Avometer

AVOmeter is a British trademark for a line of multimeters and electrical measuring instruments; the brand is now owned by the Megger Group Limited.

After around three years production, the volume of sales was sufficient to justify a redesign of the instrument, now with a movement whose full-scale current was 6 mA.

After copper oxide instrument rectifiers became available in the late 1920s, a 20-range "Universal" version of the AVOmeter was introduced in 1931 having both direct and alternating voltage current ranges.

From 1933, the number of available voltage and current ranges in Universal AVOmeters was doubled by incorporating a dual sensitivity movement circuit.

[10] The higher sensitivity was selected by a push button switch marked ÷2 (Divide by two) signifying that the pointer indication should be halved.

[11] Its maximum current draw of 50 μA at full-scale deflection (corresponding to 20,000 ohms per volt) is sufficient in most cases to reduce voltage measurement error due to circuit loading by the meter to an acceptable level.

The Model 8 Mark V, 6 & 7 were provided with an "OFF" position on the DC switch which both disconnected the meter's terminals and short-circuited the moving coil.

AVOmeters designed from 1936 onwards were fitted with an overload cut-out operated by the moving coil frame hitting either forward or reverse sprung end stops.

The earlier versions of models 7, 8 and 9 had a design flaw which resulted in many instruments sustaining damage to the movement in transit.

Despite continuing demand from customers, production was stopped in 2008, reportedly due to increasing problems with suppliers of mechanical parts.

[16] The last meter to leave the factory was an AVOmeter Eight Mk 7 (Serial Number 6110-610/081208/5166) which was presented in February 2010 to the winner of a competition run by the Megger company.

A development of the 36-range Universal AVOmeter incorporating automatic cut-out and internal construction similar to the Model 7 (Basic ranges to 12 A and 1200 V, the former extendable with accessory current shunts).

Universal AVOminor (1936 to 1952) - A small instrument with AC & DC ranges selected by plugging leads into required socket.

The Automatic Coil Winder & Electrical Equipment Co. Ltd. made many other types of instruments, including a line of valve (vacuum tube) testers.