Circular Motion
Forces and Motion

Fine beam tube

Practical Activity for 14-16 PRACTICAL PHYISCS


This shows an electron beam being deflected by a magnetic field into a circular path.

Apparatus and Materials

  • Fine beam tube and stand
  • HT Power supply, 0-350 V and 6.3 V for heater
  • Rheostat (10-15 ohms)
  • Power supply, low voltage, variable for field coils
  • Magnadur magnets, 2
  • Helmholtz coils, 2
  • Leads, 4mm shrouded, 6

Health & Safety and Technical Notes

Take care when using the HT supply. Make all connections, using shrouded connectors, with the HT turned off. Once switched on, do not make changes to the connections. An electric shock from a HT supply can be severe, possibly fatal.

Take care when handling the fine beam tube. Use the purpose-made holder and stand to avoid damage.

Read our standard health & safety guidance

Set up the fine beam tube as in the Leybold or Teltron manufacturer’s instructions. No voltage should be connected to the deflecting plates. These should both be connected to the anode.


  1. Use a single Magnadur magnet to deflect the beam. A pair of magnets will then give a bigger and more symmetrical deflection, as shown.
  2. Connect the low voltage power supply to the Helmholtz coils (combined resistance 4 ohms) and set to about 8 V, giving currents of 0.5 - 2.0 amps.
  3. Adjust the supply voltage so that the beam becomes circular.
  4. Increase the current, pointing out that this decreases the radius of the electron beam path.
  5. Turn the tube slightly so that the beam moves in a spiral. This shows that the circular motion produced by this field does not stop after one revolution.

Teaching Notes

  • These demonstrations show that the beam is bent where the magnetic field is strongest - and that the force always acts at right angles to the motion of the beam. For steady bending a large uniform field is needed and the Helmholtz coils are used for this.
  • The track left by the electrons shows that they move in a straight line until the magnetic field is applied in a plane at right angles. The beam then moves in a circular path, with a radius dependent on the strength of the magnetic field.
Circular Motion
can be analysed using the quantity Centripetal Acceleration
can be described by the relation F=m(v^2)/R
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