I'm trying to understand electricity and there for electrons. And so far i believe i am grasping it. But there are a few parts i have a hard time understanding.

For example in as i understand it, in electricity there must be a closed loop in order for the electrons to move. But when i apply this knowledge to CRT's and Electron guns it kinda gets messy. I get most parts of the electron gun, but not how it actually produces electrons, because it dosnt seem to be a closed circut. I read somewhere that most electrons get back to the anode but if only "most" electrons get back wouldt the crt or electron gun actually stop working after a while, because it would bleed to many electrons in the end ?

In a old fashined crt, with a cathode and anode on each side (the class tube one) i can grasp how it can keep going forever, or at least as long as you provide it with current. But in new type of crt where you have an electric gun, i cant really understand how it can keep going.


1 Answer 1


The electron gun produces electrons by heating the cathode; this shakes out electrons from the metal ("boils them off"), and as soon as they're out, they are repelled and accelerate away to do your bidding. This is called Thermionic Emission. When an electron is emitted, another one comes in from the cable connecting the cathode to the power source.

The positively charged anodes inside the electron gun (there can be several) accelerate the emitted electrons towards the screen, and this can be conceptually annoying as the electrons that make up the beam don't actually end up on the anodes in the electron gun (obviously). The simple cathode<->anode concept breaks down due to kinematics - the electron is relatively heavy and once accelerated towards an anode in the electron gun, if you put a hole in the anode it will just run through it without colliding back into the anode.

At this point you have an electron beam, and it's true that electrons are used from the power source without being sent back to the source. Compared to the other power drains (like driving the cathode heater) the lost electrons constitute a very small part though.

However, in a TV / CRT, there is yet another anode, at the screen, called a collector. I couldn't find a good illustration for this, but it's layered in the screen just next to the phosphorous layer. Just like the other anodes it has a high positive potential, but unlike the other anodes this one does collect most of the electrons that were emitted and return them to the source (after they have lit up the phosphor).

The electrons that are not collected immediately, build up a negative charge where they hit, and eventually are collected as the charge grows or dissipates. So I think the collecting anode is the missing element in your picture of a CRT.

  • $\begingroup$ Thank you for this explanation, been having a hard time understanding exactly how this works. Does this mean that during the Thermionic Emission that the metal actually has a theoretical lifetime ? E.g. it can be emitting electrons for xx time until it gets burned out like a light bulb ? $\endgroup$
    – mutiju
    Feb 17, 2015 at 13:01
  • $\begingroup$ @mutiju Hi! Normally no, the negative feed from the power source will keep feeding electrons into the metal. Maybe if you don't connect it anywhere, but just heat it up, maybe you'll shake off some electrons but I guess they will just fall back into the metal.. it's not really an electron gun then anyway.. $\endgroup$
    – BjornW
    Feb 17, 2015 at 22:52

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