The wavemeter was an indispensable tool for making sure that a Transmitter or a Receiver were TUNED to the correct operating frequency.  There were many of them and a well trained and experienced W/T operator could quickly change frequency guided by the tell-tale indicators of such machines.  Individual transmitter and receivers had free tuning oscillators which did the job, oscillating that is,  albeit without the associated and necessary accuracy demanded.  Eventually, wavemeters used quartz crystals to control the measuring source and afforded the accuracy we all take for granted today.   This ultimately led to each separate piece of equipment [transmitter and receiver] having their own crystal controlled oscillators which many years later were supplanted by frequency standards and frequency synthesisers.

Usually, Wavemeters associated with the accuracy of transmitters were built into the transmitters interface as a permanent instrument.

 Wavemeters for tuning receivers were portable equipments, and any wavemeter could be used to tune a receiver as long as it was used in conjunction with an oscillator, a buzzer, and a heterodyne set. 

The GN was one such Wavemeter Outfit, and every member of the W/T Brach knew of its capabilities and how to operate it.  This file gives one a good and clear understanding of how important this device was especially during in World War Two.  During this time there were long  periods of radio silence, but when lifted, especially to transmit enemy reports which conveyed critically important data for Command and Control, W/T personnel had to be sure that their transmitter were EXACTLY on the correct frequency so that distant stations would not miss these 'win or lose'  messages which were transmitted inside a tiny and unheralded 'window' and desirably, only the once.  Likewise, a ship setting watch on a circuit which might at that time be subjected to radio silence in the hope of hearing any enemy reports,  if transmitted, had to make sure that they were indeed ready in all respects on the correct frequency, the correct emission set and the correct bandwidth set.

Here is the file which contains text, pictures, schematics and line drawings OUTFIT GN.pdf