While working at EEV Chelmsford Raymond Beurle identified a specific need to express electromagnetic phenomena in the time domain. He had used an early computer to simulate the propagation of activity in neural networks and later had experience of using Southwell’s relaxation technique to solve electrostatic field problems. These two apparently unrelated themes coalesced to suggest that propagation in a matrix of transmission lines might be used to simulate propagation in space, to enable high frequency field distributions to be calculated in arbitrarily shaped cavities.