at the dot position, the extension of the dash erased the charge initially stored at the starting point). This worked because the negative halo around the second dot would fill in the positive center of the first dot. A collection of dots or spaces, often one horizontal row on the display, represents a computer word. Increasing beam energy made the dots bigger and last longer, but required them to be further apart, since nearby dots would erase each other. The beam energy had to be large enough to produce dots with a usable lifetime. This places an upper limit on the memory density, and each Williams tube could typically store about 256 to 2560 bits of data. Because the electron beam is essentially inertia-free and can be moved anywhere on the display, the computer can access any location, making it a random access memory. Typically, the computer would load the address as an X and Y pair into the driver circuitry and then trigger a time base generator that would sweep the selected locations, reading from or writing to the internal registers, normally implemented as flip-flops. Reading the memory took place via a secondary effect caused by the writing operation. During the short period when the write takes place, the redistribution of charges in the phosphor creates an electrical current that induces voltage in any nearby conductors. This is read by placing a thin metal sheet just in front of the display side of the CRT. During a read operation, the beam writes to the selected bit locations on the display. Those locations that were previously written to are already depleted of electrons, so no current flows, and no voltage appears on the plate. This allows the computer to determine there was a "1" in that location. If the location had not been written to previously, the write process will create a well and a pulse will be read on the plate, indicating a "0". Reading a memory location creates a charge well whether or not one was previously there, destroying the original contents of that location, and so any read has to be followed by a rewrite to reinstate the original data. In some systems this was accomplished using a second electron gun inside the CRT that could write to one location while the other was reading the next. Since the display would fade over time, the entire display