One of the most commonly used forms of memory in early computers like
ENIAC. The unusual thing is that the idea of delay lines has resurfaced in the area of
optical computers.
Anyway, what would happen is that you would have tube of mercury about 5 feet long; with a piezoelectric crystals at each end. You would apply a current to one of the crystals, and it would set up a vibration in the mercury, which would be picked up at the other end by the other crystal about one millisecond later. You would then pick up the electric current from the crystal, and turn it back into a digital value, most frequently feeding it back in again at the other end, but sometimes actually processing it. Depending on the type of pulse you caused, you'd either be encoding a "1" or a "0". You would then set up a whole train of vibrations, in fact you could fit about 1000 bits in a 5-foot delay line.
This functioned as your main memory. Whenever you needed to read, you would just pull the value from the receiving crystal. When you wanted to write, you would change the value going into the transmitting crystal. Of course, you had to somehow work out when the correct pulse would be arriving.