Cambridge Instruments Portable Potentiometer

ser no L163780 


This instrument contains a Weston standard cell (1.018 volts)dated 1972. It is intended to be used for accurately measuring low DC voltages and typically those generated by thermocouples to measure temperature. Measurement ranges: 0-20.1mV and 0-100.5mV

I was given this instrument which formed part of a collection of many instruments collected by Bob Evans by his daughter Alice Kirby back in 2015. The main problem when I first tried it was that the taut wire of the galvanometer suspension was broken but when I connected a digital multimeter in its place and connected it to a 1.5 Volt cell I was able to standardise it using the rheostat control and use it to measure millivolts. More recently, after dismantling another redundant Cambridge Instrument I was able to replace the galvanometer completely. This was easily done but when I came to test it I found that I could still standardise it but it steadfastly refused to work in the test mode. No amount of switch cleaner improved the situation so the only solution had to be the removal of the switch and try and effect a repair. This proved to be a nightmare, all the wires are black and single stand and the switch must have been one of the first components fitted to the front panel. Once opened up I found that the copper wipers were green and rather worn. these were cleaned by dipping in Harpic lavatory cleaner and the switch re-assembled wired back in place.  As can be seen this was not straightforward and unfortunately I found that I had inserted the moving portion upside down and so the switch markings were incorrect so it all had to be done again.

The instrument now works and proved to be accurate when checked against the 20mV voltage output of my Model CR-103/J Current and Voltage Standard balance was obtained at  a stetting of 19.99 on the dials.

How much is it worth? Instruments such as this are of little use today and not many people seem interested in them. Expect to pay �20-�30


Close up view of STANDARDISE/TEST switch and rotating contacts

The standard cell is on the right, the circular disc is the drive for the rheostat.