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Q & A: Cathode ray oscilloscope vacuum

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Most recent answer: 10/22/2007
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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