Radiography, the use of X-rays to view unseen or hard-to-see objects, began in 1895 when the first X-ray machine was built by Wilhelm Roentgen. This technology was almost immediately taken up by medicine, and became an important medical speciality called radiology. Radiography is also used industrially, to detect faults or welds in piping. X-rays began to be used in airports for bomb detection in the 1960s.
Discovery
X-rays were discovered in 1895 by Wilhelm Conrad Roentgen, a professor at Wurzburg University, Germany. He was experimenting with cathode-ray tubes and noticed a kind of light being emitted, one that could pass through most--but not all--solid objects.
This discovery received extraordinary interest from the scientific community and the press. Other scientists dropped what they were working on in order to follow up on it.
Medical Use
"Within a month after the announcement of the discovery, several medical radiographs had been made in Europe and the United States, which were used by surgeons to guide them in their work," says the Center for Nondestructive Testing. "In June 1896, only 6 months after Roentgen announced his discovery, X-rays were being used by battlefield physicians to locate bullets in wounded soldiers."
Radiography--in the medical context, called radiology--is essential to modern medicine and dentistry, where it is used to detect cavities in teeth.
The Thermionic Tube
Various methods were at first used to produce X-rays. U.S. engineer William Coolidge devised, in 1913, the type that is still being used today. It involves the cathode rays being produced by a tungsten filament, whose current can be changed to adjust the intensity of the X-ray beam.
Industrial Use
Coolidge's thermionic tube allowed X-rays with much higher penetrating power to be used, operating at power levels up to 100 kilovolts. In 1922 a 200 kilovolt X-ray tube was produced, and in 1931 General Electric developed generators that could produce 1,000 kilovolts of power.
At high-enough power levels, X-rays can see through certain types of metal. Industrial X-rays are very useful for inspecting welds in areas that would otherwise be very difficult to access, such as in pipes. They're also used for probing through concrete (to find rebar or conduits), and through pipe wall.
Security
In the 1960s, X-ray screening machines were introduced alongside metal detectors at airports to detect bombs in luggage.
Since then, they've become a standard fixture not only in airports, but in many government buildings.
Dangers
Exposure to X-rays leaves very small residual amounts of radiation in the subject. These build up cumulatively over a lifetime, causing cancer at high-enough levels.
Because it does take a long time for this damage to happen, X-rays were not immediately suspected as the cause. The first recorded death from X-ray radiation damage was Clarence Dally, one of Thomas Edison's assistants.
Through the first half of the Twentieth Century, intensive research was done into the effects of this radiation, and protective measures (such as lead shielding) developed to reduce exposure to it.
Interesting Fact
Before radiation dangers were fully understood, X-rays were used for fitting shoes. These machines--"shoe-fitting fluoroscopes"--were popular from the 1920s through the 1940s.
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