Rubicon Instruments Model 2736 Potentiometer

This device is a very interesting piece of mid-century test equipment. The Model 2736 Portable Potentiometer is primarily designed for interfacing with and measuring from thermocouples, sensors that vary voltage in relation to temperature. I've been able to find no datasheet or other official documentation for the unit, but a few scattered references to it can be found in forum posts and other documents online, primarily pertaining to its use in scientific studies. There are a few variants of this model, as well as a few different models with a similar circuit and layout.

There will unfortunately be no live testing performed here, only a rough teardown. For reasons that may become obvious, these units do not stand the test of time particularly well.

It was produced by Rubicon Instruments, a Philly company that had been a division of Honeywell since 1957. The nameplate betrays Rubicon's status as a victim of Honeywell acquisition, with the large red Honeywell logo featuring most prominent compared to the cursive Rubicon logotype.

The case of this device seems to be a type of MDF or other wood particle composite, though it is very nicely covered from all angles by smooth faux wood paneling. Aside from the fancy stamped metal nameplate, it has a latch with a lock (my unit included the key!), a free-swinging handle with plastic grip, and sported two sets of rubber feet which I removed shortly after acquisition due to the black stains they were leaving on my table. The only accessory I don't seem to have are the thermocouple probes.

The lid of the unit opens up to reveal the interface of the instrument, mounted on a pair of very smooth black bakelite panels. A detailed instruction sheet can also be found on the inside of the lid.

Highlights of the above instruction sheet include the recommended application of petrolium jelly to the potentiometer for cleaning, and precisely zero safety notifications regarding the hazardous nature of the standard cell inside of the device. Also note that the black knob visible at the upper left of the instruction sheet toggles the lamp switch on the main panel when the lid is closed, preventing the battery from draining.

The instrument panel is made up of two discrete panels, a galvonometer panel and a potentiometer panel. The internal wiring is fantastic looking and very clean, with each panel sporting its own point-to-point circuitry and vintage musty scent.

Below can be seen the rear circuitry of the potentiometer panel. The large flat cylinder is actually the rotor of the potentiometer, with the conductive winding running along the curved face. The wiring has delightfully precise 90° bends allowing it to run across the panel as well as normal to it so it can reach the eyelets at the end of the leaf switches.

Below is a closeup of the wiper contact for the potentiometer, which is a discrete component from the actual rotor, mounted separately on the panel. The brass posts at the upper right are the rotational limits for the potentiometer.

Both panels are connected to the internal standard cell and battery circuit via tube socket connectors - one noctal and one heptal. The standard cell is mounted directly to the wooden case with a clip. The batteries would have been wired inside of a separate compartment in the case, accessible via a back panel. Per the instructions above, this unit used two No. 6 Dry Cell batteries, which produce 1.5VDC each.

The galvonometer assembly actually utilizes a mirror galvonometer, as evident by the transparent readout and inclusion of an internal lamp. The lamp sits inside a reflective tube, which channels the light towards an array of mirrors. A tiny mirror suspended in the galvonometer tube reflects this light towards the measured value on the scale above. The galvonometer tube is actually made of plastic, and the galvonometer assembly has a label indicating it was manufactured by Honeywell in Denver, CO, and has a part number of 16790817-001, for whatever that's worth.

Another fun discovery was a line of text printed on the back of the glass scale for the galvonometer, instructing the user to clean it with carbon tetrachloride. To be clear, don't use carbon tetrachloride for anything.

The Weston standard cell is held in place with a c-clip against the wall of the enclosure. It's an EPLAB model, made by Eppley Laboratory, Inc., with the serial number 754805. Removing it was somewhat precarious, mainly due to the looming threat of breaking the glass tube, which in the best case scenario would spill cadmium salt electrolyte solution, assuming of course that the elemental mercury remained immobilized by the mercurous sulfate binder, which is of course itself toxic to touch and inhale. At any rate, with careful prying, the cell can be removed from the clip. This must be done first as there is a nut underneath the cell that fastens the clip to the enclosure, so the clip cannot be removed with the cell in place. This specific cell is an unsaturated cell, as the Model 2736 is a mobile instrument and transportation would inevitably result in the inversion of the cell. This feature does come with the implication that the cell is no longer a viable reference source, as unsaturated cells have a useful life of little more than a decade, and this instrument is certainly at least that old.

Given that the standard cell is not viable as an electronic component and is just hazardous enough to be uncomfortable as a novelty item, I disposed of it by emptying its liquid contents down the sink and throwing away the remaining glass and solid matter.

...I'm joking, of course. I've disposed of these before, they're best dropped off at municipal hazardous waste collection sites inside a sealed container with padding. They're used to receiving things like mercury thermometers and lead-acid batteries, though they're unlikely to recognize a component like this. When dropping it off, I mark the container it's in with a 'best-of' list of it's toxic chemical contents - namely mercury, mercurous sulfate, and cadmium. I'll also usually describe it as a 'battery', so that the processing staff have at least some framework for what it is and don't try to open it haphazardly.