[Boatanchors] Help - I'm stumped!

[Michael D. Harmon]

I have the parts of an old Simpson Model 375 ammeter.  Some time in its 
checkered past, it was dropped, thrown, or run over, smashing the meter 
movement beyond repair.  I decided to save the shunts and reconstruct 
the meter in a new box.

When I downloaded the manual, I discovered that the original meter was 
described two different ways.  In the electrical specs at the front of 
the manual, the meter movement is described as a "1 mA annular 
instrument with integral shunt".  The (external) current shunts for each 
range are designed to provide a 100 mV drop at full scale on the meter 
movement.  In the schematic on the back page however, the meter is shown 
as a 100 mV meter.

If the meter is designed to be 1 mA full scale, and provides a 100 mV 
drop, then according to Ohms Law, the internal resistance must be 100 
ohms. I have boxes of meters, but I have NOT found a 1 mA meter that has 
an internal resistance of 100 ohms!  I have measured them all!

  If you try to use a microammeter as a millivoltmeter, using the the 
multiplier formula (R= Efs/Ifs -Rm), most of the time you end up with a 
negative number (depending on the internal resistance of the meter under 
test)!  You just can't build a millivolt meter out of a microammeter 
without some trick of mathematics!

What am I missing here???  Was this some kind of special meter?  I have 
Jim Tonne's meter face design  program, but it doesn't do much for me 
until I find a suitable meter movement!

Anyone know where I can find a 1 mA meter movement with a 100 ohm 
internal resistance??

Thanks for the help!
Mike Harmon, WB0LDJ
mharmon at att dot net