Help me light this 1923 voltmeter

[BinaryMike]

How would I re-scale it, by reducing the 500k resistor on it?

That sounds right. A 1mA movement needs a 500K calibration resistor to read 500V full-scale. If it was to be calibrated for a much lower voltage, then you would have to take coil resistance into account, but at 50V you should be able to ignore that issue.

 

[Aerobat]

I did some experimenting and found with a 47.5K resistor the meter reads 44vdc as 480v. Close enough. I could play with values and get it right on 440v but don't have the resistors on hand, probably needs 50k or so.

 

[merlynski]

Use a 5k-10kΩ trimmer pot in series with your 47.5kΩ resistor and dial it right in exactly.

 

[gadget73]

yep, fixed resistor and a trim pot to fine-tune should make it right.

 

[Aerobat]

I was watching the needle bounce around last night while playing music. Hard to believe the meter is in this shape after almost 100 years. The back is covered in some kind of carbon from where I assume it was installed.

 

[gadget73]

oh, random thought, do you have a pair of 100k? That'll make 50k in parallel. Should be a little closer to right even without the trimmer

 

[Aerobat]

I think I do but I put heat shrink on the connection and will have to get some motivation to cut it off and re-do it. Y'all can be wowed by it on Sunday lol.

 

[Aerobat]

For your amusement, here's the 1923 500K resistor that was on the back of the voltmeter.

 

[gadget73]

Likely wirewound and high precision. That was the only practical method of making a precise stable resistor a century ago.