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William Crookes

English scientist William Crookes was very innovative in his investigations with vacuum tubes and designed a variety of different types to be used in his experimental work.

William Crookes

William Crookes was born in London, England, on June 17, 1832, the son of Joseph Crookes and his second wife, Mary Scott. At the age of 15, Crookes enrolled at the Royal College of Chemistry, marking the beginning of a lifelong interest in the subject. In 1850, he began working as an assistant to August Wilhelm von Hoffman, which enabled him to undertake serious research. His earliest published papers, which appeared when he was 19, recounted his work with selenocyanides, newly discovered compounds of selenium. A sample of a deposit from a sulfuric acid factory was the source of the selenocyanides he studied.

After four years at the Royal College, Crookes accepted a position superintending the meteorological department of the Radcliffe Observatory at Oxford. Subsequently, in 1855, he began teaching chemistry at the College of Science in Chester. His tenure at the school was brief since he soon gained a sizable inheritance from his father that enabled him to retire to his own private laboratory in 1856. From this laboratory, located in London, Crookes carried out a number of investigations, primarily in chemistry and physics. Also in 1856, Crookes married Ellen Humphrey, who would bear him four children.

In 1861, when reexamining the residues he had saved from his early study of selenocyanides, Crookes discovered the element thallium through spectroscopic methods. He chose the name of the element based on its bright green emission line, the Greek word thallos referring to "a young, green shoot" or "twig." He continued his study of thallium for many years, succeeding in isolating it, characterizing the properties of its compounds and eventually, in 1873, ascertaining its atomic weight. The discovery of thallium brought Crookes a significant amount of recognition and precipitated his election into the Royal Society of London in 1863. His investigations with the element also led to his discovery of the principle upon which he built the Crookes radiometer, a device that produces rotary motion from light. This instrument was the predecessor of a number of more sensitive types of radiant energy detectors.

It is not known precisely when Crookes began his work with the evacuated glass tubes that are commonly associated with his name, but it was presumably during his attempts to measure the weight of thallium in a vacuum. Crookes tubes are glass vacuum chambers that contain a positive electrode (anode) and a negative electrode (cathode). When an electrical current is passed between the electrodes of one of the tubes, a glow can be seen in the chamber. It is now known that the glow is due to the interaction of electrons, which travel from the cathode to the anode, with residual gas present in the device. At the time of Crookes’s initial studies, however, the fluorescent light was poorly understood, being attributed to enigmatic rays, which were called cathode rays because they appeared to be emitted by the negative electrode. His experiments, however, helped reveal important new information about the nature of the mysterious rays.

It is notable that several other scientists had devised tubes similar to those used by Crookes in the years before he embarked on his studies in the area. Michael Faraday had used a comparable device in the late 1830s that was evacuated to a pressure of about 2 millimeters when he discovered the dark area in the tube near the cathode that came to be known as Faraday’s dark space. Heinrich Geissler produced a large number of vacuum tubes beginning the 1850s that were used both for study and entertainment, many people being fascinated by the pink glow characteristic of the tubes when they contained air at about 5 millimeters of mercury. Julius Plücker increased the vacuum in the tubes and found that this resulted in a differently colored glow and an increase in the Faraday dark space in the late 1850s, and his student Johann Hittorf noticed that an object placed in front of the cathode produced a shadow. Nevertheless, sealed glass tubes encasing a cathode and an anode are most commonly called Crookes tubes, possibly because the tubes used by Crookes during his investigations, which were formed by a skilled instrument maker and could be evacuated to about 1/100 millimeters of mercury, were the best produced up to that time, or perhaps because Crookes published so many foundational papers recounting his experiments with the tubes that they became indelibly linked with his name.

Crookes was very innovative in his investigations with vacuum tubes and designed a variety of different types to be used in his experimental work. One of the most famous is the Maltese cross Crookes tube, which he used to demonstrate that cathode rays travel in straight lines. The cross placed in the tube could be laid flat or stood up vertically, and when it was in the latter position, its shadow could be seen at the end of the tube when a voltage was applied to the device. In another tube, Crookes included a paddle wheel that turned when bombarded with cathode rays, and he used a Y-shaped tube to show that cathode rays would travel towards an anode even if it was not located directly in front of the cathode. Crookes further demonstrated that cathode rays can be bent by a magnetic field and carefully investigated the changing colors and effects in the tubes as they became increasingly evacuated. He was the first to note the dark space (now termed the Crookes dark space) that appears near the cathode at very low pressure, and correctly speculated that the space was a region in which the cathode rays moved freely before colliding with the gas molecules present in the tube, which he believed was responsible for producing the characteristic glow.

Overall, however, much of Crookes’s theoretical work has been discounted. When J.J. Thomson, for instance, discovered the electron and its subatomic nature was established, Crookes’s supposition that cathode rays are a fourth state of matter consisting of normal-sized molecules was proven wrong. Crookes was a very open-minded scientist, however, and despite his advanced years at the time, he willingly acknowledged the ascendancy of better interpretations of his experimental work than he had been able to develop at the time. Crookes’s flexible nature was also exemplified by his scientific approach to phenomena that many other investigators considered beneath their notice. In particular, he was fascinated by psychic phenomena and undertook the study of several famous spiritual mediums. For his work in this area, Crookes was often ridiculed, but throughout his life he steadfastly maintained the reality of the psychic phenomena he witnessed.

A less controversial interest of Crookes was journalism. He was founder of the Chemical News, which he edited from its inception in 1859 until 1906. Other notable achievements include the invention of the spinthariscope, the study of radioactive materials, the design of protective eyewear for use by glassblowers and the publication of texts on chemical analysis and diamonds. For his numerous achievements, Crookes received many honors. He was knighted in 1897, awarded the order of merit in 1910 and was elected president of the Royal Society, the Chemical Society and several other organizations.