@freemo The Bendix G-15d computer used delay lines on a magnetic drum. The spacing between read and write heads determined the delay.
Those delay lines contained the main memory *and* the registers (such as the arithmetic accumulator).

@hendrikboom3 I suspect you mean magnetostrictive delay lines. They were the next generation right after mercury delay lines. They operated via torsional waves in a wire. very cool stuff.

@freemo No, I did mean a real magnetic drum on this machine, acquired by the university in 1957. I got to use it in1962. I spent a happy summer hanging about the university's air-conditioned computer room reading its circuit diagrams to learn how it operated. It was a truly magnificent kludge.

Main memory consisred of a number of these recirculating tracks, each was 108 words of 29 bits each -- 7 hex digits and a sign bit. The instructions had a source memory line, a destination memory line, and specifications when the instruction was to be executed and when the next one was to be read. These times were essential -- they determined which words were being read and rewritten and thus possibly modified as the drum turned.

@hendrikboom3 A magnetic drum isnt a delay line though, so I'm confused what you mean then.

@freemo I wasn't aruond for the mercury delay lines. They were from the 40's and 50's, right? And you can still get them nowadays? I didn't get into computing until '62. I remember reading about William's tube memories. Why do I get the idea they were grasping at straws back then to get anything, just anything, to work?

@hendrikboom3 mercury delay lines were at the birth of computing. ENIAC had one as its memory. I highly doubt you can get them today. They are insanely expensive, bulky, poisonous, and hold very little memory. I cant imagine any use for them still being made today.