[jpk]

Back in the early 90's, my transportation engineering class got to go to the control tower at the airport. They showed us the computers used to track the planes and put the little ID numbers on the radar screens. They were these huge old Burroughs vacuum tube machines. I speak in plural because there were redundant systems with automatic failsafe switchover. They used field core memory - each bit of memory was a ferrite-ceramic ring with a coil of wire around it; depending on which way the current ran, the core would be magnetized with a positive or negative charge. There were racks and racks of panels about the size of a poster board with several hundred of these little rings on them.
I remember the guide telling us that these were originally designed for aircraft carriers and had proven reliability in less that ideal conditions, so the FAA was still using these systems, even though a programmable calculator could run circles around them for performance.