Electron beams (cathode rays)
The experiments in this collection demonstrate the existence and some of the properties of electrons and electron beams. They are appropriate for advanced level courses, and sometimes intermediate level courses.
Each of the experiments uses an electron tube (see 'Types of electron tube') with slightly different features from the others. However, all electron tubes have the same basic construction: an evacuated glass bulb with a metal plate that is heated by a small filament. Your choice of which tubes and experiments to use is likely to be determined by what apparatus you have available.
The basic ideas are as follows.
- Hot cathodes (which are negative) produce beams of electrons.
- Beams of electrons can be deflected in an electric field (in parabolas).
- Beams of electrons can be deflected in magnetic fields (in circles).
- Electrons are absorbed by metals.
- Beams of electrons or cathode rays have applications – like televisions and cathode ray oscilloscope (CRO) tubes.
Most of these ideas can be demonstrated by either a fine beam tube or a deflection tube. The deflection tube has the advantage that you can, if you want, analyze the shape of the path.
- The "electron gun" or valve diode
- Deflecting an electron beam
- Fine Beam Tube: a naked oscilloscope
- Maltese cross: casting shadows
- Electron deflection tube: straight line streams
- Electron deflection tube: using an electric field