Mercury Delay-Line Memory

The flip-flop blueprint ended on an economic problem: two active devices per bit means a kilobyte of valve memory is sixteen thousand valves, and nobody could build or power that. The delay line is one of the strangest answers anyone has produced. Instead of storing bits in devices, it stores them in FLIGHT — a train of acoustic pulses is launched into a tube of mercury, takes about a millisecond to travel the length, is picked up at the far end, amplified, reshaped and launched back in at the start. The data is never at rest anywhere; it is a signal endlessly circulating, and the tube is not really memory but a delay that has been closed into a loop. Storage becomes almost free — one tube holds hundreds of bits — and the price is that a bit is only readable at the instant it happens to arrive.