Answer: Williams-Kilburn Tubes
A distant cousin to our modern DDR RAM modules, Williams-Kilburn Tubes were the first random-access memory device. Invented in 1946 by Freddie Williams and Tom Kilburn, the memory units were a microcosmic example of computers of the age. They were bulky, terribly sensitive to environmental conditions, had to be hand-tuned, and were housed inside vacuum-sealed cathode ray tube.
The tubes wrote binary data by projecting it onto the front of the tube just like a television CRT tube projects an image, only in the case of the Williams-Kilburn tubes they were creating negative and positive charges that could be read by a plate positioned over the end of the tube, not an image that would readily recognizable to an observer–some rare tubes did had a phosphor coating which allowed computer operators to see where the tube was writing for diagnostic purposes. Each tube was capable of storing 512-1024 bits of data.
The Williams-Kilburn was used in many early computers, most notably the Manchester Small-Scale Experimental Machine–the first computer with electronically stored programs.