Wavemeter, Air Ministry type W1117 (10D/10220), c. 1939. This battery-operated set could detect frequencies between 125kc/s and 20mc/s, or 125kHz-20MHz.

Wavemeters are a device which can be used to measure the frequency of electromagnetic waves and first being to be used for wireless telegraphy (radio using Morse code) in the early twentieth century. They were used to set British miltary wireless transmitters and receivers to the correct frequencies during the First World War and continued to be developed and used in the interwar period, and into the Second World War and the Cold War - the latter for radar.

This particular wavemeter was designed for use by the Royal Air Force (RAF) during the Second World War.