The Admiralty and the Air Ministry
procured many thousands of AVOmeters during WW2. These were
substantially the same as the commercial instruments of the time, but
modified to suit their particular requirements. The Admiralty
Pattern model 48A used by the Royal Navy was
a
variant of the AVO Industrial Test Set
which included a model 40 meter and accessories in a wooden box. Two
versions of the Industrial Test set were made:
Model 1.
A test set comprising a Model 40 Avometer with the following
4,800V Multiplier, 120A Shunt, 480A Shunt and 60/240A Transformer.
Model 2.
As an alternative, a test set can be supplied comprising a Model 7
Avometer with the following:
4,000V Multiplier, 100A Shunt, 400A Shunt and 50/200A Transformer.
The Admiralty pattern instrument was very
similar to the Model 1 but has:
3,600V Multiplier, 120A Shunt, 480A Shunt
and 60/240A Transformer.
The serial number on the box indicates
that it was made in July 1944
The meter in this box is a model 40 Mk2
manufactured in June 1970 the other items are original
The multiplier resistors on glass tubes
are housed in a ventilated wooden box.
Shunts and current transformer
This was given to me by Warren
Jackson. It belonged to hisfather,
Fred Jackson who worked as a maintenance electrician at Searles refrigeration near
Gosport. It was in need of a good clean and and some repairs to the
box, the meter worked on all ranges but was dirty and had been modified
to include a fuse and a home made resistor. The front panel was washed
with detergent and a tooth bush, rinsed and washed again and allowed to
dry. The engravings were filled with off white emulsion paint and the
panel then given a thin coat of light machine oil. A fuse holder had
been fitted to the left hand side of the case and this was wired in
series with the home made resistor which measured 17 ohms. Both were
removed, the latter with a selected 5% tolerance 15 ohm half watt
resistor and the hole left by the fuse holder with a two part epoxy
resin mixed with black powder intended for the colouring of cement. The
label giving the serial number was almost unreadable, though I was able
to establish that it was made in June 1970 and the date codes on the
internal blue Welwyn resistors lend support to this.
I imagine that Fred Jackson may have purchased the box (maybe
without the original model 48A meter) at a government surplus sale many
years ago and subsequently obtained the much later AVO 40 Mk2 and
removed the instruction plate which would have been screwed in the lid.
The shunts, current transformer and voltage multiplier were cleaned
but have not been tested, though the resistance of the latter was found
to be 5200 ohms.
The firm also produced a
range of smaller multimeters, the AVO
Minor and later the AVO
Multiminor as well as a number of special instruments such as
the Heavy Duty
model. Production of the iconic model 8 meter ceased in 2008. You will
find more information
here about these instruments and other similar models. AVO also produced a range of
smaller multimeters which are described
here.
Batteries The model 48A used a rectangular 1.5 volt cell with
a brass terminal and a flying lead for the negative connection this can
be replaced with a single cell size D with soldered connections.
How old is it? Many AVO meters can be dated from the last 3
or 4
digits of the serial number under the right hand end of the scale.
These define the month and year of manufacture. For example this one
was made in July 1944.
What is it worth? One in this condition with leads, probes
and case is unusual so maybe �30. The model
8 being more sensitive is probably a better investment, you should be
able to buy a model 8 MkII for �20- �30 which you will find equally
as
good as the later models though without leads and probes.
