Mercury delay-line store, c 1950.

This is a memory device for storing digital information in computers. Its operation is based on the fact that sound waves travel more slowly than electrical impulses. At one end electrical signals are converted to sound waves and sent down the tube containing mercury. They are reconverted at the far end to electrical signals. By recirculating the signals round the tube continuously the device stores information. Acoustic delay lines were succesfully used until the late 1950s.