Cathode Ray Tube
In the mid-to late-1800s, the world was experiencing a scientific revolution. Phenomena that had never before been truly understood, such as light, heat, and radiation, were systematically unravelled, and by such great scientists as Henri Becquerel, Marie Curie and Thomas Young. Among these phenomena was the nature of electricity: how it worked, and why. The early experiments to solve the riddle of electricity often included the use of anode-cathode tubes--glass tubes containing an anode at one end and a cathode at the other. When most of the air was evacuated from this tube, an electrical charge could be observed jumping across the gap between the two electrodes. one scientist who performed such an experiment was Michael Faraday; he noticed that, as the amount of air within the tube decreased, a faint glow could be detected between the anode and the cathode. However, the technology of the time was not sufficient to produce a high vacuum within the glass tube, and so Faraday was unable to further explore this effect.
The pioneers in the study of cathode-ray tubes were the German team of Heinrich Geissler and Julius Plucker. Geissler, a skilled glassworker, was employed by the University of Bonn as a maker of scientific instruments. While at the university he met Plucker, then a young professor. Some time around 1855, Plucker convinced Geissler to design an apparatus for evacuating a glass tube. Geissler did just that, constructing a hand-crank mercury pump that could (after a laborious session of pumping) 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--much brighter than any achieved in previous experiments. Second, he found that the glow responded to a magnetic field, and that it could be moved by a powerful magnet. This discovery was monumental, for it 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 conducted in 1869 by Johann Hittorf. 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 by partially obscured by these obstacles, casting shadows. This further confirmed the idea that the glow was caused by a particle emission.
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, finding the glow easily deflected. He also installed tiny vanes within his tubes; as the current was applied the vanes would turn slightly, as if they were blown by a gust of wind. These experiments incontrovertibly showed that a stream of particles travelled through the tube. Crookes believed that cathode rays were a "fourth state of matter," possibly associated with an invisible aether.
German scientist Eugen Goldstein first dubbed Crookes's rays cathode rays in 1876. In 1892, Phillip Lenard, following up on Heinrich Hertz 's discovery that under certain conditions cathode rays could penetrate metal, 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, showing that cathode rays were not a phenomenon exclusive to a vacuum. While performing a similar experiment in 1895, the German physicist Wilhelm Roentgen accidentally discovered an even more penetrating form of radiation, which he called X-ray radiation.
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. About the same time, the final proof of the particulate nature of cathode rays was provided by the great British physicist J. J. Thomson. Thomson also succeeded in measuring the mass and charge of the particles, which were shown to be smaller than an atom. The cathode-ray particles became known as electrons, and the cathode ray tube as an electron gun. For his discovery of the first subatomic particle, Thomson was awarded the 1906 Nobel Prize in Physics.
During the first three decades of the 20th 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, almost every form of image-viewing device is based upon cathode-ray technology.
Electron guns are used widely in scientific and medical applications, and also based on the principals of cathode ray tubes. One application of particular importance has been the electron microscope, invented in 1928 by Ernst Ruska. The electron microscope uses a stream of electrons, rather than light, to magnify an image. Because electrons have a much smaller wavelength, they can be used to magnify objects, such as ultramicroscopic viruses, 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.
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