Experiment: Thomson Tube (115 V, 50/60 Hz)

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Thematic focus:_..

•  Investigate the deflection of an electron beam by a magnetic field.
• Investigate the deflection of an electron beam by an electric field.
•  Construct a velocity filter using orthogonal electric and magnetic fields.
•  Estimate the specific charge of an electron.

Experiment description:_..

In a Thomson tube electrons pass horizontally through a slit behind the anode and impinge upon a fluorescent screen placed at an angle to the electron beam on which they can be observed. Beyond the slot there is a plate capacitor. The electric field between its two plates deflects the electron beam in a vertical direction. In addition Helmholtz coils can be used to rate a magnetic field in a horizontal direction perpendicular to the motion of the electrons that also deflects them in a vertical direction.
If the direction of motion is perpendicular to a uniform magnetic field, electrons are deflected in a circular path with a centripetal force resulting from the Lorentz-force.
The velocity of the electrons depends on the anode voltage U_A. This means that measuring the radius of the path allows the specific charge of an electron to be determined as long as the homogenous magnetic field B and the anode voltage U_Aare both known.