A friend of mine picked this up at a garage sale where an elderly electrical engineer was selling a bunch of old test equipment.

It's missing a faceplate where it should name what it is, and he's googled the company name and model number, all to no avail.

Several of the engineers in my company have been by to take a look at it, and so far the consensus seems to be that it's some sort of tunable filter (it has what appears to be a large variable capacitor and large variable resistor).

I was wondering if anybody here had seen one of these before and had any idea of what it was used for, and possibly some links to relevant documentation.

UPDATE: Added image of interior of device

Wideview Closeup Inside

  • \$\begingroup\$ Whats below the holes on the top left. Speaker? or vent? (Two screws seem to be holding something up) \$\endgroup\$
    – Doc
    Commented Aug 10, 2012 at 15:10
  • \$\begingroup\$ We think it's a vent, under it is a flat strip of metal that snakes back and forth like a radiator. Multiple taps come off the strip and go to the multiply by knob. We think it's some sort of variable resistor. Under the big knob is what looks like a variable capacitor, wikipedia seems to think the construction is called a spiral torsion spring. \$\endgroup\$
    – TehBam
    Commented Aug 10, 2012 at 15:20
  • \$\begingroup\$ I should also mention, it's similarly vented on the bottom in the exact same place \$\endgroup\$
    – TehBam
    Commented Aug 10, 2012 at 15:23
  • \$\begingroup\$ People have been coming into the office all day to take a look at it, and the general feeling seems to have shifted to the large dial being a variable resistance, not capacitance. The thinking is now that the spring we thought was the variable capacitance element is simply the electrical connection for the wiper, with the outer metal band of the large circular piece being the variable resistance element. \$\endgroup\$
    – TehBam
    Commented Aug 10, 2012 at 16:10
  • 2
    \$\begingroup\$ @TehBam - Measure the connections to the big rotary thing with an ohmmeter. That should tell you if it's a cap or a potentiometer. Heck, just measure what you can, and report the resistance or capacitance (if any). \$\endgroup\$ Commented Aug 11, 2012 at 5:55

1 Answer 1


It is precision potentiometer with very low resistance and very high ratio accuracy. It could be used for say in bridge with "zero reading" galvanometer to match 2 currents or voltages. It was possibly marketed for chemical labs doing thermocouples measurements.

I'd estimate it is nearly as good as low-end Kelvin-Varley divider. And can still be very useful and very stable thermally.

One clue comes from googling the "Rubicon Company Philadelhia". There are similar products (from 1960s) like "standard 1 Ohm", potentiometers, galvanometers (with mirror for light beam!) and other high end lab devices of the era.

  • \$\begingroup\$ Must be worth something :-) \$\endgroup\$
    – Piotr Kula
    Commented Aug 17, 2012 at 11:25

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