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

Electron deflection tube: straight line streams


This demonstration shows that electron streams travel in straight lines. It is useful preparation for measuring the deflection in an electric field. These two experiments are similar to Deflecting an electron beam. However, the advantage of the deflection tube over the fine beam tube is that you can take measurements from its scale if you wish.

Apparatus and materials

Power supply, 6.3 V AC, for the heater filament (often included on the EHT supply)

Deflection tube

Stand for tube

Power supply, EHT

Health & Safety and Technical notes

A school EHT supply is limited to a maximum current of 5 mA, which is regarded as safe. 
Although the school EHT supply is safe, shocks can make the demonstrator jump. It is therefore wise to see that there are no bare high voltage conductors; use female 4 mm connectors where required. 

1 Set the tube up according to the manufacturer’s instructions. 
Ensure that you can identify the following: 

  • The 6.3 V supply to the cathode heater. (If you connect the wrong voltage to the heater you can easily damage the tube beyond repair.) 
  • The EHT supply for the anode. Set this to zero. The cathode is often one of the heater terminals. 
  • The terminals for the deflecting plates. 

2 The experiment works well without any point connected to earth. In this case it is likely that leakage between different points of the circuit and earth will cause the negative terminal to be at some negative potential (e.g. - 1 kV) and the positive terminal to be at some positive potential (e.g. + 2 kV). 
However, it is better to earth some point of the circuit, so that all potentials are fixed with respect to earth. With tubes such as this one where the electron beam is used after it has passed through the anode, it is best to earth the anode. 
With the tube anode at earth potential, the heater circuit will be 5 kV below earth potential, and therefore the heater circuit connectors should be made so that accidental contact with the circuit is highly unlikely. The connectors and cables should be rated at better than 5 kV. Use a 6.3 V AC supply designed for valve heater circuits. Ensure the transformer isolation is rated to withstand 6 kV across the secondary and primary winding, and secondary winding to earth. 
Avoid the use of batteries or general power supplies for the heater circuit. 
Some power supplies have moving coil voltmeters incorporated in them. This type is helpful in this experiment. 
Make sure you connect the 6.3 V heater supply to the heating filament. Too big a voltage can cause irreparable damage. 
3 The beam from the deflection tube is produced by a horizontal slit in the anode. The beam fans out to produce a ‘V’ of electrons in the horizontal plane. This is aimed at a vertical fluorescent screen inside the tube. The vertical screen is at an angle to the beam direction, so the fan of electrons cuts across the screen, producing a straight line along it. 



a Set up the deflection tube in its special stand. 

Deflection tube set-up

b Connect the 6.3 V supply to the filament. Make sure you connect a 6.3 V supply to the filament. (See technical note 2 above.) 

Deflecting plates
c You won’t use the deflecting plates in this experiment. Connect them together and then to the anode on the tube. 
d Connect the negative terminal of the EHT supply to the filament and the positive terminal to the anode. 
e Set the EHT to zero volts and switch on the 6.3 V supply to the heater filament. 
f With no output from the EHT supply, the light from the filament produces a line on the inclined fluorescent screen. 
g Increase the potential difference (p.d.) to about 3 kV: a fluorescent line appears. This is the path of the electron beam. 
h Show that the electron beam travels in a straight horizontal line.

Teaching notes

1 This experiment is best demonstrated to the students in groups of four to five in a darkened room if full value is to be obtained. 

2 Always reduce the anode to zero volts when not actually observing the beam, because the tube has a finite life time. 
3 The line on the fluorescent screen is formed when the electrons strike the vertical fluorescent screen. 
4 The fact that the electrons seem to travel in a straight line suggests that they are moving extremely fast. 
This experiment was safety-tested in August 2007


Related guidance

Classroom management in semi-darkness

Electron guns

The speed of electrons

Types of electron tube


Related experiments

Fine Beam Tube: a naked oscilloscope

Electron deflection tube: using an electric field

Deflecting an electron beam