1950's- Delay Line Memory
Magnetostriction Delay Line
Ferranti Electric In, Plainview NY
Ser No: 10952 Type No: 5801
National Company Inc 31091
Delay line memory was a form of computer memory used on some of the earliest digital computers. Like many modern forms of electronic computer memory, delay line memory was a refreshable memory, but as opposed to modern random-access memory, delay line memory was serial-access. In the earliest forms of delay line memory, information introduced to the memory in the form of electric pulses was transduced into mechanical waves that propagated relatively slowly through a medium, such as a cylinder filled with a liquid like mercury, a magnetostrictive coil, or a piezoelectric crystal. The propagation medium could support the propagation of hundreds or thousands of pulses at any one time. Upon reaching the other end of the propagation medium, the waves were re-transduced into electric pulses, amplified, shaped, and reintroduced to the propagation medium at the beginning, thus refreshing the memory. Accessing a desired part of the propagation medium's memory contents required waiting for the pulses of interest to reach the end of the medium, a wait typically on the order of microseconds. Use of a delay line for a computer memory was invented by J. Presper Eckert in the mid-1940s for use in computers such as the EDVAC and the UNIVAC I.
A later version of the delay line used metal wires as the storage medium. Transducers were built by applying the magnetostrictive effect; small pieces of a magnetostrictive material, typically nickel, were attached to either side of the end of the wire, inside an electromagnet. When bits from the computer entered the magnets the nickel would contract or expand (based on the polarity) and twist the end of the wire. The resulting torsional wave would then move down the wire just as the sound wave did down the mercury column. In most cases the entire wire was made of the same material.
Unlike the compressive wave, however, the torsional waves are considerably more resistant to problems caused by mechanical imperfections, so much so that the wires could be wound into a loose coil and pinned to a board. Due to their ability to be coiled, the wire-based systems could be built as "long" as needed, and tended to hold considerably more data per unit; 1k units were typical on a board only 1 foot square. Of course this also meant that the time needed to find a particular bit was somewhat longer as it traveled through the wire, and access times on the order of 500 microseconds were typical.