sentences:  After the war, the success of Colossus encouraged him to contemplate the possibility of telephone exchanges each using tens of thousands of valves.He was told that this was impossible and he could not say he had already done it with Colossus because he was bound by the Official Secrets Act.However, a fully electronic prototype Time-Division Multiplex Model Exchange was constructed at the Post Office Research Station at Dollis Hill and then an experimental TDM exchange system was built and tested at Highgate Wood in 1962, but it was found to be beyond the technology of the time