Avometer & magger

The Avometer was a British brand multimeter (USA), still in production today in the form of the Model 8 Mk 7 by Megger. It is often called simply an AVO and derives its name from the first letter of the words amperes, volts, ohms. It was by far the best instrument of its kind in the UK from the 1930s to at least the 1960s. Almost uniquely for a radio repairman's multimeter it measures alternating current up to 10 A as well as the standard AC and DC voltages up to 1000 V; as an ohmeter it measures from 0.1 Ω up to 200 kΩ in three ranges. The instrument had an accuracy of ±1% of FSD on DC ranges and ±2% on AC ranges. Its maximum current consumption was 50 μA (corresponding to 20,000 ohms per volt), which was sufficient (in most circuits) to reduce the voltage measurement error, caused by connection of the meter, to an acceptable level. A pair of rotary switches are used to select the range to be measured, being arranged in such a way as to minimise the risk of damage to the instrument should the wrong range be selected. Further protection is provided by an overload cut-out. It was a superb example of British radio engineering in its heyday and was found in virtually every radio repair workshop throughout the country and can still be found in regular use.A megger (or sometimes meggar) is often used as an alternate term for an insulation tester - a circuit tester which puts a very high voltage at a very low current across two conductors to make sure that they are properly insulated. The word is short for megohm-meter.
It is in fact the registered trade mark of Megger Group Ltd who have manufactured insulation testers since 1889.
Older types of insulation testers have a small built-in generator turned by the handle. Modern types work with batteries and circuitry to generate the voltage required, typically 500V DC.