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Q:
In a Cathode ray oscilloscope, why is there a vacuum?
- Sally
N.H.S., Northampton
A:
Hi Sally,
Cathode ray tubes emit electrons from a negatively charged metal
electron emitter (the "cathode"). These are accelerated to high speeds
by a voltage difference between the cathode and the positively charged
anode, shaped around where the electron beam goes. The beam is steered
side-to-side either by charged plates or magnetic fields. In an analog
oscilloscope, these are electrostatic plates because they are faster
than the steering magnet coils which are used in the picture tubes of
television sets. These days, fancy oscilloscopes have digital memories
and can display the image to the screen at a slower speed than the
original signal came in, allowing the use of ordinary computer monitors
instead of specially designed high-speed imaging tubes.
The vacuum in the oscilloscope's picture tube is needed to keep
the electrons in the beam from being scattered by gas molecules which
would blur and dim the image on the screen. Electrons are very light
and have a large charge, and so they feel strong forces when they
travel past any gas molecules because of all of the electrons and
protons in the molecules. Whenever an electron comes close to a gas
molecule, it would be bent off course. All picture tubes have vacuum
inside, and they would perform badly if they ever developed a leak.
Tom
(published on 10/22/2007)
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