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.
The Air Ministry publications A.P.1186E vol1 Section 9 March 1946 and A.P. 10955H Vol1 Section2 Nov1943 describe five different test meters: types C,D,E,F and H. (See this page)
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, a previous owner has partially erased the 'AM' before the word TESTMETER Although the instructions refer to stores reference10S/10610 the meter movement is marked 10A/10610
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 mine was made in August 1945 but that belonging to Donald Berry can be dated from the first two digits i.e May 1939
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 �20- �30 which you will find equally as good as the later models though without leads and probes.
The Test meter Type D Ref 10S/10610 was also made with the same specification though different mechanically by other manufacturers. John Kirby who has such an instrument made by SEI Ltd (Salford Electrical Industries) sent me these pictures. You will note that some of the wire wound resistors have been replaced. This and one housed in a Bakelite case also made by SEI are described as Type D2.
I have a similar instrument (serial no.6416) which was in a very poor state which has a working meter movement but very little else!
The basic movement is OK (100 mV 1.44mA 70 ohms as far as I can tell)
The lowest DC voltage range seems to indicate that the movement is shunted to measure voltage at 3 mA. DC current ranges work but read high.
It seems this is a less sensitive model than the AVO model D. I have traced out the heart of the circuit which indicates that that 3mA is required for FSD for K=1 as the current shunt chain is always in circuit.
I felt there must be something adrift in the resistors directly in
series with the meter. Like the AVO model D this meter is protected by
a fuse which I assume must be interchangeable so should be 4 ohms. As
you might expect the fuse had been replaced with a piece of wire but
even with 4 ohm resistor in place the meter still read high. Using a
length of resistance wire I found that a value of 8.1 ohms gave the
correct readings on the 150 mV scale and the k=1 k=2 produced the
expected indications. A small bobbin resistor of this value is now
wired in place of the fuse and I have made up resistors for the 1.5V
and 15V ranges. For the record the terminal resistance on the three
(six) DC ranges which are now working
51.0Ω / 50.5Ω, 508Ω / 508Ω, 5.07kΩ /5.08kΩ
I don't envisage going any further with it so it is now back together having been cleaned and is sleeping on a shelf.
What is it worth? Fairly rare model possibly about £20 perhaps more if case and leads are included. A model 8 would be a better buy.