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 ar

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.