Direct Line Memory was used as memory in computers before RAM was invented.
Gizmodo has a small article about what type of memory was used before the invention of Random Access Memory in the modern computer.
How does Delay Line Memory work?
“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.”