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The History of X-rays
 Wilhelm Conrad Roentgen (pictured at right), a Bavarian physicist discovered x rays. He was working with sealed glass vacuum tubes that contained a cathode and an anode. During his experiments, he applied voltage to these tubes and noticed that a screen near the tubes was glowing. He blocked the path of these newfound rays to see what would prevent the screen from glowing. When he placed his own hand there, he could see the outline of his bones on the screen. This historic discovery on November 8, 1895 dramatically changed diagnostic procedures for both medicine and dentistry. Roentgen (in some references spelled “Röntgen”) received the first Nobel Prize in physics in 1901 for his discovery of x rays.
Roentgen's Hand
The X-ray Tube
 The dental x-ray tube is a vacuum-sealed glass enclosure surrounded by lead shielding. The shielding has only one opening in it to allow the escape of the x-ray beam. The glass enclosure houses a cathode made up of a tungsten filament wire with a focusing cup, and an anode composed of a thin slab of tungsten, molybdenum, palladium or another heavy metal embedded in copper. The anode gets very hot, so heat is dissipated by oil which circulates through the copper. The tungsten wire in the cathode is heated by an electric current and emits electrons. The focusing cup focuses the negatively charged electrons generated by the cathode and directs them across the gap to a small spot on the positively charged anode. The point where the electrons are focused is called the focal spot. Two types of energy are generated at the focal spot: heat and x rays. (The process is very inefficient, with 99% of the energy supplied to the tube dissipated as heat.) The x rays escape the housing through an aluminum filter at the opening of the tube head. The collimator at the opening of the tube restricts the x-ray beam to less than 2 ¼ inches at the patient's skin surface. A lead-lined cone collimates the beam further.
The image to the left is a Coolidge X-ray tube manufactured by General Electric in 1913. There are numerous newer designs for medical and industrial x-ray units, but the Coolidge tube has remained the design used in modern dental x-ray units.
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