Cathode Ray
The research that led to the discovery and understanding of cathode rays involved many different scientists from several countries and spanned more than a decade. Today, devices ranging from the oscilloscope to the electron microscope to the television set rely upon cathode rays and cathode-ray tubes.
Pioneers in the field of cathode rays, such as Michael Faraday, performed considerable research involving electricity; in particular, they were interested in the behavior of electrical current as it crossed the gap between an anode and a cathode. In order for this to occur, the two electrodes were placed within a tube from which much of the air was evacuated. During the early 1800s the technology necessary to create a true vacuum within the tube did not exist. Still, Faraday and others noticed that, even in a partially evacuated tube, a slight fluorescent glow could be detected when a current was applied.
The first great advance in cathode ray research came in 1855, when Johann Heinrich Geissler (1815-1879) invented an improved method for evacuating tubes. These geissler tubes operated at very low pressures, making it easier for the current to jump across the two electrodes. Using these new tubes, physicists worldwide performed inspired research that often produced astounding results. One of the most famous researchers was the German scientist Julius Plücker (1801-1868), who found that the position of the fluorescent glow within the tube could be changed by placing a magnet near the glass. This seemed to indicate that the glow was caused by charged particles rather than electromagnetic waves. A student of Plücker, Johann Hittorf (1824-1914), continued this line of research, discovering that the nature of the glow changed as the pressure within the tube was further decreased. He also found that objects placed within the tube would cast a "shadow"--another indication that these "rays" could be particulate in nature.
The definitive research on cathode rays was performed during the 1870s by the English physicist William Crookes. Crookes began by designing his own tube (called the crookes tube) that could be evacuated nearly completely. He found that, when the pressure was sufficiently low, the glow stretching between the anode and cathode would disappear, only to be replaced by a strange luminescence that covered the inside walls of the glass tube. Though Eugen Goldstein (1850-1930) dubbed them cathode rays, Crookes did not fully understand their nature, and he proposed that they represented a "fourth state of matter""--not liquid, not gas, not solid, but perhaps a part of the ectoplasmic ether. A few years later the French scientist Jean Baptiste Perrin provided the final proof that cathode rays were negatively charged particles by allowing them to collect upon a metal plate within a Crookes tube.
As the twentieth century approached it was clear that cathode rays were some sort of very small, negatively-charged particle. However, important questions remained: how small? how greatly charged? The answers were to be provided by Joseph John Thomson, the renowned English physicist. While experimenting with electromagnetic radiation, Thomson became intrigued by the nature of cathode rays. He improved the vacuum tube yet again and, in 1897, showed that the rays could be deflected in an electrical field as well as a magnetic field, proving beyond doubt that cathode rays were composed of particles. Using an extensive set of calculations, Thomson determined the ratio of the cathode-ray particles' charge to its mass, discovering that the mass had to be nearly two thousand times smaller than that of an atom.
What Thomson had done was discover the existence of subatomic particles. Many scientists had suspected for years that the atom was surrounded by particles smaller than it, but no research seemed advanced enough to detect them. The negatively-charged subatomic particle was called the electron, and for its discovery Thomson was awarded the 1906 Nobel Prize for physics.
With its nature revealed, the cathode ray became an important tool for scientific use. Its first practical application was an oscilloscope, a device that used a moving beam of electrons to illustrate signal patterns, that was developed by Karl Ferdinand Braun (1850-1918). The cathode-ray tube was modified into an electron gun, the heart of the electron microscope; by using focused electrons in place of focused light, much smaller objects can be observed in this microscope. Probably the most familiar application of cathode rays is in television sets. Both television cameras and receivers use electron guns to scan the image to be broadcast, while the typical television receiver utilizes a modified cathode-ray tube (called a CRT) as a display. Also, the greater understanding of the electron that stemmed from cathode ray research led to many of the important advances in atomic theory.
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