A second switch brought a rheostat into circuit in series with the instrument and could be used to control the current through a device under test and the meter. All ranges could be selected by a single rotary switch which set both the function and the range value.
The original AVOmeter was designed to measure direct current (3 ranges, 0.12, 1.2 & 12 A), direct voltage (3 ranges, 12, 120 & 600 V) and resistance (single range, 0 - 10,000 ohms, 225 ohms mid-scale). The design concept is due to the Post Office engineer Donald Macadie, who at the time of the introduction of the original AVOmeter in 1923 was a senior officer in the Post Office Factories Department in London. The multimeter is often called simply an AVO, because the company logo carries the first letters of 'amps', 'volts' and 'ohms'. Possibly the best known multimeter of the range was the Model 8, which was produced in various versions from May 1951 until 2008 the last version was the Mark 7. in 1923, and measured direct voltage, direct current and resistance. The first Avometer was made by the Automatic Coil Winder and Electrical Equipment Co. AVOmeter is a British trademark for a line of multimeters and electrical measuring instruments the brand is now owned by the Megger Group Limited.
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