Early 20th Century Potentiometer

[SurreyNick]

I have this rather attractive early 20th century Potentiometer Type 3387B made by H Tinsley & Co., London still in its original mahogany case.

In the centre of the ebonite panel there is a large knob with a rotational scale from -0.005 - 0 - +0.105 in divisions of 0.0005.
The volts selector is adjustable from 0 to 1.8 in .1 increments
The selector switch has options for S, 1 or 2
The Rheostat knob will turn ten full rotations
The Galv switch is of the push/pull type
There are three sockets marked XBY 1, .1 and .01

There are also five pairs of terminals. These are:
GALV.
BATT +/-
STD. CELL +/-
1 +/-
2 +/-

I have read a potentiometer can be a measuring instrument which is essentially a voltage divider used for measuring electric potential and I am assuming that this is what this item is. I also know there is one of these in the University of Aberdeen, Natural Philosophy Collection of Historical Scientific Instruments, so I guess it is of some historic interest too.

Sadly, I am going to have to pass it on to another collector, but before doing so I would actually quite like to know how this bit of equipment was used and for what practical purpose.

Can anyone cast any light?

Thanks

Nick
   

[ppppenguin]

The wikipedia entry is pretty good: https://en.wikipedia.org/wiki/Potentiome...trument%29

A potentiometer, as its name implies, is used to measure electrical potential. It does this by comparing a known fraction of one potential with a another potential. If one of the potentials is known (eg a Weston standard cell) then so is the other potential.

There is no current drawn from one of the sources of potential, effectively infinite input impedance.

The modern usage of potentiometer as a 3 terminal variable resistor is simply because it is logically identical to the potential measuring device. Albeit without accurate calibration.

[Terry]

Your estimate of the age of your potentiometer may be teensy weensy bit (technical term) out ...

This reference: http://collectionsonline.nmsi.ac.uk/deta...?t=objects&type=browse&f=CATEGORY1&s=SCM+-+Electricity+and+Magnetism&record=919 suggests 1960 - 79!

Another webpage states "This Tinsley instrument uses an external galvanometer to indicate the null, achieved when a voltage having been standardised, is divided by the correct ratio to equal the unknown using the resistance switches on which a value can be selected to four significant figures of precision. The painstaking care with which equipment of this type was handmade is remarkable by today's standards."

It obviously wouldn't have been cheap, either!

[Refugee]

Those old instruments are lovely. It must be difficult to get those standard batteries these days.
In modern times they use a very special silicon device to get the voltage and a couple of OP amps in place of the potentiometer.
Later the manufacturers wrapped the galvo and power supply up in a single box and used the same principal in order to measure resistance.
This does not even need a regulated power supply as it is measuring the ratio of the resistors and not the absolute value.
I have got one that in built into a Megger.

[Ed_Dinning]

Hi Nick, we used these in the Reyrolle standards lab as well as in the production test areas (not such a precision device there) when I worked there in the 60's.
A useful instrument.
Wait until you see an AC one, that has adjustments for phase as well as voltage, gets a bit more complex.

Surely you will have space for these at your new abode?

Ed

[Yorkie]

Here are some pics of one made by Cambridge Instruments which a neighbour asked me if I wanted before he took it to the tip some time ago. I couldn't let that happen.

I think I posted a thread on it on the forum some time back.

The consensus of opinion was that it wasn't anything like as old as its appearance might suggest - probably 1970s, and most likely and ex university lab item.

Beautifully hand made, with wire wound pots, and many inductors as the pics illustrate - goodness knows at what cost. (It's minus its Mallory standard cell).

I still have it, and really, it needs re-homing to a museum, but which establishment would want such a thing?