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Cathode Ray Tube (CRT)

Cathode ray tube (CRT)

In the mid to late 1800s, the world experienced a scientific revolution. Phenomena that had never before been truly understood, such as light, heat, and electricity, were systematically explored. Great scientists like Henri Becquerel (1820-1891; French chemist and an authority on luminescent, or light caused by radiant energy, phenomena), Marie Curie (1867-1934; Polish-born French physicist who worked extensively with radium) and Thomas Young (1773-1829; English physician and physicist) led the way.

Early Experimentation

Early experiments to solve the riddle of electricity often included the use of anode-cathode tubes (glass tubes that contained an anode at one end and a cathode at the other). When most of the air was evacuated from the tube, an electrical charge could be seen jumping across the gap between the two electrodes. It was English physician and chemist Michael Faraday (1791-1867) who noticed that as the amount of air in the tube decreased, a faint glow between the electrodes could be seen. Faraday was not able to explore this effect completely because technology was not advanced enough to produce a high vacuum within the tube.

Vacuum Tubes

The German team of Heinrich Geissler (1815-1879) and Julius Plucker (1801-1868) pioneered the study of cathode-ray tubes. Geissler was a skilled glassworker, employed by the University of Bonn (Germany) as a maker of scientific instruments. While at the university he met Plucker, then a young professor. Sometime around 1855, Plucker convinced Geissler to design an apparatus for evacuating (completely emptying) a glass tube. Geissler did just that. He constructed a hand-crank mercury pump that could remove most of the air from a tube. The new vacuum tubes were very popular, and became known as "Geissler's tubes".

Using the improved vacuum tube, Plucker made some startling discoveries. First, he was able to produce a bright stream-like glow between the electrodes. The glow was much brighter than any achieved in previous experiments. Second, he found that the glow responded to a magnetic field. The glow could be moved by a powerful magnet. The discovery indicated that the stream crossing the vacuum was composed of particles rather than rays.

The next scientist to conduct important research using vacuum tubes was Johann Hittorf ((1824-1914) in 1869. A student of Plucker's, Hittorf further improved the method for creating a vacuum within glass tubes of his own design. He observed that the luminescent glow increased dramatically as the pressure within the tube continued to decrease. He also placed tiny obstacles inside the tube in the path between the two electrodes. When a current was applied, the glow would be partially obscured by these obstacles, casting shadows. This further confirmed the idea that the glow was caused by a particle emission.

Crookes' Tube

Probably the most important research using cathode-ray tubes was performed in 1875 by the English physicist William Crookes. In order to confirm the experiments of Plucker and Hittorf, Crookes designed his own vacuum tube from which the air could be almost completely removed. So great an improvement over Geissler's tubes were these that the "Crookes tube" quickly became the standard vacuum tube for use in scientific experiments. Crookes continued Plucker's experiments with magnetic fields, confirming the glow was easily deflected. He also installed tiny vanes within his tubes. As the current was applied the vanes would turn slightly (it was as if they were blown by a gust of wind).

German scientist Eugen Goldstein (1850-1930) first dubbed Crookes's rays "cathode rays" in 1876. In 1892, Phillip Lenard followed up on Heinrich Hertz's discovery that under certain conditions cathode rays could penetrate metal. Lenard succeeded in passing cathode rays through a window of thin metal set into the side of a Crookes tube. The rays exited the tube through the window into the air. Lenard proved that cathode rays were not a phenomenon exclusive to a vacuum. While performing a similar experiment in 1895, the German physicist Wilhelm Roentgen (1845-1923) accidentally discovered an even more penetrating form of radiation, which he called X-ray radiation.

Practical Uses for Cathode Rays

While many scientists were busy trying to unlock the secrets of cathode rays, others were searching for ways to apply them toward practical ends. The first such application came in 1897 in the form of Karl Ferdinand Braun's oscilloscope. This device used a cathode ray tube to produce luminescence on a chemically treated screen. The cathode rays were allowed to pass through a narrow aperture, effectively focusing them into a beam which appeared on the screen as a dot. The dot was then made to "scan" across the screen according to the frequency of an incoming signal. An observer viewing the oscilloscope's screen would then see a visual representation of an electrical pulse.

During the first three decades of the twentieth century, inventors continued to devise uses for cathode ray technology. Inspired by Braun's oscilloscope, A. A. Campbell-Swinton suggested that a cathode ray tube could be used to project a video image upon a screen. Unfortunately, the technology of the time was unable to match Campbell-Swinton's vision. It was not until 1922 that Philo T. Farnsworth used a magnet to focus a stream of electrons onto a screen, producing a crude image. Though the first of its kind, Farnsworth's invention was quickly superseded by Vladimir Zworykin's kinescope, the ancestor of the modern television.

Today, most forms of image-viewing devices are based upon cathode-ray technology. In addition, electron guns are used widely in scientific and medical applications. One important use for cathode-ray research has been the electron microscope, invented in 1928 by Ernst Ruska. The electron microscope uses a stream of electrons to magnify an image. Because electrons have a very small wavelength, they can be used to magnify objects that are too small to be resolved by visible light. Just as Plucker and Crookes did, Ruska used a strong magnetic field to focus the electron stream into an image.

[See also X-ray ]

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