In 1895 Wilhelm Conrad Röntgen, a mechanical engineer and physicist, was working in his lab in Würzburg, Germany. Röntgen was testing the electrical discharges from newly developed vacuum tube equipment made by a variety of manufacturers. He noticed something very strange. One of the vacuum tubes was covered with a cardboard shield, so that it could emit no light. The point was to protect the sensitive aluminum that the tube was made of. Although the tube was completely covered, when it was turned on, it created a fluorescent effect on a screen painted with barium.
Röntgen had no idea what was causing this mysterious “exposure,” so he named the emanation an “x-ray,” with the “x” the unknown, as in an algebraic equation.