PhysicsLAB Resource Lesson
Famous Experiments: Cathode Rays

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In 1897, J. J. Thomson was refining experiments that dealt  with the glowing paths revealed when currents of electricity provided by high voltage sources passed through evacuated glass tubes. He was to eventually declare that these mysterious "cathode rays" were actually beams of electrons, small building blocks of matter.
 
 
 
In the picture above, the cathode is on the left. It is where the electrons originate. The anode, or positive terminal, is on the right and is the electrode towards which the electrons are being accelerated by the electric potential placed across the tube. A metal plate coated by phosphors is positioned inside the tube to detect the path of the electrons. It emits a green glow when struck by electrons.
 
Scientists had discovered as early as the 1850's that when a magnet was held up next to the cathode ray tube, the beam would be deflected. This result can be seen in the following video.
 
Although the properties that J. J. Thomson defined for the electron are not quite what we know to be true today, his work with the discharge of electricity in gases earned him the 1906 Nobel Prize in Physics.
 
 
Cathode Rays
AVI film clip (1.2 Meg)
 
Refer to the following information for the next two questions.

After you have watched the video, determine whether the side of the magnet closest to the camera for each captured frame would be the North pole or the South pole. Remember that electrons obey a left-hand rule since they are negatively charged!
 

 



Further Developments
 
 
An interesting "fashion" development for home decorations based on this theory of accelerating electron beams were the intricate Crookes' phosphorescent flower tubes.
 
These tubes, produced in the early 1900's, had a rotating wane and were 30-cm high. The colors were based on using different phosphorescent minerals that re-emit the KE of the colliding electrons at different wavelengths. The tube shown was probably made by Gundelach, a family of master German glass blowers, or by the Pressler Companys. They were available for purchase at the 1904 World's Fair.
 
Notice that at the top of the tube turning vanes deflect the electron beam so that only the flowers coming into contact with the beam "light up." This shows that the electrons travel in straight lines unless their direction is changed.
 
In the 1970's-1990's, computer monitors were large and extremely heavy CRTs, or cathode ray tubes. They were also based on the previous technology of television picture tubes. LCD and plasma television screens emerged in the mass market around 2009. In recent years, these monitors and screens have become lighter, larger, and less expensive.
 
pictures courtesy of The Cathode Ray Tube Site  
 
Later Gessler, during the early 20th century, introduced intricate gas discharge tubes which are the precursor of today's neon tubes. Geisseler tubes require, on average, 10 kV for proper operation. Different colors are achieved by interlinking tubes containing different gases under different pressures.
 

 
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