Delay Line Memory: How Computers Remembered Before RAM

Before there was random access memory, there was delay line memory. It was random in a different sense; it involved turning electrical pulses into sound waves, sending them through long tubes of mercury, and re-electrifying them at the other end.

The technology had its roots in the work of engineer J. Presper Eckert, who developed line delay systems to improve radar during World War II. Instead of storing data in individual bits, it was compressed down to sound waves and sent through a medium that slowed them down (initially mercury, then other substances, and finally wire). At the other end, they were re-electrified, processed, and then sent back through the tube. Because of slowing that occurred in the tube's substance, hundreds of pulses of data could be sent in a single tube—hence the name "delay line"—bouncing back and forth until they were needed.

This post on Make offers a helpful analogy:

If you had a hard time remembering things for very long, and happened to live in a cave, you could just shout out what you didn't want to forget, and a few seconds later you would hear an echo to remind you. Of course, the problem with this is that an echo doesn't stick around for long, so you would have to shout again every time that you heard the echo, so that you could remember again in a few seconds. Assuming you could keep this up, you would never forget your idea.

Until computers like the 305 RAMAC introduced random access memory, machines like the UNIVAC I, shown above, used serial-access systems like delay line memory. What this meant is that a certain piece of data couldn't be called up at any time; you had to wait until it was ready. In this case, it was ready when it had bounced to the "read" end of the tube. It only took a matter of milliseconds, but it was a fundamentally different system of computing, and it wasn't until the development of RAM some years later that our modern sense of computer memory started to take shape. [Wikipedia, Make]

Memory [Forever] is our week-long consideration of what it really means when our memories, encoded in bits, flow in a million directions, and might truly live forever.