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 Post subject: What is an Antique Potentiometer
PostPosted: Apr Wed 25, 2018 4:54 pm 
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Joined: Jan Sun 07, 2018 3:02 pm
Posts: 135
Greetings,

I just found this listing...

https://newyork.craigslist.org/brx/clt/ ... 66030.html

...for an Antique Potentiometer, for $40.

Image

For that price I am itching to just buy it, cause it looks great, but I don't know what it's for.

Any ideas?

Thanks...


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 Post subject: Re: What is an Antique Potentiometer
PostPosted: Apr Wed 25, 2018 5:06 pm 
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Location: Boiling Springs, PA
Google is your friend

https://www.worthpoint.com/worthopedia/ ... -142533679

Rubicon Portable Precision Potentiometer
Model 2730 ca. 1930
This is a piece of electrical engineering test equipment from the early 1930's. It is a battery-powered precision potentiometer used for measuring low voltages (0-1.61 volts) very precisely, and was manufactured by the Rubicon Company of Philadelphia. There are two galvanometers; one with an adjustable standard meter movement, and the other an incandescent back-lit meter movement known as a "pointerlite".
Controls are: Power on/off
Lamp on/off
1/0.1 multiplier switch
Volts, 0-1.5 in 0.1 volt steps (16-position)
Coarse galvanometer rheostat (R1)
Fine galvanometer rheostat (R2)
E.M.F. momentary switch
S.C. momentary switch
Adjustable voltage slidewire, 0-.11 with 0.0005 graduations
Connections are: +E.M.F. and -E.M.F. (circuit to be tested)
Uses two #6 batteries for measurement and two #2 batteries for the lamp (not included); both accessed by unscrewing the door in the back. Unit is clean and complete, except for batteries. Has solid wooden case with handle and lock, key included. Case has some minor dents and scratches, but overall is in very good condition considering its age. Panel is polished black polymer with nickel-plated screws and hinges; no missing hardware. Instruction sheet in lid is perfectly intact. Unit is assumed functional.
This is a rare piece of equipment for an electronic enthusiast.

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 Post subject: Re: What is an Antique Potentiometer
PostPosted: Apr Wed 25, 2018 5:55 pm 
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They were used in chemistry and physics labs of old to measure the small emf's during potentiometric titrations until the 1970's or so


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 Post subject: Re: What is an Antique Potentiometer
PostPosted: Apr Wed 25, 2018 6:52 pm 
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Great! Thanks for your replies.

Sounds like something I could use with my kid, when he's old enough to understand.

Thanks again.


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 Post subject: Re: What is an Antique Potentiometer
PostPosted: Apr Wed 25, 2018 11:25 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
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Location: Sunbury, Ohio 43074
Got one similar ... have not had a chance to research it yet. Appears to be an older version of yours perhaps ??

Attachment:
RUBICON  WHEATSTONE BR MEBBEEs.JPG
RUBICON WHEATSTONE BR MEBBEEs.JPG [ 159.69 KiB | Viewed 1204 times ]

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 Post subject: Re: What is an Antique Potentiometer
PostPosted: Apr Wed 25, 2018 11:47 pm 
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Location: S. Dartmouth MA 02748-1225 USA
Be aware there may be a mercury reference cell inside. Some Potentiometers used an external cell.

Chas

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 Post subject: Re: What is an Antique Potentiometer
PostPosted: Apr Wed 25, 2018 11:52 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island
It is a voltage measuring device. They are often calibrated in temperature, for use with thermocouples. This one is calibrated in millivolts for chem lab/physics experiments, and also as a voltage reference for general use.

The basic principle is that a bias current is passed through a high precision resistance wire potentiometer. The bias voltage is established by dry cells in the instrument and turned on and off by the toggle switch in the upper left hand corner. The two small knobs to the right are the coarse and fine current adjustments for the bias circuit. By passing a current through the potentiometer from bias cells, no power is taken from the source being measured. This is essential for chemical reactions, physics experiments, and other measurements where no loading of the source voltage can be tolerated. It's really quite simple; the potentiometer is essentially an adjustable voltage divider. The galvanometer is connected between the adjustable arm of the pot and the EMF being measured. When the two are at exactly the same potential, no current can flow and the galvanometer will not deflect.

There is also a standard cell in the box. The voltage of this would have been previously measured to a high degree of accuracy when the instrument was calibrated. The main millivolt dial and multiplier dial are set to the standard cell voltage and the "SC" button is pressed. The coarse and fine bias current rheostats are then adjusted for zero deflection of the galvanometer. This standardizes the dial to the cell. Then the EMF button is pressed and the main dial and multiplier are adjusted for zero deflection of the galvanometer again. The voltage is then read from the dial and multiplier.

The standard cells are the biggest problem with this kind of instrument. Most of them are dead by now, or certainly no longer accurate. They also tend to get broken if the instrument has been dropped or subjected to extremely cold temperatures. Cells of this type are no longer made. It is possible, however, to retrofit precision band gap semiconductor voltage references if one is so inclined.

It is worth noting that Rubicon instruments with light beam scales do not have two galvanometers. There's only one but it is very shrewdly designed to have both a reflecting mirror and a pointer with a scale on it. A flashlight bulb illuminates the mirror when in use. This way you can use the light beam which gives greater magnification when conditions permit, or the pointer and scale if the ambient illumination is too bright.

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 Post subject: Re: What is an Antique Potentiometer
PostPosted: Apr Thu 26, 2018 5:57 am 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
A potentiometer doesn't measure voltage. It compares voltages.

The only way to measure voltage is to convert it to a current.

Poggendorf's method may well make Schrödinger's cat smile

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Amateur Radio Literacy Club. May we help you read better.
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 Post subject: Re: What is an Antique Potentiometer
PostPosted: Apr Thu 26, 2018 2:57 pm 
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Location: Long Island
Ever hear of a differential voltmeter?

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 Post subject: Re: What is an Antique Potentiometer
PostPosted: Apr Thu 26, 2018 4:37 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
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Location: Sunbury, Ohio 43074
I think there's a disconnect in terminology here.... "back in the day" this "precision potentiometer" was named this way perhaps taking some poetic license by the manufacturer, but it actually does function as one regardless of the internal system. It was probably done because any other name would have had to consist of 15 words or more :)

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 Post subject: Re: What is an Antique Potentiometer
PostPosted: Apr Fri 27, 2018 12:38 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11412
Location: Powell River BC Canada
Fun with potentiometers. I have one and years ago it was for thermocouples. The
instrument came with a thermometer.

Here is some words. I reached for a book, and found the words almost instantly.
(that alone is scary) Would the world be here without google ?
Attachment:
Potentiometer master i.jpg
Potentiometer master i.jpg [ 233.1 KiB | Viewed 1042 times ]
Attachment:
Potentiometer master ii.jpg
Potentiometer master ii.jpg [ 231.27 KiB | Viewed 1042 times ]

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de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: What is an Antique Potentiometer
PostPosted: Jun Tue 18, 2019 4:32 pm 
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Posts: 183
I have seen some old Leeds - Northrup units with equally impressive build quality. I have a late 1930s Standco megohmmeter with similar construction. Still works. A boatanchorish Dressen Barnes lab high voltage (~450 VDC) laboratory power resides on a shelf here, having SOTA, TOTL 1950s - 1960's style wiring that differs from that of your Rubicon but is, when looked at, obviously of the highest quality achievable in a man-made instrument. Just amazing, in contrast to the ultra high tech but plastic throwaway stuff they make these days. Must have been uber spendy when new.

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 Post subject: Re: What is an Antique Potentiometer
PostPosted: Jun Tue 18, 2019 5:10 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11412
Location: Powell River BC Canada
Wood box instruments are great display items The texts for certain studies,
had lab experiments. Someone doing these experiments today could gain
insight that would come in handy in rationalizing what a modern piece of automatic
measuring gear was doing, or not doing.

It might be better than watching a video. Especially if sly pitfalls lurk in something
as simple as a loose wire lug that only works sometimes.

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de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: What is an Antique Potentiometer
PostPosted: Jun Wed 19, 2019 2:19 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 181
Location: Slidell, LA
I believe Rubicon was taken over by Leeds & Northrup at some point. So the "newer" versions would look pretty much the same.

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 Post subject: Re: What is an Antique Potentiometer
PostPosted: Jun Wed 19, 2019 5:48 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11412
Location: Powell River BC Canada
It would probably fetch good money if it could be part of Radioclast apparatus. :roll:

Does Abrams appear on a name tag somewhere ?

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VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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