1. First Vacuum Tubes – Cathode Ray
Towards the end of the 19th century Joseph J.Thomson ( ) was studying electric discharges at the well-known Cavendish laboratory in Cambridge, England. Several people had been studying the intriguing effects in electric discharge tubes before him. Spectacular glows could be observed when a high voltage was applied in a gas volume at low pressure. It was known that the discharge and the glow in the gas were due to something coming from the cathode, the negative pole of the applied high voltage. Thomson made a series of experiments to study the properties of the rays coming from the cathode. He observed that the cathode rays were deflected by both electric and magnetic fields - they were obviously electrically charged. By carefully measuring how the cathode rays were deflected by electric and magnetic fields, Thomson was able to determine the ratio between the electric charge (e) and the mass (m) of the rays. Thomson's result was
e/m = coulombs/kg.
The particle that J.J.Thomson discovered in 1897, the electron, is a constituent of all the matter we are surrounded by. All atoms are made of a nucleus and electrons. He received the Nobel Prize in 1906 for the discovery of the electron, the first elementary particle.
William Crookes ( ). This cartoon by Spy, which appeared in the magazine Vanity Fair in 1903, shows Crookes holding one of the glass tubes named after him.
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2. Magnetic Effect
Designed to demonstrate the deflection of cathode rays in a magnetic field. The electrons pass through a horizontal slit at the right end and can be seen on the fluorescent screen
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3. Magnetic Effect
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4. Maltese Cross Tube The tube's most distinctive feature is the
Maltese Cross that could be laid flat or stood
up so that the cathode ray beam impinged
upon it.
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5. Maltese Cross Tube
When a high voltage was applied, the flat end of the tube fluoresced (and although it wasn’t yet recognized, emitted x-rays).
If the cross was in the cathode ray beam, its shadow appeared on the end of the tube.
This indicated that cathode rays traveled in straight lines and that they could be stopped by a thin piece of metal.
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6. Maltese Cross Tube
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7. Crookes Railway Tubes
When the high voltage is applied, the paddle wheel travels down the glass "railway tracks" towards the anode.
This demonstrates the fact that the cathode rays possess momentum.
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8. Crookes Railway Tubes
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9. Crookes Radiometers
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10. How Radiometers work
The radiometer consists of a partially evacuated glass bulb that contains four diamond-shaped vanes. One side of each vane is painted black and the other side is white.
When exposed to light, the vanes of the radiometer rotate so that the black sides move away from the light source.
Since the black sides of the vanes are slightly hotter than the white sides, the gas molecules close to the black sides have more kinetic energy than the gas molecules next to the white sides.
The collisions of these gas molecules against the black sides of the vanes is responsible for the movement.
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