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Widget Of The Week
Evershed & Vignoles Series 3 ‘Wee’ Megger, 1960?
 Testing the insulation of wires and cables isn’t as common as it
used to be in days gone by, for the simple reason that it isn’t as much of a
problem as it used to be. That’s mainly due to vast improvements in insulating
materials, stricter standards and more consistent manufacturing techniques, but
when electrical insulation fails -- as it does -- sparks fly and very bad
things can happen…
 Testing electrical insulation is not as straightforward as you
might think. The low voltages that most common or garden multimeters use to
measure electrical resistance simply don’t cut the mustard when checking the
integrity of cables that may be called upon to carry several hundred volts. You
need something with a bit of muscle, and that’s where test instruments,
generically known as ‘Meggers’ come in. They’ve been around for almost as long
as cables have carried electricity, since the late nineteenth century, and
Evershed & Vignoles Ltd, of Acton Lane in Chiswick, London, who made this
Series 3 Megger, have been in the insulation testing business since the very
beginning. We’ll come back to their story a little later on.
 One of the two key components in any Megger is a high voltage
generator that can expose otherwise invisible weaknesses and flaws in a cable’s
insulation layer, that a low voltage test would miss. In the Evershed and
Vignoles Series 3 it comes from a hand-cranked dynamo that puts out a steady
500 volts. Legend has it they’ve been used for less benign applications, such
as instruments of torture, but we needn’t dwell on that… The other important
element in a Megger is the meter, which gives an indication about the
effectiveness of a cable’s insulation layer, measured in thousands and millions
of ohms. As a matter of interest this Series 3 Megger is a high-resistance
model, with the meter scaled from zero to infinity ohms; other Series 3 models
were also produced, configured for lower or narrow resistance ranges.
 There are no controls as such and to use it for simple insulation
testing a pair of probes is connected to spring loaded terminals on the sides
of the case. The positive (red) probe goes to the cable’s inner conductor and
the black probe connects to Earth, or the metal chassis of the equipment being
tested. The idea is to crank the handle at around 160 turns a minute, for one
minute, whilst noting down the reading on the meter. In general very high to
infinity is often a good result; anything less might suggest a problem, though
the engineer carrying out the test should know beforehand what sort of reading
to expect. Cranking speed and hence the output voltage is kept reasonably steady by a
centrifugal clutch on the dynamo, that helps the user to maintain a steady
rhythm.
The Series 3 model is a real old timer and affectionately known by
generations electrical engineers as the ‘Wee’ Megger. Production began in 1932
and lasted until the mid 1960s. Over the years the only significant changes to
the design, in the late 1940s, was to switch to an AC generator. Along with a rectifier circuit and
capacitors it provided a more stable test voltage. This
one I found at a car boot sale around 10 years ago; I am fairly sure it only
cost £2 or £3, thanks to the stallholder having little or no knowledge of what
it was. The generator handle turned easily and the general condition was very
good, which suggests it had seen little use and was one of the later models, probably dating from the
early to mid 1960s. It
is housed in a crack-free Bakelite case. This is a tough thermo-setting plastic
that can survive all but heaviest drops and blows. It only required some light
cleaning to get it into near showroom condition. Fortunately there’s very
little to go wrong and after a little light lubrication it proved convincingly
that it was still working by giving me a lively jolt, whilst poking around
inside the case and unwisely giving the handle a few experimental turns…
What Happened To It?
The story begins in 1888 when Sidney Evershed and Ernest
Vignoles were both working for Goolden and Trotter, a UK company making test
equipment and dynamos for the telegraph and mining industries. Evershed was a
talented engineer and responsible for many technical refinements. In 1895
Evershed teamed up with Ernest Vignoles, a director of Goolden and Trotter.
Together they purchased the company’s instrument division to set up their own
business, using their expertise and marketing savvy to make test instruments.
Production of insulation testers started in 1903 and in the same year they
registered the Megger name and trademark. Evershed and Vignoles made Meggers
with hand generators until well into the 1980s. However, they kept up with
developments in technology and in the early 1960s were one of the first to
market a battery powered Megger, using a transistorised high voltage generator.
Over the years ownership of the company changed several times,
most recently in 1986 when it was acquired by AVO. They were then, and still
are, a leading maker of test instruments and shortly after the buy-out they re-branded
themselves as Megger Instruments Ltd. High performance insulation test instruments
still feature prominently in the company’s extensive product range.
Aside from a small number of specialist collectors, workaday test
instruments – in their original state – tend to attract little interest, but
there are exceptions. Vintage test instruments, especially if they are
in wooden cases and have lots of shiny brass fittings and trim, are popular. Sadly a lot of them are ‘up-cycled’ into daft looking
table lamps, or worse still, stripped of parts for Steam Punk contraptions.
Series 3 Meggers are somewhat dull looking and until now have largely escaped
this terrible fate. They were made in quite large numbers and at the time of
writing (mid 2021) there is usually half a dozen or more of them on ebay at any
one time, typically selling for between £10 and £30, depending on the
condition. I have, however seen several half hearted table lamp conversions using a series 3,
but it’s not a pretty sight. Nevertheless makers and sellers seem pleased with their efforts and clearly feel they
are worthy works of art. One example I spotted at a large Midlands
antique fair recently, had a £120 price tag…
DUSTY DATA
First Seen: 1932
Original Price: £?
Value Today: £20 (0621)
Features: Hand-cranked DC generator with
centrifugal clutch speed governor, 500 volts test supply, twin coil, moving
coil meter, zero to infinity logarithmic scale, spring terminals for test leads
Power req.
Integral DC generator
Dimensions: 137 x 100 x 64mm
Weight: 1.6kg
Made (assembled) in: England
Hen's Teeth (10 rarest) 6
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