ser no 21272-845
The Admiralty and the Air Ministry procured many thousands of multimeters during WW2. Those made by AVO were substantially the same as the commercial instruments of the time, but modified to suit their particular requirements. The model D was a variant of 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 and had different voltage and current ranges. The Q knob has been retained but is non functional and the divide by 2 button of the model 40 has been replaced with a switch K=1 K=2. This meter does not have the more usual AM and crown logo at the top of the scale plate.
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 D ser
no 21272-845 in Bakelite case
This belonged to the late uncle of Imogen
Radford. He was a collector and hoarder of all sorts of things,
and while she was helping her aunt with clearing out her house she
thought this might be of interest. It was in excellent condition and
complete with cloth covered leads, prods and clips and a nice leather
case. She thought his national service in
the Royal Air Force, might have prompted him to buy it. I made a
donation to the charity she had been doing a sponsored swim for.
As with all avometers an abbreviated set of instructions can be found on the underside
The leads for early meters were cloth covered like these
Avometer model D ser no 59-8431 1939 in aluminium case
Donald Berry sent me these pictures of his model D
meter which can be seen to have a fuse rather than a cut out and have a
very different internal component layout to my later version. This
early model is housed in an aluminium case with a wooden battery box.
Batteries The model D 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 August 1945.
What is it worth? One in this condition with leads, probes and case is unusual so maybe £20. 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 though without leads and probes.
The Test meter Type D Ref 10/S 10610 was also made with the same specification though different mechanically by other manufacturers. John Kirby who has such an instrument sent me these pictures.
It seems clear that whoever made this were attempting to make a meter were starting from scratch. For AVO this would have been just a modification of a well tested existing design.