Determining the internal resistance of a meter..

This from a .pdf document on meter shunts posted on the internet.

Clearly, though, you need to know the resistance of the meter itself in order to work out the value of the shunt you're going to need. Luckily the internal resistance of most meters is specified when you buy them new.

For example the resistance of the MU-45 type 0-1mA meter movement sold by Electus (QP-5010) is specified as 200 ohms, while the more sensitive 0-50uA model (QP-5012) has a specified resistance of 3500 ohms. So it's not hard to work out the value of shunts needed to adapt either of these meters to read higher currents. But what if you have a meter movement salvaged from a piece of equipment, and you don't know its resistance? Or even its current sensitivity? (It might already have an internal shunt, for example.) The simplest way is to measure it -- but don't just slap a multimeter across it, because that could damage the meter by passing too much current through it. The best way is to use the simple test circuit shown below.

Here the meter is connected to a 12V battery via a DMM (set to a suitable current range) and adjustable resistor RV1, which is carefully adjusted until the meter is just giving its full-scale reading. The DMM will then be showing the current needed to produce this current FSD -- in other words, the meter's sensitivity.

Now switch SW1 is closed, to connect the second adjustable resistor RV2 directly across the meter, as a shunt. RV2 is then adjusted until the meter reading is reduced to exactly half scale, while monitoring the total current via the DMM and, if necessary, adjusting RV1 to maintain it at the original FSD figure.

When the meter reading is exactly 50%, the value of RV2 will be equal to the meter's own resistance, because the current is being split equally through the two paths. So you can then disconnect the battery and use the DMM to measure the value of RV2 (by itself), to learn the meter's resistance.

Note that even if the meter should have a current shunt built inside it already, this test will still give you its effective sensitivity and resistance including any internal shunt. So the figures you get will allow you to work out the value of any new shunt you need to adapt it for measuring higher currents, regardless of what may be inside the meter case.