X-rays are a form of electromagnetic radiation created when high-speed electrons bombard and interact with the atoms in a metal target. The two essential parts of an x-ray machine are an x-ray tube and a source of high voltage electricity (the induction coil). These were common pieces of apparatus in the physics laboratories of the late 19th century.

The first x-ray tube was the Crookes tube, a partially evacuated glass bulb containing two electrodes, designed by British experimenter William Crookes. When an electric current passes through the tube, a beam of electrons is emitted from the cathode, and travels toward the anode. Experimenters in the 19th century called this a ""cathode ray."" The interaction of these rays with the gas molecules in the tube and the glass walls of the tube caused a fluorescence. These were the phenomena that Roentgen was studying. X-rays were a side-effect, produced when the high-speed electrons were suddenly stopped as they hit the anode. Crookes tubes produced "soft" x-rays of low energy.

An x-ray machine is much like a camera. X-rays cause chemical changes that darken photographic plates. Since x-rays pass through some tissues and bone more easily than others, when a body part is placed between the source of x-rays and the photographic plate, an image of the interior structure is produced. For example, the lungs contain mostly air and do not absorb x-rays. Hence they appear black on the x-ray pictures. On the other hand, bones absorb x-rays and appear white.
Canadian Heritage Information Network
Canada Museum of Science and Technology, Musée de la civilisation, Stewart Museum, Canadian Medical Hall of Fame, Museum of Health Care at Kingston, University Health Network Artifact Collection, University of Toronto Museum of Scientific Instruments, University of Toronto Museum Studies Program

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