Cambridge Portable Potentiometer.

[sparkymike]

Cambridge Potentiometer model L251229
I could not resist taking this one under my wing.
Nice case and mostly in top condition, but at some time in its life it must have been dropped or been clouted as there are three items that need repairing.
The pedestal for the push button switch has been broken. The meter movement is damaged and there is a small coil that has become detached, but not sure where it should be soldered to.
re. the movement, The movement hangs on a fine wire. Anyone know if this wire is a special material or gauge ? I hope to have a go at repairing it if possible, and if my eyesight is up to the task.
I have marked the coil in question on one of the thumbnails.
What year would this be ?
One other question, there is a round tube cavity in the box. Is this for a battery and if so, what voltage would it be.? Quality of the build is superb.
Was Cambridge instruments part of the Pye organisation ?
Mike.

[David G4EBT]

That brings back happy memories for me.

The history of the company is quite interesting (well it is to me at any rate!). See here:

http://www.richardsradios.co.uk/cambhistory.html

I spent my career with British Gas, during which - in 1968 - I spent a year in the Natural Gas Conversion laboratory based at Wellingborough, designing and testing burners and other components to convert obsolete gas appliances to natural gas, when they'd been designed to work only on coal gas, This obviated the expense of BG having to replace the whole appliance.

To test oven thermostats we used a black ball thermocouple (a black globe about 2.5" diameter with the thermocouple inside it), coupled to a Cambridge Potentiometer. The black ball was placed in the centre of the oven and would test the thermostat at various settings to ensure that the temperature at each setting was correct and remained constant. Rather more to that than it may sound, and it had to operate satisfactorily on five types of gas of different flame speeds and propensity to generate incomplete combustion, (and hence, carbon monoxide - an instant fail of course).

The thermocouple was brought into balance on the instrument, but quite how this was related to temperature I can't recall. All a dim memory after half a century!

It was a very challenging and interesting role with much job satisfaction, from both a practical and theoretical perspective.

I'm not sure if it used a 'Weston Cell' or something similar as the Voltage reference, which produces a highly accurate and stable 1.018638V. (I never took the lid off!).

https://en.wikipedia.org/wiki/Weston_cell

[sparkymike]

Hi David,
yes you are right. Weston cell is there and in one piece and not (as far as I can tell broken in any way).
At first I thought it was some kind of mercury switch !!
Mike.

[Ed_Dinning]

Hi Mike, I have one of these minus it's box, so could act as a donor for yours.

Ed

[sparkymike]

Thanks Ed, I have sent you a pm.
Mike.

[WME_bill]

Cambridge Potentiometer. The meter is taut band, suspended on a filament of phosphor bronze. Try using a watch or meter hair spring stretched out straight, or use the suspension band for a 400 day clock. Or else see if you can find a professional mender.

I have replaced hairspring coils in the past on moving coil meters, but have hesitated over these taut band jobs. Got my springs from a watch and clock trade stockist in Bristol. He tells me the only other one in UK is in Kent ?Whitstable. I have over 40 years got tweezers, eye-glass, non-drying oil, watchmakers screwdrivers etc and hairsprings from him or his father before him. I will try to remember the name of the business. I could take you to the shop, but I am 80 miles away, and you even further.

The coil in the picture looks like the compensation coil which connects in series with the meter coil. It will be of manganin or constantin, with a temperature coefficient to compensate for the copper of the meter coil itself, so that the meter unit has a contant resistance with temperature. This keeps shunts and range switching accurate.
The batteries are two EverReady AD4, each of 1.5V. The manual says other types can be used. As you will see, it also has a WestonStandard Cell as the reference.
And to finally cheer you up, I attach a copy of the manual. I suspect the date is 1957.
No, not Pye Radios. I suspect Cambridge Instrument Co was part of the original W G Pye & Co, from which C O Stanley bought the radios business in 1928.

wme_bill.

[MotorBikeLes]

There will be as already noted a Standard Weston Cadmium cell for voltage reference. There will also be a single leclanche cell in there, maybe a U2. In use, the leclanche is first standardised againt the standard cell, as you must NOT draw any current from it. from time to time, you recheck against the standard.
The meter movement is obviously a delicate device, but it does not need to have any specified accuracy, since all measurements are taken at null balance.
You must correct all readings for cold junction, for which there will be a mercury in glass thermometer built in.
Dave, I was NOT with BG then, but I worked in the pottery industry. We had some kilns which "could not be converted", not because they were too old, but because the conversion company was stumped. In the end, I masterminded the thing myself.
I remember when the intended contractors were still involved, you (BG) brought about 30 high pressure bottle of natural gas with all the control gear to get a supply to the kiln in question.
I remember two of the BG engineers were named Faith and Hope, we just needed the Charity for a full complement.
Les.

[lesmw0sec]

Used these many times while working at AEE Winfrith. Always rather liked them. I seem to recall using a similar but non-portable device for bench work, which was spec'd to 0.01 %. Not bad for the time..................Then digital voltmeters came in and spoiled all the fun!

The other Les.

[Ed_Dinning]

Hi Gents, looking at the manual it looks as if I have the higher spec workshop version, sadly without its case.

Ed

[sparkymike]

Hi Ed,
is there a the circuit in the manual ?
I was surprised to find that the Weston cell is giving a voltage reading.
Do these stay charged ad infinitum ?
Mike.

[Ed_Dinning]

Hi Mike, I think the circuit in the manual a couple of posts up is for both versions. The "workshop" version had additional fine setting pots I believe.
The standard cells are pretty robust if not shorted or physically damaged. Leaving them to settle often helps. A DVM with 10Meg input impedance will measure them without harm.

Ed

[TonyDuell]

I have a related instument. It claims to be a 'Cambridge Slide Wire Potentiometer' and was clearly not designed to be portable. It's a flat wooden box with various switches and variable resistors, no internal cells or galvanometer.

There's a row of pairs of terminals for a standard cell, galvo, 3 inputs (there's a switch to select each one of these) and a 2V supply (presumably a lead-acid cell). I seem to remember you switch to 'standardise', set the standard cell voltage on one of the controls, and balance it using the 'coarse' and 'fine' knobs. Then switch to measure, select one of the inputs and balance using the mV switch (in 10's from 10mV to 170mV) and the slidewire dial (which seems to be calibrated in 40uV divisions).

Alas I don't have a galvanometer, but I do remember borrowing one from school many years ago, and finding this thing worked, but would not standardise with a 1.5V or 3V supply, you had to use a 2V cell. So I never really used it.

I have seen the old threads here on that unit, but nobody seems to have a manual for it. Does anyone have one now?

[sparkymike]

Hi Tony,
I downloaded the instructions for a similar instrument, "The Cambridge workshop potentiometer" It is in pdf form. I will try and find the link if that is a help.
Mike.

[sparkymike]

https://www.vintage-radio.net/forum/...3&d=1207937544

The link to the manual (I hope)
Mike.

[TonyDuell]

No, alas that's not the instrument I have.

Mine is not a portable case, it's a wooden box with an ebonite (?) top panel with the knobs and terminals on it, no cover to protect them when not in use. No internal batteries, standard cell or galvanometer.

Mine is clearly a laboratory instrument, to be set up and used in one place.

I'll dig it out sometime and take some photos...

[McMurdo]

Went to Dad's yesterday and he showed me a polished wooden box he'd bought at the auction..'is this any use to you?'. It was one of these! Except it had 4 multiplier dials across the bottom marked 1's, 10's, 100's and 1000's. I demurred and left it in his museum of stuff.

[WME_bill]

Cambridge Portable Potentiometer.
Mike "Sparkymike" need not not be hopeful about whether his reference to an earlier posting of the circuit can strill be found. If he looks at the circuit attached to my posting of a couple of days earlier, he will find the self same manual with circuit.
There are quite a lot of variants around this basic standard cell potentiometer voltage reference idea. Tony Duell has found another. At one time I had several, all different. But with shortage of space and the arrival of a Solartron 7075 digital voltmeter to 1 uv, they all went on their way.
The ebonite and mahogany case and resistance box brass plugs now makes me wish I had kept them, even from 30 years ago.
For there was huge advantage with these old instruments. They did not go wrong, or require recalibrating every 2 years. The Weston Standard Cell does alter a little with age. It is worth checking them every 10 years.
wme_bill