In 1923 a Post Office engineer called Donald Macadie got fed up with having to use different instruments to check voltage, resistance and amperes so designed the first multimeter. This he named the Avometer, combining A for Amps, V for Voltage and O for Ohms, the measure of resistance. The Automatic Coil Winder and Electrical Equipment Company (ACWEECO), founded in 1923, was set up to manufacture the Avometer and by the Second World War was well established and supplying the instrument to British forces. The Admiralty bought the instrument at the start of the war in the form of the Pattern 40 Avometer. This was upgraded during the war and this led to the Pattern 47a:This avometer was originally issued in a wooden box with a selection of accessories:Sadly I only have the avometer itself, but it is a striking and impressive instrument. The front of the avometer has the important dials and gauges:At the top we have a window with the scales for reading off voltage, amperes etc.:The model number is printed at the top of the dial, whilst a serial number is marked at the bottom right of the dial, the last three digits indicate that this avometer was produced in June 1944:Underneath this dial are the controls to set the instrument for different purposes, whilst the bottom two corners have the terminals to allow the instrument to be wired up to a piece of work.On the rear is a printed panel describing how to use the instrument:A leather carrying handle is fitted to the top of the avometer:Underneath this is a removable cover to access the battery compartment:Note also the loop to hold the two separate aluminium probes, sadly missing from this set. The avometer uses a small 1.5V dry cell battery, this looks like a post war example from the packaging, but is similar to the type used during World War Two:The voltage range of these avometers is a little crude and looking online it has been suggested that they were for use with electronics such as engine starters and vehicle electrical systems rather than more delicate electronics such as wireless or radar systems.