Tuning early wireless sets was not easy and it was not uncommon for radios to drift off station and no longer be able to communicate with other sets on a ‘net’. Amongst the improvements made to the WS38 set when the Mk III was developed, was to allow a crystal calibrator to be attached to the front face of the radio. Here is can be seen attached to the front of the wireless unit: 

The crystal calibrator No9 had six reference frequencies evenly spaced out over the frequency range of the set, marked A through F. These allowed the radio to be set accurately and repeatedly to within 10kHz. The system also provided an audible tone on incoming signals to allow accurate netting. Taking the unit off the main radio we can examine it more closely. It is a metal box that was originally painted green to match the radio, although this example is rather beaten up: 

The unit is attached to the main radio using the screws in each corner. The circular fitting below is a desiccator to allow the internal parts to be dried out without dismantling the whole unit: 

The designation of the unit is moulded into the case, with the individual serial number on a copper plate rivetted to top: 

A cable comes off the unit to allow it to be attached to the main radio unit. The calibrator button can be seen next to this: 

At the opposite end of the calibrator can be seen a socket to allow it to be wired into the battery of the radio set: 

The manual issued to accompany the WS38 Mk III set includes this illustration showing the correct setting up of the crystal calibrator on the wireless set: 

This fantastic accessory for my radio was a very generous gift from an old friend and it is great to be able to add yet another piece to this largely overlooked radio set.