Science Diction: How 'X-Ray' Got Its 'X'

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Dr. Howard Markel, a medical historian at the University of Michigan, discusses how the German physicist William Roentgen stumbled across the phenomenon of X-rays while playing with a cathode tube in his lab, and why Roentgen gave the electromagnetic beams the name "X-rays."


Up next, Science Diction. It's our monthly history of scientific words. And whether it was for a cavity or a broken bone, chances are you've had one - an X-ray. Why don't we call - why do we call them that?

Joining me now for this month's episode of Science Diction is my guest Howard Markel. He is professor of history of medicine at University of Michigan in Ann Arbor and he's also director for the Center for the History of Medicine. Welcome back to SCIENCE FRIDAY, Dr. Markel.

Dr. HOWARD MARKEL (History of Medicine, University of Michigan; Director, Center for the History of Medicine): Well, thanks for having me back, Ira.

FLATOW: X-rays. It's used in science fiction all over the place. Why do we call them X-rays?

Dr. MARKEL: Well, it's a great story. You know, it takes a lot of history. But, you know, it's the most - one of the most important technologies in the history of science and medicine because it allows doctors to really look into the, you know, lift up the hood and look at the engine while it's still running and not do a lot of invasion.

But really, around 1895, electrons and electricity was all the rage. And there was a physicist, William Roentgen, who was working in Wurzburg, who was playing with a cathode tube. And this particular tube had a thin aluminum window that allowed these electronic beams to escape. And he was playing around in his lab, covering the cathode tube with black cardboard and noticed that there were several beams escaping several feet away to a nearby lab bench, and was hitting a screen that he was also going to use to test against these waves that was made out of barium platinocyanide. And, lo and behold, it glowed in the dark.

It was a eureka-like moment. He locked himself in his laboratory, stayed there for six weeks, except for meals and I imagine to go to the bathroom. And then he found that these newly discovered beams would pass through opaque objects and the effected photographic film he put underneath it. And photographic film was a very common thing in laboratories at that time. It was used to document phenomenon. And first, it was, you know, a set of weights and then a piece of metal and, most famously, it was the bones of his wife's hand.

FLATOW: Ah, that famous picture with the ring on it.

Dr. MARKEL: Yeah, his wife. It was December 22, 1895.

FLATOW: And so how did the name X-ray come out of it?

Dr. MARKEL: Well, X was a very common thing to - for unknowns. And a lot of scientists were using that for unknown phenomenon. But where that really comes from - Roentgen, of course, was a physicist. He was very skilled in mathematics. And while he may not have been a Cartesian philosopher, he was almost certain to have read Rene Descartes' 1637 text "Geometry," which is a very important geometry text. And it was Descartes who actually introduced the terms X and then followed by Y and Z...


Dr. MARKEL: unknown quantities to match the symbols of knowns, which would be A, B and C, respectively, in an equation.

FLATOW: So the X was the unknown and we didn't know what those X-rays were made out of.

Dr. MARKEL: Exactly. That was found out a lot later on, that it was found to be in a precise wave length...

FLATOW: All right.

Dr. MARKEL: ...and so on and so forth physically.

FLATOW: We've run out of time, Dr. Markel. Thank you very much for the -for our Science Diction word of the month. We'll have you back next month.

Dr. MARKEL: Great. Thanks so much.

FLATOW: Have a great holiday weekend. You could check out Dr. Markel's post about the history of X-rays on our website. If there's a word you like to hear on our next episode of Science Diction, leave us a comment. Go over to our website at

Greg Smith composed our theme music. We had help from NPR librarian Kee Malesky. Surf over to our website at Podcasting, blogging, all this kind of stuff. Have a great and safe Father's Day holiday weekend. We'll see you back here next week. I'm Ira Flatow in New York.

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