V per 10 µA of current.

 

I can see this can get quite involved with many factors possibly affecting the measurement

I'll try this out on the bench when i get time and see what results i come up with.

thanks.

 

this can get quite involved with many factors

In particular, check out the burden voltage of your multimeter - it is often disappointingly large so can in itself influence the current being measured.

 

In particular, check out the burden voltage of your multimeter - it is often disappointingly large so can in itself influence the current being measured.

Yes, this is something to consider.
The full-scale voltage for common multimeters that have a 200mV minimum DC voltage range is 200mV when measuring current.
This can thus affect the accuracy of the current measurement if measuring current in low voltage circuits.

 

Just to put that in perspective: On a range that measures uA (up to 200), that means a 1K resistor is inserted in the circuit.

 

Back in the day, when I was learning physics at school, we used a galvanometer which is a very sensitive moving coil ammeter, typically with a small mirror to deflect a beam of light giving a large optical movement for small deflections. Still probably a very practical way to measure small currents. Analogue moving coil meters typically have negligible resistance so they can be used to measure voltage with a high value resistor in series. If you are looking for a reliable way to measure low currents with minimal circuit disturbance I’d seriously consider using analogue