Evershed & Vignoles Hand Driven Generators, and insulation testers

An early Evershed and Vignoles megger of the type first patented in 1906.

Changes over the years

The following picture has been taken from the 'Megger pocket book on insulation and continuity testers' dated 1960. They are described as Series 1, 2 and 3 as indicated on the illustration. The same basic constructions and principles were used for a wide variety of insulation, conductivity and continuity testers. Continuity testers are sometimes called Ducters [a term registered by Evershed in 1908], presumably a shortened version of 'conductor tester.' Earth Testers using the same housing and construction but equipped with four terminals were introduced in 1926. The earliest wooden cased combined meggers had a hinged top cover, on the later ones the top cover was held in place by four machined screwed rods an nuts. The 'Meg Tester' was a metal cased predecessor to the series 2 range of instruments, these date from 1923 onwards and the latter from about 1932. The series 4 range of instruments date from around 1954 onwards.

Originally the generators were all direct current (DC), but by using an alternating current (AC) generator with a rotating magnet it avoided the need for carbon brushes. The output of the generator was rectified with diodes and capacitors and limited by a silicon carbide stabilizer. This also meant that by using AC, voltage- multiplying circuits could be employed to give a choice of DC output voltages usually in the range of 50 to 1000 volts. This change took place as a result of a 1947 patent.

How old is my megger?

People regularly contact me asking how old their newly acquired instrument might be. I have tried to establish whether it might be possible to estimate the date of manufacture from the serial numbers. I have looked at information given to me by contributors and searched the internet and examined the entries on auction sites. From what I have been able to glean it would seem that the serial numers start from a low number and increase over the years. There does not appear to be any kind of date code included in the numbers, though it seems likely that blocks of serial numbers were allocated for instruments of a particular type.

I have so far found 34 instruments and pieces of equipment [not exclusively meggers] where serial numbers have been given or are visible on photos. The earliest, an ammeter dated 1895 has a serial number 1656 and the latest, a megger has a serial number 2606230. By correlating the numbers with the dates for the various patents etc. I have produced this very rough guide to those manufactured at the Acton works between 1903 and 1986 or thereabouts:

9 000 after 1903
48 000 after1910
170 000 after1928
260 000 after1935
800 000 after1950