[Boatanchors] Help - I'm stumped!

[Bill Cromwell]

Well I'm confised. You said it is a 100 mA meter and THEN you said it is 
a microammeter. Maybe that is the source of the trouble you are having.

Meters are spec'ed for full scale current. I have never seen a meter 
movement spec'ed for full scale voltage. For that to happen there MUST 
be additional parts even if they are mounted inside the housing.

73,

Bill  KU8H

On 10/05/2018 12:21 AM, Michael D. Harmon wrote:
> 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
>
>
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bark less - wag more