Uniselector
The telephone exchange's rotary selector, stepping one notch per pulse — and the cheapest read-only memory the early computers could get hold of.
An electromagnet with a pawl and ratchet drags a wiper arm one step across a bank of fixed contacts for every pulse it receives, and the arm stays wherever the current stopped. It was developed for automatic telephone exchanges from the 1890s, where pulses from the dial stepped the selector to the right subscriber, and it was mature, cheap and manufactured by the million long before anyone needed one for a computer.
So the computer builders simply took it. EDSAC's initial orders — the thirty-one instructions that taught the machine to read in its own programs — sat on uniselectors, which meant changing the boot code was a job for a soldering iron rather than a programmer. Zuse and Bell Labs built entire machines out of telephone relays for the same reason: the parts existed, worked, and cost almost nothing. Slow and deafening, but it held, and the uniselector went on living in pinball machines and vending machines for decades after computers moved on to valves.