The wirelesses of World War II required constant work from signalmen to keep them functioning correctly. One continual problem was the radio losing its frequency calibration and so being unable to receive and respond to other wirelesses in the net at the correct frequency. This was clearly a serious problem and so recalibration devices were provided to set the wireless to the correct frequency. One of these is the Class D Wavemeter, used with 19 and 22 set radios:

This wavemeter could be used to preset a transmitter of receiver to any frequency between 1.9 M/cs to 8.0 M/cs. The wavemeter is housed in a sturdy metal case to ensure it does not get damaged in use:

A pair of spring clips hold the lid in place, whilst a pair of loops allow a shoulder strap to be attached:

The wavemeter’s designation and stores code is stenciled onto the underside of the outer case:

Opening the lid of the carry box reveals the controls of the wavemeter:

Some wavemeters had internal power supplies, but this example has a cable fitted to the bottom left hand corner. A socket for the earphones is fitted in the opposite corner. The manual shows the controls:

A set of operating instructions are pasted into the lid of the wavemeter:

The internal parts of the wavemeter are held on a metal ‘sled’ that fits into the outer box and is secured with a pair of thumb screws. Undoing these allows the sled with its multitude of wires, valves and capacitors to be pulled out for maintenance and repair:

The use of one of these devices seems quite complicated, but would presumably have been second nature to those using them on a daily basis. The operations manual explains how to maintain a Class D Wavemeter: