beam concept In the most basic implementation, the holding beam tube uses three electron guns; one for writing, one for reading, and a third "holding gun" that maintains the pattern. The general operation is very similar to the Williams tube in concept. The main difference was the holding gun, which fired continually and unfocussed so it covered the entire storage area on the phosphor. This caused the phosphor to be continually charged to a selected voltage, somewhat below that of the secondary emission threshold. Writing was accomplished by firing the writing gun at low voltage in a fashion similar to the Williams tube, adding a further voltage to the phosphor. Thus the storage pattern was the slight difference between two voltages stored on the tube, typically only a few tens of volts different. In comparison, the Williams tube used much higher voltages, producing a pattern that could only be stored for a short period before it decayed below readability. Reading was accomplished by scanning the reading gun across the storage area. This gun was set to a voltage that would cross the secondary emission threshold for the entire display. If the scanned area held the holding gun potential a certain number of electrons would be released, if it held the writing gun potential the number would be higher. The electrons were read on a grid of fine wires placed behind the display, making the system entirely self-contained. In contrast, the Williams tube's read plate was in front of the tube, and required continual mechanical adjustment to work properly. The grid also had the advantage of breaking the display into individual spots without requiring the tight focus of the Williams system. General operation was the same as the Williams system, but the holding concept had two major advantages. One was that it operated at much lower voltage differences and was thus able to safely store data for a longer period of time. The other was that the same deflection magnet drivers could be sent to several electron guns to produce a single larger device with no increase in complexity of the electronics. Design The Selectron further modified the basic holding gun concept through the use of individual metal eyelets that were used to store additional charge in a more predictable and long-lasting fashion. Unlike a CRT where the electron gun is a single point source consisting of a filament and single