Answer: Williams-Kilburn Tubes
Williams-Kilburn Tubes, a distant cousin of our modern DDR RAM modules, 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 time. They were bulky, terribly sensitive to environmental conditions, had to be manually adjusted and housed in a vacuum-sealed cathode ray tube.
The tubes wrote binary data by projecting it on the front of the tube, just like a television CRT tube projects an image, only in the case of the Williams-Kilburn tubes they created negative and positive charges that could be read by a plate placed over the end of the tube, not an image that would be easily recognizable to an observer ̵
The Williams-Kilburn was used in many early computers, most notably the Manchester Small-Scale Experimental Machine – the first computer with electronically stored programs.