This is a very simple simulation of the way the selecting electrodes (selection bars) control electron flow.
(150 kb Java applet)

The previous simulations showed how secondary electron emission can be used to manipulate stored charges. Another key function of the Selectron tube is the way that the electron flow from the cathodes can be admitted or blocked passage through "windows" on their way to the storage electrodes. First, more (overly simplified) background...

Electrons will be attracted to a more positive potential; that is, electrons emitted from a lower voltage electrode will move toward a higher voltage electrode. Electrons will be repelled from electrodes that are more negative. The simulation on the right depicts a source of electrons (cathode) and four (4) selecting electrodes. (Think of the electrodes as slats in a picket fence.) There are three (3) gaps between the electrodes where electrons may pass. The electrons are continuously "boiling off" the cathode and directed toward the selecting electrodes. Clicking your mouse cursor on the electrodes will alternate the polarity. Try it!

"OK, What am I seeing?" Well, the slats are close together, making the opening window small. If both electrodes are negative, electrons can not enter -- they are repelled. If one electrode is positive and one electrode is negative no electrons will pass. The electrons passing near the negative electrode will be repelled and those near the positive electrode will be attracted and collected. None shall pass!

If the electrodes on both sides of the gap are positive the incoming electrons will pass through. "Huh? Why aren't they sucked into the positive electrodes? How can they make it through?" Well, that's easy to explain: I over simplified the description. I'll clarify that now. The electrons are not attracted to electrodes, they are attracted to potential fields. If there is one lone positive electrode it will have a potential field around it of concentric rings, like a bullseye target. The field strength is higher near the electrode. An electron passing by will move toward the higher fields, crossing the rings, and travel toward the electrode...and strike it.

If two electrodes are near each other their fields will interact. If two electrodes with the same voltage are close to each other, the potential in the gap between them will be the same as the electrodes themselves. Imagine a rubber band stretched between the two electrodes. An electron headed toward the gap will not be attracted to the electrodes, but be attracted toward the straight line between the electrodes. They can pass right through.

So, if the selection electrodes are closely spaced it is necessary for both electrodes to be positive. And that is useful: it forms a two-input AND-gate. And when we consider the other (orthogonal) pair of selecting electrodes, admittance of electrons to the storage elements are controlled by a four-input AND-gate. And back in the 1940s a four-input gate would require a half-dozen electron tubes and a fistful of resistors, capacitors, tube sockets, wires, and other components. And the associated power consumption and decreased reliability and lifetime. If we consider the 4,096-bit Selectron tube, well, the savings of external parts is astounding! The internal all-of-four gating eliminates tens of thousands of electron tubes! This was one amazing concept. The Selectron was one amazing tube.