Rubicon 2732 Precision Potentiometer (Wheatstone bridge?)

[Dave92F1]

I picked this up at a local ham flea market.



It's Rubicon 2732 Precision Potentiometer. I'm not sure exactly what it does, but I'd like to get it going. I think it's a sort of Wheatstone bridge and/or accurate voltmeter. I think the design dates from the late 1940s, but parts inside say 1963.

On the left is a mirror galvanometer. It was all messed up when I got it (the beam missed the ground-glass screen completely) but I adjusted it and it seems OK now.

On the right is a giant graduated slidewire pot, two pairs of EMF terminals and a switch to choose between them, a small unmarked knob that turns thru about 30 degrees, a big clickly rotary switch marked "MILLIVOLTS" that goes from 0 to 160 in steps of 10, a switch that chooses 0.1 vs. 1.0, and two buttons "EMF" and "SC".



Inside is what appears to be a Weston cell (reads 1.017 volts, which seems a little low, or my 5.5 digit DMM is off).

The instructions:



They say it runs on a #6 dry cell and two #2 dry cells.

A #6 appears to be a giant 1.5 volt carbon-zinc cell. I hooked up an alkaline D cell - something in there is drawing 15 mA.

I can't tell what a #2 was, but it appears to only run the 3v light bulb in the mirror galvanometer, so I connected 2 C cells in series - it seems happy with that (I get a nice spot and hairline).

So it says to do the "Standardization" by tapping the SC button and adjusting the "standardizing rheostat" (whichever that is) "until galvanometer stays on scale when button is tapped".

When I tap the SC button the beam goes way off to the right. No adjustments seem to change that.

So I'm stuck. Which one is the "standardizing rheostat", and what is going on here?

Underside of right side:



The Weston cell:



View toward underside of galvanometer:

[bob91343]

From what I recall regarding stuff like this, what you have is a standard voltage source.  The standard cell calibrates it, and once calibrated, you can generate any voltage from zero to, say, one volt.

So picture a potentiometer with one volt exactly across it.  Since the calibrator, the standard cell, is a bit more than one volt, you put a variable resistor in series with the potentiometer to absorb the excess.  The power supply, the large cell, supplies the current through the pot.  The extra resistor, variable, adjusts the voltage to match the standard cell.

You read the cell voltage marked on the cell and adjust the standardization pot for the excess over one volt.  Then the calibrating resistor is moved to get a null on the galvo.

Once done, you have a source of millivolts with good accuracy.  Today's meters easily outperform this but in its day it was a good thing to have in a metrology lab.  Very expensive item back then.

All of the above is from the top of my head, since I have never seen a unit like this and it's been a long time since I studied this stuff.  I have a textbook called Basic Electrical Measurements or something similar.  Lots of pics of Leeds and Northrup and Rubicon and Weston and other high end stuff.

[bob91343]

Check this out:

https://www.ebay.com/itm/Rubicon-Potentiometer-2732-Locking-Wooden-Cabinet-Portable-Precision-Instruments/183225854459?hash=item2aa91ca9fb:g:l9gAAOSwbbda-cYG

[Dave92F1]

Check this out:

https://www.ebay.com/itm/Rubicon-Potentiometer-2732-Locking-Wooden-Cabinet-Portable-Precision-Instruments/183225854459?hash=item2aa91ca9fb:g:l9gAAOSwbbda-cYG

Very similar. I paid $30 for mine. 

[beanflying]

Cool box there is/has been a similar for sale in Oz over the last few weeks eBay auction: #264300212982 In spite of the listing saying pickup only I just got three Weston Cells from the same seller so if asked he may ship.

Mainly a word of warning about measuring the Cell for voltage. Only ever use a GOhm impedance meter on it or you risk killing it! So no basic handhelds ever.

[Calambres]

Those "Universal Potentiometers" are a sort of primitive DVMs. From a constant low voltage power source (Westone Cell), a Wheatstone bridge and a sensitive galvanometer you could measure millivolts and even microvolts very precisely. They also had mercury thermometers to adapt the readings to the ambient temperature because a standard cell has about -40µV/°C

I've got one that was originally used to calibrate thermocouples. The Weston Cell dates back to 1947 and is still working good. Unfortunately the galvanometer's metallic chassis has sort of disintegrated rendering the instrument unusable.

[bob91343]

I just did some research on Weston cells.  According to the NBS (now NIST) you can load the cell with 10 Megohms but must allow it to stabilize after removing the load.  So a 10 Megohm voltmeter will change the cell voltage enough to defeat some of its value.

Strangely enough, you can even short circuit the cell briefly but must allow a few days for it to come back.

Perhaps a voltmeter such as an HP 3456A could work to measure a Weston cell, since its input resistance is presumably infinite.

I don't envy the guys who try to establish standards.