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Electricity and Magnetism
 
The Cathode Ray Tube
 
The diagram below shows the main parts of the type of c.r.t. used in oscilloscopes.
 
The filament is heated by a low voltage supply.
This causes it to emit electrons.
The emission of electrons by heated metals is called thermionic emission.
 
A high voltage supply is connected across the filament and the accelerating anode (negative to filament and positive to anode).
This causes the electrons to be accelerated towards the front of the tube forming an electron beam.
The beam can be concentrated or focused by focusing anodes (not shown in this simplified diagram).
 
The front of the tube consists of a screen coated with a fluorescent material.
This material gives out light when it is hit by fast moving electrons so we see a spot of light at the point where the electrons hit the screen.
 
If a voltage is applied to the Y plates the beam can be deflected vertically.
Similarly, a voltage applied to the X plates can deflect the beam horizontally.
The displacement of the spot of light on the screen is directly proportional to the voltage applied to the plates.
Therefore a c. r. t. can be used as a voltmeter.
The advantage of using a c.r.t. instead of a voltmeter is that it can respond very rapidly to changes in voltage.
This fast response means that the c.r.t. can be used to visualize rapidly changing alternating voltages.
 
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