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The Double Beam Tube

The double beam tube consists of a set of Helmholtz coils surrounding a bulb of helium gas at low pressure.

Inside the bulb there are two electron guns. One emits electrons straight into the glass bulb (the radial electron gun), and the other shoots electrons tangentially to the coil. Using the radial electron gun, the range of the electrons is show to be proportional to the accelerating voltage and the energy of the electrons. When a current is passed through the Helmholtz coils, the magnetic field generated acts on the electron. The tangential beam can be bent into a circle.

For a field of flux densityfor the electrons to travel in a circle of radiuswe have(1)

The kinetic energy of the electron is equal toand this is equal to the potential energy gained,whereis the voltage difference between anode and cathode, so(2)

From (2)

cancelfrom both sides to givethen

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