Proficient in the fields of chemistry and physics, Crookes is best rememberedfor his invention of the Crookes tube, a cathode-ray tube that was the precursor to modern television and video tubes.
Crookes was born in London in 1832, the oldest of 16 children of a wealthy tailor and real estate investor. At age 16 he entered the Royal College of Chemistry in the hopes of studying organic chemistry. While there he became the assistant to August Wilhelm von Hofmann, a position which allowed him to attend meetings at the Royal Institution. It was at one such meeting that Crookesmet the eminent physicist Michael Faraday who convinced him to change his area of concentration from chemistry to physics, and particularly to optics.
After graduation from the Royal College of Chemistry, Crookes briefly attempted two positions in academia: superintendent of the meteorological departmentat the Radcliffe Observatory in 1854, and Lecturer in Chemistry at Chester Training College in 1855. Upon his father's death he received a substantial inheritance and was able to open his own laboratory and concentrate on physics.
It was not long before this investment paid off. While experimenting with spectroscopy, physics pertaining to the theory and interpretation of interactions between matter and radiation, Crookes discovered a lime green band in the spectrum of selenium, a band that belongs to no known element at that time. After several years he succeeded in isolating the element, which he named thallium (from the Greek word thallos, meaning "green twig "). At the sametime, the French scientist C. A. Lamy had also isolated a sample of thallium,sparking a small controversy over who would be attributed with the discovery. Today, the discovery is usually credited to Crookes. In 1861 he published his discovery.
While trying to determine the precise atomic weight of thallium, Crookes became interested in the use of vacuum tubes, which had recently been improved upon by Johann Heinrich Geissler. By placing a tiny amount of thallium within avacuum tube and weighing it there, a very accurate measurement could be obtained. However, he noticed that the arms of the scale would occasionally jerkunexpectedly, even though no visible force was acting upon them. Crookes later discovered that very small amounts of air had remained within the tube, andwhen struck by sunlight or lamplight it became disrupted, thus striking andmoving the sensitive scale. In order to demonstrate this phenomenon, Crookesinvented a device called a radiometer, consisting of a series of four small vanes balanced upon a pin. The two sides of each vane were painted different colors--one side black, the other silver. The entire assembly was then sealedin a vacuum bulb. When light struck the vanes the black sides would heat up,causing them to turn as the excited air molecules struck them. This device was (and still is) mostly a science toy, but it was also used by Scottish physicist James Clerk Maxwell (1831-1879) to prove the kinetic theory of gases.
Perhaps Crookes' most important work also stemmed from his experiments with vacuum tubes, particularly with cathode-ray tubes. At that time a great deal of research was being done on cathode rays; Heinrich Geissler (1815-1879) hadinvented an improved vacuum tube in which two electrodes could be placed, oneat each end. An electrical current was sent through the system, and as the air in the tube was evacuated, it would begin to glow. Crookes made further improvements to the system and in the late 1870s the first practical cathode-ray tube was designed. It was comprised of a vacuum tube with two electrodes, acathode and an anode, one at each end of the tube. When an electric currentwas introduced and the tube evacuated, a green glow would appear. While thiseffect had been observed before, it was Crookes who first noted the dark areaclosest to the cathode that he called Crookes' dark space. More importantly,he noted that when he placed a pivoting vane within the tube, the vane wouldturn slightly, as if within the current of a stream when the tube was evacuated.
At the time it was not known just what made up this invisible stream. Crookeshimself postulated an ultragaseous fourth state of matter that manifested itself within the tube. It was not until 1897 that English physicist Joseph Joseph J. Thomson (1856-1940) announced that a stream of electrons was created by the Crookes tube. Today, the same kind of technology exists in the electronguns used in televisions and electron microscopes.
In his later years, Crookes was fascinated by the emerging field of radioactivity. After researching uranium for several years he invented an instrument designed to detect alpha-rays called a spinthariscope, which means "spark-viewer." When an alpha particle strikes the instrument's zinc sulphide covered screen, a microscopic flash of light can be observed. During his career Crookeswas awarded numerous accolades, including the Royal Medal, and was knightedin 1897. For some, however, his reputation is tarnished by the fact that he was a spiritualist, publishing several papers on the validity of psychic phonomena and the occult.
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