Editing Delay-line memory (section)

===Mercury delay lines===
[[File:Mercury memory.jpg|thumb|Mercury memory of [[UNIVAC I]] (1951)]]
After the war, Eckert turned his attention to computer development, which was a topic of some interest at the time. One problem with practical development was the lack of a suitable memory device, and Eckert's work on the radar delays gave him a major advantage over other researchers in this regard.

For a computer application the timing was still critical, but for a different reason. Conventional computers have a [[clock period]] needed to complete an operation, which typically start and end with reading or writing memory. Thus the delay lines had to be timed such that the pulses would arrive at the receiver just as the computer was ready to read it. Many pulses would be in-flight through the delay, and the computer would count the pulses by comparing to a master [[Clock signal|clock]] to find the particular bit it was looking for.

[[File:SEACComputer 010.png|thumb|Diagram of mercury delay line as used in [[SEAC (computer)|SEAC computer]]]]
[[Mercury (element)|Mercury]] was used because its [[acoustic impedance]] is close to that of the piezoelectric [[quartz crystals]]; this minimized the energy loss and the echoes when the signal was transmitted from crystal to medium and back again. The high [[speed of sound]] in mercury (1450&nbsp;m/s) meant that the time needed to wait for a pulse to arrive at the receiving end was less than it would have been with a slower medium, such as air (343.2&nbsp;m/s), but it also meant that the total number of pulses that could be stored in any reasonably sized column of mercury was limited. Other technical drawbacks of mercury included its weight, its cost, and its toxicity. Moreover, to get the acoustic impedances to match as closely as possible, the mercury had to be kept at a constant temperature. The system heated the mercury to a uniform above-room temperature setting of 40&nbsp;°C (104&nbsp;°F), which made servicing the tubes hot and uncomfortable work. ([[Alan Turing]] proposed the use of [[gin]] as an ultrasonic delay medium, claiming that it had the necessary acoustic properties.<ref>{{cite journal |last1=Wilkes |first1=Maurice V. |title=Computers Then and Now |journal=Journal of the ACM |date=January 1968 |volume=15 |issue=1 |pages=1–7 |ref=wilkes |doi=10.1145/321439.321440 |s2cid=9846847|doi-access=free }}</ref>)

A considerable amount of engineering was needed to maintain a clean signal inside the tube. Large transducers were used to generate a very tight beam of sound that would not touch the walls of the tube, and care had to be taken to eliminate reflections from the far end of the tubes. The tightness of the beam then required considerable tuning to make sure that both transducers were pointed directly at each other. Since the speed of sound changes with temperature, the tubes were heated in large ovens to keep them at a precise temperature. Other systems{{specify|date=September 2019}} instead adjusted the computer clock rate according to the ambient temperature to achieve the same effect.

[[EDSAC]], the second full-scale [[stored-program]] [[digital computer]], began operation with 256 35-[[bit]] [[Word (data type)|words]] of memory, stored in 16 delay lines holding 560&nbsp;bits each (words in the delay line were composed from 36 pulses, one pulse was used as a space between consecutive numbers).<ref>{{cite magazine |last1=Wilkes |first1=M. V. |last2=Renwick |first2=W. |title=An Ultrasonic Memory Unit for the EDSAC |magazine=Electronic Engineering |date=July 1948 |pages=209–210 |url=http://www.cs.man.ac.uk/CCS/Archive/misc/EDSAC/EDSAC%20Memory%20Image.pdf}}</ref> The memory was later expanded to 512&nbsp;words by adding a second set of 16 delay lines. In the [[UNIVAC&nbsp;I]] the capacity of an individual delay line was smaller, each column stored 120&nbsp;bits, requiring seven large memory units with 18 columns each to make up a 1000-word store. Combined with their support circuitry and [[amplifier]]s, the memory subsystem formed its own walk-in [[Room (architecture)|room]]. The average access time was about 222&nbsp;[[microsecond]]s, which was considerably faster than the mechanical systems used on earlier computers.

[[CSIRAC]], completed in November 1949, also used delay-line memory.

Some mercury delay-line memory devices produced audible sounds, which were described as akin to a human voice mumbling. This property gave rise to the slang term [http://www.rfcafe.com/references/popular-electronics/electronic-mind-remembers-popular-electronics-august-1956.htm "mumble-tub"] for these devices.