charges with light, which was much more resistant to external influence. Writing Writing to the cell was accomplished by an external cathode ray tube (CRT) arranged in front of the photoemissive side of the grid. Cells were activated by using the deflection coils in the CRT to pull the beam into position in front of the cell, lighting up the front of the tube in that location. This initial pulse of light, focussed through a lens, would set the cell to the "on" state. Due to the way the photoemissive layer worked, focusing light on it again when it was already "lit up" would overload the material, stopping electrons from flowing out the other side into the interior of the cell. When the external light was then removed, the cell was dark, turning it off. Reading Reading the cells was accomplished by a grid of photocells arranged behind the phosphorescent layer, which emitted photons omnidirectionally. This allowed the cells to be read from the back of the device, as long as the phosphorescent layer was thin enough. To form a complete memory the system was arranged to be regenerative, with the output of the photocells being amplified and sent back into the CRT to refresh the cells periodically. References Echert Jr., J.P., "A Survey of Digital Computer Memory Systems", Proceedings of the IRE, October 1953. Reprinted in IEEE Annals of the History of Computing, Vol.20, No.4, 1998 Computer memory