R W Paul Mains Test Set This is a unipivot pattern U instrument housed in a polished wooden case with brass fittings. The large moving coil meter movement has an offset zero covering 50 - 0 - 750 microamps full scale. The lid is used to store three shunts and has a compartment to store test leads. Brief instructions and a schematic diagram are on the lid of this compartment. I have reproduced an enlarged copy of this below.
This is one of many instruments collected by Jack Davidson C Eng FIEE which I have been given by Dr Graham Winbolt. The pictures here were taken after dismantling and cleaning the woodwork and the brass fittings.
The front panel is engraved Robt. W. Paul London. N. although the meter is labelled Cambridge Instrument Co Ltd. The calibration certificates I have for this instrument are dated 1936. As the instrument has such a low serial number I think it must have been made some years earlier. Perhaps it was made before RW Paul was taken over and modified sometime after 1924 when a new scale was inserted and the instrument re-calibrated. The resistance figures on the instruction label which have clearly been added by hand might support this explanation. In the light of this I guess it was made in about 1920.
The box is equipped with a brass cover [which I have been unable to remove] attached to a chain. This appears to be a lamp housing which could be used to illuminate the scale. The paper label reads as follows: "PAUL UNIPIVOT If this instrument is opened, great care must be taken to exclude the minutest DUST OR FLUFF." This is no idle warning, the meter kept on getting stuck when I first tried it, and when I removed the circular brass cover was I able to see [with a magnifying glass] that a couple of small of corrosion particles had lodged in the air gap. These were removed with a thin sliver of paper and a a small sable brush. With the instrument out of the case the slightest air movement caused the needle to deflect. The meter works on all ranges,though it appears to read between 97% and 99% of the correct full scale value value depending upon the range selected. These values were obtained at about 16 degrees Celsius.
two views of the inside
Enlarged view of the instructions and diagram which can be seen on the first picture on this page
Robert William Paul (1869-1943) who had previously worked for Elliott Bros trained as a scientific instrument maker at City and Guilds College in London. He set up in business in Hatton garden in 1891, producing a range of instruments, mostly electrical. In 1903 he introduced the 'Unipivot' galvanometer. This was a robust, easy-to-use, single pivot moving-coil very sensitive instrument. About this time the business moved to a new factory at Newton Avenue Works, New Southgate, London N10 [This is now part of Muswell Hill.] The 'Unipivot' galvanometer proved to be very successful and within a few years a large number of variants and accessories for a wide range of electrical measurement were being made. They were assembled from the large-scale manufacture of identical and interchangeable parts that could be assembled economically.
In December 1919 his firm was taken over by the Cambridge Scientific Instrument Co and the name changed to The Cambridge and Paul Instrument Company Ltd. The name was shortened to the Cambridge Instrument Co Ltd in 1924 when it was converted to a public company.The latter was sold to George Kent in 1968 and after the merger it was organised into four divisions, each being eventually separately sold off. The premises in Newton Avenue and the adjacent Sydney Street in Muswell Hill remained in use until 1975 when the company moved to St Neots in Cambridgeshire. In 1974 the Kent Group combined with BBC [Brown Boveri & Co. Ltd], the latter merged with the Swedish firm ASEA in 1988 to form ABB Group. Robert Paul continued to work within the enlarged organisation for the rest of his life. He also became a cinematic pioneer in the 1890s. For some interesting history on this aspect see here:www.screenonline.org.uk and here://www.thecinetourist.net/my-local-filmmaker.html