Working Principle
The Williams tube depends on an effect called secondary emission. When a dot is drawn on a cathode ray tube, the area of the dot becomes slightly positively charged and the area immediately around it becomes slightly negatively charged, creating a charge well. The charge well remains on the surface of the tube for a fraction of a second, allowing the device to act as a computer memory. The lifetime of the charge well depends on the electrical resistance of the inside of the tube.
The dot can be erased by drawing a second dot immediately next to the first one, thus filling the charge well. Most systems did this by drawing a short dash starting at the dot position, so that the extension of the dash erased the charge initially stored at the starting point.
The computer reads information from the tube by means of a metal pickup plate that covers the front of the tube. Each time a dot is created or erased, the change in electrical charge induces a voltage pulse in the pickup plate. Since the computer knows which location on the screen is being targeted at that instant, it can use the voltage pulse from the plate to read the data stored on the screen.
Reading a memory location creates a new charge well, destroying the original contents of that location, and so any read has to be followed by a write to reinstate the original data. Since the charge gradually leaked away, it was necessary to scan the tube periodically and rewrite every dot (similar to the memory refresh cycles of DRAM in modern systems).
Some Williams tubes were made from radar-type cathode ray tubes with a phosphor coating that made the data visible, while other tubes were purpose-built without such a coating. The presence or absence of this coating had no effect on the operation of the tube, and was of no importance to the operators since the face of the tube was covered by the pickup plate. If a visible output was needed, a second tube with a phosphor coating was used as a display device.
Each Williams tube could store about 512–1024 bits of data.
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