X-rays—the technology that allows doctors to peer into the human body while the engine is still running—represent one of the most revolutionary advances in the history of science. But where did its quaint name come from? The short answer is that it is really a story of connections. Howard Markel, a a physician, medical educator, and historian of medicine at the University of Michigan, explains:

The X-ray was born in November of 1895 when the phenomenon of electricity and electrons was all the rage. A physics professor at the University of Wurzberg named William Röntgen was playing with a cathode tube featuring a thin aluminum window that allowed some of the electromagnetic rays to escape. He covered the tube with black cardboard to shield its fluorescent glow, and, shortly after, noticed beams escaping several feet to a nearby plate made of barium platino-cyanide. Secreted in his lab for 6 weeks, pausing only to take meals, he experimented with other fluorescent screens and photographic paper, the latter a common item in laboratories for purposes of documentation and illustration.

Röntgen’s "Eureka moment" arrived shortly after noticing that his newly discovered beams passed through opaque objects and affected the film beneath. The results included shadowy “radiographs” of a set of weights, a piece of metal, and, most famously, the bones of his wife’s hand and her wedding ring. When she underwent the world’s first x-ray on a human, Mrs. Röntgen exclaimed, “I have seen my death.”

Because he did not know the precise physical nature of these electromagnetic beams, Röntgen referred to them as x-rays. In later years, some preferred the now lost moniker, Röntgen Rays.

Röntgen published his initial findings in late December of 1895. It took a matter of weeks before such news spread around the world and X-rays became the media sensation of the day. Thomas Edison and others manufactured commercial fluoroscopes, carnivals employed them as novelties for fair-goers wanting a radiograph of their bones, shoe stores used them to measure the size of feet, and eventually, physicians, surgeons and dentists learned to employ them widely as central tools in their diagnostic arsenal. In 1901, Röntgen was awarded the first Nobel Prize in physics. It is no understatement that his discovery changed the modern practice of medicine in ways the physicist could have never imagined.

Long before Röntgen named his rays, however, mathematicians—and then scientists—used the letter X to describe any unknown. The French philosopher, physicist and mathematician René Descartes introduced x, y and z as unknown quantities in his famed 1637 treatise La Géométrie. In order to provide symbols of unknowns corresponding with the symbols of knowns, a, b and c, Descartes took the last letter, x, for the first unknown and proceeded backwards to y and z for the second and third respectively.

Writers from Samuel Coleridge and William Thackeray to Sinclair Lewis favored the term X to designate a person, thing, agency or factor that was unknown. In 1894, the American astronomer Percival Lowell began his search for the ninth planet in the solar system, which he called Planet X. His successors identified it in 1930 and subsequently named it Pluto. More recently, X has been a label of choice for a slew of supernatural comic books, science fiction thrillers, motion pictures and television shows.

How ironic—at this late date—that this entirely familiar form of ionizing radiation continues to bear the mark of the unknown.

In this segment, we'll talk about the origins of the word 'x-ray' and the history of its meaning.