4 ms·

"Mercury was used because the acoustic impedance of mercury is almost exactly the same as that of the piezoelectric quartz crystals; this minimized the energy l

"Mercury was used because the acoustic impedance of mercury is almost exactly the same as 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 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 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 °C (104 °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.)" https://en.m.wikipedia.org/wiki/Delay_line_memory https://en.m.wikipedia.org/wiki/Delay_line_memory

CamTin 8y ago
Also, they're not "loops" in the physical sense, though they do operate logically as loops: pulses go in one end of the tube, travel through the mercury, and are received at the other end, where they are repeated elecrtonically back to the transducer at the starting end. "Reading" a bit means waiting for the moment when it is hitting the pickup end, and pulling it out of the loop at the same time as repeating to the transmitting end. "Writing" means waiting for the same moment in the "loop" and replacing the output of the pickup transducer with a signal that represents the data you wish to write. Thus, there is an inherent tension between making the tubes longer (more storage per tube) versus keeping them short (lower access times). This tension is inherent to all forms of "delay" memory, including the wire torsion memory grandparent mentions.

CamTin 8y ago

Also, they're not "loops" in the physical sense, though they do operate logically as loops: pulses go in one end of the tube, travel through the mercury, and are received at the other end, where they are repeated elecrtonically back to the transducer at the starting end. "Reading" a bit means waiting for the moment when it is hitting the pickup end, and pulling it out of the loop at the same time as repeating to the transmitting end. "Writing" means waiting for the same moment in the "loop" and replacing the output of the pickup transducer with a signal that represents the data you wish to write. Thus, there is an inherent tension between making the tubes longer (more storage per tube) versus keeping them short (lower access times). This tension is inherent to all forms of "delay" memory, including the wire torsion memory grandparent mentions.