Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Fine beam tube


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.

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.



a Use a single Magnadur magnet to deflect the beam. A pair of magnets will then give a bigger and more symmetrical deflection, as shown. 

b 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. 
Adjust the supply voltage so that the beam becomes circular. 
c Increase the current, pointing out that this decreases the radius of the electron beam path. 
d 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. 

Apparatus 1

Apparatus 2


Teaching notes

1 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. 

2 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. 


Related guidance

Types of electron tube


Related experiments

CO2 puck on a glass plate

The motion of the Moon around the Earth

Whirling a rubber bung on a string

Whirling a rubber bung and letting go


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