Cathode ray

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A schematic diagram of a Crookes tube apparatus. A is a low voltage power supply to heat cathode C (a "cold cathode" was used by Crookes). B is a high voltage power supply to energize the phosphor-coated anode P. Shadow mask M is connected to the cathode potential and its image is seen on the phosphor as a non-glowing area.

Cathode rays are streams of electrons observed in vacuum tubes, i.e. evacuated glass tubes that are equipped with at least two electrodes, a cathode (negative electrode) and an anode (positive electrode) in a configuration known as a diode.

When the cathode is heated, it emits radiation, which travels to the anode. If the inner glass walls behind the anode are coated with a phosphorescent material the incident electrons induce a glow. The prescence of cathode rays was first postulated in early studies in vacuum tubes by placing metal shapes between the electrodes, thereby casting a shadow on the phosphorescent coating. This suggested that the cause of the light emission was due to rays emitted by the cathode and hitting the coating. They travel towards the anode in straight lines and continue past it for some distance.

1 History
2 Properties of Cathode Rays
3 See also
4 References
5 External links

[edit] History

After the 1650 invention of the vacuum pump by Otto von Guericke, physicists began to experiment with mixtures of rarefied air and electricity. In 1705, it was noted that electrostatic generator sparks travel a longer distance in rarefied air than in standard air. The scientists of the day did not think this could happen. In 1838, Michael Faraday passed current through a rarefied air filled glass tube and noticed a strange light arc with its beginning at the cathode (negative electrode) and its end almost at the anode (positive electrode). The only place where there was no luminescence was just in front of the cathode, which came to be called the "cathode dark space", "Faraday dark space" or "Crookes dark space". Hence, it became known that whenever a voltage is applied to rarefied air, light is produced.

Scientists began traveling from town-to-town delighting audiences by making light glow in glass tubes. They did this by first taking an air-filled glass tube of which they would pump the air out. Next, wires would be attached at the opposite ends of the tube, and then the voltage would be turned up. This would make the tube glow in lovely patterns. In 1857, German physicist and Glass blower Heinrich Geissler sucked even more air out with an improved pump and noticed a fluorescent glow, thus inventing the Geissler tube. While Geissler tubes are intended to cause an enclosed low pressure gas to glow, observers noticed that certain glasses used in the tube envelope (enclosure) would glow, but only at the end connected to the positive side of the power supply. Special tubes were developed for the study of these rays by William Crookes and are called Crookes tubes.

Toward the end of the 19th century, this phenomenon was studied in great detail by physicists, yielding a Nobel Prize, for example, to Philipp von Lenard. It was soon understood that cathode rays consist of the actual carriers of electricity which are now known as electrons. The fact that the cathode emits the rays showed that electrons have negative charge.

[edit] Properties of Cathode Rays

Like a wave:

they travelled in straight lines
Produced a shadow when obstructed by objects
could pass through thin metal foils without disturbing them (Tested by New Zealander Ernst Rutherford using a gold sheet)

Like a particle:

Left the surface of the cathode at 90 degrees, not radiating like a wave
Deflected by magnetic fields
Could turn a wheel in the path of the ray
Travelled far slower than light

These conflicting properties caused disruptions when trying to classify it as a wave or a particle. Crookes insisted it was a particle, whilst Hertz maintained it was a wave. The debate was resolved when an electric field was used to deflect the rays by J. J. Thomson. This evidence was strong because scientists knew it was impossible to deflect electromagnetic waves with an electric field.