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Double beam tube Fine beam tube

The double beam tube (or fine beam tube) is different from the Maltese cross tube and the deflection tube in two important ways
(a) the double beam tube has two electron guns
(b) the tube is not evacuated but is filled with low pressure helium gas

As the electrons pass through the gas they collide with atoms of helium which then give out a greenish light.

Using electron gun 1 you can see that for a bigger anode voltage the electron beam can travel further through the helium gas (See Figure 1(a) and 1(b)).

Using electron gun 2 (see Figure 2) you can fire an electron beam along a tangent to the radius of the tube. A magnetic field applied at right angles to the tube will bend the electron beam into a circle. The amount of bending and so the diameter of circle depends on the speed of the electrons (controlled by the anode voltage) and the strength of the magnetic field.

A low speed (low anode voltage) and/or a strong magnetic field will bend the electron beam into a small circle (see Figure 3(a)) while a high anode voltage and/or a low magnetic field will give a large circle (see Figure 3(b)).

© Keith Gibbs 2011