Accelerating Field

Once the electrons are out of the filament we use an electric field to accelerate them. This is done by placing a metal cap with a hole in the end over the end of the filament as shown in Figure (1). The filament and cap are attached to a battery as shown in Figure (2) so that the cap is positively charged relative to the filament. Intuitively the gun works as follows. The electrons are repelled by the negatively charged filament and are attracted to the positively charged cap. Most of the electrons rush over, strike, and are absorbed by the cap as shown in Figure (3). But an electron headed for the hole in the cap discovers too late that it has missed the cap and goes on out to form the electron beam.
A picture of the resulting electron beam is seen in Figure (4). The beam is visible because some air remains inside the tube, and the air molecules glow when they are struck by an electron.

Figure 1: To create a beam of electrons, we start by placing a metal cap with a hole in it, over the filament.

Figure 2: We then attach a battery to the metal cap so that the cap has a positive voltage relative to the filament.

Figure 3: Electrons flow from the negative filament to the positive cap. The beam of electrons is formed by the electrons that miss the cap and go out through the hole.

Figure 4: Resulting electron beam.