Megger circuit testing ohmmeters by
Evershed & Vignoles Ltd of London, patented in 1933. Thiey
are housed in mottled green coloured, compression moulded, phenol
formaldehyde cases. The outer surfaces have faded to a dull brown
colour. The battery compartment cover is attached to the body by means
of two large metal screws with coin sized slots. Connection to the
terminals on the instruments is via holes in the side and the spring
loaded buttons visible on the front. Evershed
and Vignoles Ltd,Acton
Lane Works, Chiswick, London W4.
These
are three of
many instruments collected by Bob Evans
which I have
been given by his daughter Alice Kirby. They were very grubby when I
received them, two worked and the one
shown opened here had an open circuit coil on the meter movement.
Although these three are similar at first glance, the ranges covered by
one (0-0.3Ω and 0-30Ω) is different to the other two (0-30Ω and
0-300Ω). Also two have three
termimals and the
other has only two. The fourth image is of a third type (not in my
collection) covering 0-100Ω and 0-1000Ω which gives an indication of
the original appearance of the first three. You will note that this on
is marked A.P.5045 which indicates it woul have been made for the Air
Ministry.
These work on the same principle as the Evershed and Vignoles megger insulation testers shown elswhere on this site (see here) but instead of using a hand operated generator a 4.5 Volt battery is used. On the two terminal models (pattern B) a button has to be depressed to bring the battery into circuit. The three terminal model (pattern S) has one special probe which has two connections and houses the push button.
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Internally all three instruments are esssentially the same.
The Detector
Spike was a useful accessory which could be used with either
tester to prevent damage should the conductor be live. It contained a
neon indicator which remained in circuit until the spring loaded spike
was depressed. For use with the 3 terminal Pattern S tester
a switched adapter was available.
