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The Cathode Ray Tube
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The
diagram below shows the main parts of the type of c.r.t. used in
oscilloscopes. |
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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.
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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.
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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.
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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.
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The advantage of using a c.r.t.
instead of a voltmeter is that it can respond very rapidly to
changes in voltage.
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© David
Hoult 2008 |
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Open Door Web Site