greater than the number supplied by the gun. In this case the localized image rapidly fades as energy leaving the display through secondary electrons is greater than the rate it is being supplied by the gun. In any CRT, images are displayed by striking the screen with electron energies between these two values, and . Below no image is formed, and above any image rapidly fades. Another side effect, initially a curiosity, is that electrons will stick to the phosphor in lit up areas. As the light emission fades, these electrons are likewise released back into the tube. The charge is generally far too small to have a visual effect, and was generally ignored in the case of displays. Storage These two effects were both utilized in the construction of a storage tube. Storage was accomplished by striking any suitably long-lived phosphor with electrons with energies just above , and erased by striking them with electrons above . There were any number of varieties of mechanical layouts used to improve focus or cause the image to be refreshed either internally to the tube or through off board storage. The easiest example to understand are the early computer memory systems as typified by the Williams tube. These consisted of World War II surplus radar display CRTs connected to a computer. The X and Y deflection plates were connected to amplifiers that converted memory locations into X and Y positions on the screen. To write a value to memory, the address was amplified and sent to the Y deflection plates, such that the beam would be fixed to a horizontal line on the screen. A time base generator then set the X deflection plate to increasing voltages, causing the beam to be scanned across the selected line. In this respect, it is similar to a conventional television scanning a single line. The gun was set to a default energy close to , and the bits from the computer fed to the gun to modulate the voltage up and down such that 0's would be below and 1's above it. By the time the beam reached the other side of the line, a pattern of short dashes was drawn for each 1, while 0's were empty locations. To read the values back out, the deflections plates were set to the same values, but the gun energy set to a value above . As