Types of electron tube
Teaching Guidance for 14-16
There are a number of different cathode ray tubes available to schools. They all use similar electron guns but have different arrangements within the tube. Each one can be used to illustrate or measure slightly different behaviours of electrons. Some of them can be used for a number of different demonstrations. Also, some effects can be demonstrated using more than one tube. Often, your choice of tube will be determined by what you already have available in your school or college.
Here follows a quick overview of each type of tube and what it is best used for.
1 Fine beam tubes
There are two main types of fine beam tube.
a Leybold style tube
These were made in Germany and have a single electron beam. The path of the electrons shows up blue because there is a residual amount of hydrogen gas in the tube. The magnetic field coils are larger than the tube and normally fixed to the base board.
This can be used for basic deflection experiments and e/m measurements. However, a Teltron tube is better adapted for making the beam go in a complete orbit.
b Teltron tube
This second type of tube is made in the UK by the scientific products supplier 3B Scientific (previously manufactured by Teltron). It has two electron beams, so that one beam fires out across the tube and the other one, at right angles to the first beam, up to the top of the tube. The beam is selected using a switch close to the cathode. The paths of the electron beams are green, because the electrons are travelling through a residual amount of helium gas.
Just outside each gun muzzle there is a pair of plates for deflecting the beam by an electric field. One plate of each pair is attached directly to the gun muzzle which supports it. The other plate of each pair is connected inside the tube to the second side terminal on the tube.
These tubes are useful for e/m measurements because, using the vertical gun, it is possible to get the electron beam to go in a closed orbit.
If the beam fails to make a clear spot then try a small potential difference to the deflecting plates. Another trick is to clean the accumulated charges off the screen by sweeping the beam up and down it and across it.
2 Maltese cross tube
The Maltese cross tube is used to show the shadow produced by a piece of metal in the path of an electron beam. The electron gun is similar to other tubes except that the beam is allowed to spread. The metal cross inside the tube casts a shadow on the fluorescent screen.
3 Deflection tube
The beam from the deflection tube is produced by a horizontal slit in the anode. So 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.
The deflection plates are positioned above and below the screen, which has its own graduated scale. So the effect of the deflecting voltage can be measured on the scale.
Using the graticule, it is possible to show that the path is parabolic in an electric field and circular in a magnetic field.
The Perrin tube
This tube has a collecting plate and terminal slightly off-axis at the target end of the tube. This is to allow you to deflect the beam and collect electrons. It is possible to show that the collected charge is negative.