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 A.D. PATT.47A and 48A (the Admiralty type used in the Royal Navy) were substantially the same as the model 40 which was introduced in 1939/40 except that that it did not contain the two 4.5 volt batteries used for the the highest resistance range.
By 1965, the company had already created over one million AVOmeters. 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. Though known for their Avometer general purpose multimeters, they made a wide range of test gear including valve testers, oscillators and light meters. 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.
Avometer model 47A ser no 8067-342 in bakelite case with model 40 panel moulding
I was given
this meter which formed part
a collection of many instruments collected by Bob Evans by his
daughter Alice Kirby. I also received a model 40 ser no 40185-642
in an aluminium case.
From the serial numbers it would seem that these two were made just a few months apart and inside they appear much the same except that the model 47A shown here has a resistor wound on glass and the other has pink hi stab ones instead.
This 47A has a bakelite casing with a model 47A instruction plate, a battery box for 1.5 and two 4.5 volt batteries. The front panel has two sets of 6BA threaded holes one of which looks as if it would have suited an aluminium case. Perhaps it is a marriage as the following site indicates (if I read it correctly) that model 40s were supplied to the Admiralty but the serial number should have letter S above the number which this one does not.
Ignore the first couple of pages, there are diagrams and nice description here:
Batteries The model 47A 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 March 1942.
What is it worth? This one was a gift, so not much! The model 8 being more sensitive is probably a better investment, you should be able to buy a model 8 MkII for £10- £20 which you will find equally as good as the later models.