The object shown is a 36-range Universal Avometer made between 1934 and 1939. The month of manufacture can be determined by the serial number which will be written on the scale plate at the extreme lower right-hand edge. It is in the form m(m)y-xxxx, where m is the month and y the year. The advertised price was 12 guineas in 1933 and 13 guineas in 1938.

This was the sixth design variant in a series of instruments which began in 1923 with a multirange test meter designed by Donald MacAdie who was a senior engineer in the Post Office Factories Department. The original instrument was granted a British patent, No. 200977 and several foreign patents. The 1923 version is believed to be the world's first "multimeter". The first Avometers measured direct voltage, direct current and resistance in 13 ranges, but from 1933 a 20-range "Universal" instrument, capable also of measuring alternating current and voltage was offered. The Universal meter was made possible by the recent introduction of a copper oxide instrument rectifier by the Westinghouse Brake and Signal Company.

The instrument shown is fitted with a "divide by two" push-button switch, the subject of British Patent No. 423199, later successfully contested by a rival manufacturer. This switch effectively doubles the number of voltage and current ranges by switching the instrument to double its normal sensitivity and therefore requiring the scale indication to be divided by two to give the correct reading. The 36-range instrument draws 6mA for full-scale deflection when the button is free and 3mA when depressed. This is often described as a "sensitivity" of 166/333 ohms per volt, which today would be considered to characterise the meter as one best suited to measurements on "heavy-current" electrical equipment. It was however designed as a general purpose instrument for all types of electrical work.

The meter movement is protected from overload by a fuse of calibrated resistance which screws into the front panel. 

As well as commercial customers, instruments of this model were supplied to a number of Government bodies including the Post Office. With some variations, they were also supplied to the Admiralty and the Air Ministry who allocated their own designations.

The 36-range meter was replaced by the largely similar "Model 40" from 1939 and this later instrument shared several patented design features and updated internal construction with the "Model 7", a 46-range instrument introduced in 1936. The "Model 7" had a sensitivity of 500/100 ohms per volt, considered then more suitable for radio work.

Both the Model 40 and the Model 7 were continually in production, with progressive detail development, until the late 1970s. Avometers, of various types, were almost universal in all branches of British industry and were widely respected. A considerable number are still in use; production of the "Model 8 Mark 7" finally ended in November 2008.

Many examples of the 36-range Universal Avometer survive, some in fully operational condition.

Further information, including a circuit diagram, is available if required. 
.......... Peter M Munro, Dundee, Scotland, 14th of January 2011