English: Wireless Set No. 10 was a portable multi-channel radio telephone system used by the British Army. It was the first system in the world to use pulse-code modulation to encode the audio signal into a radio signal, and the first to multiplex multiple such signals into a single signal in order to support up to eight telephone connections at a time.

The system was made possible by the invention of the cavity magnetron in 1940, which could produce very short pulses of microwave frequency radio signals. Using the high-frequency signals allowed the resulting radio signal to be tightly focused using parabolic reflectors, making it almost impossible for the signal to be intercepted as the beams were only about 3 degrees wide. Post-war analysis showed the Germans never even knew the system existed, let alone intercepted its signals.

Links could be made between two such stations at distances on the order of 50 miles, although due to line-of-sight these were generally shorter on the order of 30 miles. Longer links were produced by placing two No 10's back-to-back and connecting them together with normal telephone wires. Stations could also be connected through the existing telephone networks if they were available. Such a system relayed information from Germany all the way to London, making the hop across the English Channel using No. 10's in Cherbourg and Beachy Head.

The image appears to show a Mark I model, as the open hatch on the left shows two identical generators, whereas the later Mark II model used a single larger generator. In practice, the sets were normally powered my the mains supply, with the generators used only as backup. During the war, Montgomery's HQ lost communications with London for less than one hour in total.