charged accelerator, in the Selectron the "gun" is a plate and the accelerator is a grid of wires (thus borrowing some design notes from the barrier-grid tube). Switching circuits allow voltages to be applied to the wires to turn them on or off. When the gun fires through the eyelets, it is slightly defocussed. Some of the electrons strike the eyelet and deposit a charge on it. The original 4096-bit Selectron was a by vacuum tube configured as 1024 by 4 bits. It had an indirectly heated cathode running up the middle, surrounded by two separate sets of wires — one radial, one axial — forming a cylindrical grid array, and finally a dielectric storage material coating on the inside of four segments of an enclosing metal cylinder, called the signal plates. The bits were stored as discrete regions of charge on the smooth surfaces of the signal plates. The two sets of orthogonal grid wires were normally "biased" slightly positive, so that the electrons from the cathode were accelerated through the grid to reach the dielectric. The continuous flow of electrons allowed the stored charge to be continuously regenerated by the secondary emission of electrons. To select a bit to be read from or written to, all but two adjacent wires on each of the two grids were biased negative, allowing current to flow to the dielectric at one location only. In this respect, the Selectron works in the opposite sense of the Williams tube. In the Williams tube, the beam is continually scanning in a read/write cycle which is also used to regenerate data. In contrast, the Selectron is almost always regenerating the entire tube, only breaking this periodically to do actual reads and writes. This not only made operation faster due to the lack of required pauses but also meant the data was much more reliable as it was constantly refreshed. Writing was accomplished by selecting a bit, as above, and then sending a pulse of potential, either positive or negative, to the signal plate. With a bit selected, electrons would be pulled onto (with a positive potential) or pushed from (negative potential) the dielectric. When the bias on the grid was dropped, the electrons were trapped on the dielectric as a spot of static electricity. To read from the device, a bit location was selected and a pulse sent from the cathode. If the dielectric for