Simpson Microammeter Internal Resistance
How Much Does This Thing Load a Circuit?
The meter being tested here is not anything special. It is an old Simpson 50

I learned earlier this year (2009) when I was testing a "joule thief" circuit, that a supposedly low resistance ammeter would markedly affect the circuit I was working with, to the point that I had to come up with a better measurement tool. (That tool turns out to be, put a 1

Although I did my "joule thief" testing with a digital ammeter, and I did try to come up with a rough idea of the internal resistance for it, I wasn't convinced that I was really calculating out the correct value. So I put those numbers aside and pulled out the analog Simpson meter to make a solid attempt at finding its internal resistance.
The Methodology
The circuit for testing this is simple. A resistor is put in series with a dry cell and the ammeter. The value of the resistor is chosen such that the voltage source will drive the meter to exactly full scale deflection, and no more. I had handy a rechargeable Ni-Cad cell with a voltage of 1.296



I did not want to break the meter by putting in too little resistance to start. I put into the circuit my Cornell-Dubilier RDA-RDB-RDC Resistance Box and set it to 30k



The next step should have been simple. Put a shunt resistance in parallel with the meter to drop the current through it to exactly one half scale. I did this, once again using the resistance box to select the correct resistance. It was 1780

Two things about this surprised me. First was that the resistance was so high. (Although it was in the ballpark with that I had calculated for the digital ammeter.) And second was that when I tried to tie all the measured numbers together, I was not exactly on target. Oh, yes, they were pretty close. And I knew that this number must be representative of what the resistance actually was. But I wanted a more convincing set of calculations. I also pulled out my Shallcross 638-R Kelvin Wheatstone Bridge to make certain of the resistances. This device can measure resistances at an accuracy level of 0.3% between 1


The graphic below may be clicked upon to enlarge it. This is the results of the experimentation as captured in an Excel workbook. I am not going to repeat the stuff I have shown below as the workbook should be self-explanatory, with a little study.
Excel Workbook Calculations and Results
Conclusion
The ammeter has 1793