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In this installment of the history of atom theory, physics professor (and my dad) Dean Zollman discusses how x-rays were discovered and later explained. – Kim

By Dean Zollman

Dean ZollmanIn the last decade of the 19th century, many researchers in addition to J.J. Thomson were using cathode ray tubes to conduct research. Before Thomson established that the cathode rays were electrons, several researchers had investigated their properties. Philip Lenard, a German physicist, had measured the range of the “rays” in air. In 1895, Wilhelm Conrad Roentgen was following up on that research by looking at what happened when the cathode rays struck some metals. In the process, he made an astonishing observation.

Wilhelm Conrad Roentgen

Wilhelm Conrad Roentgen by Nobel Foundation

Roentgen had recently moved in the University of Würzburg in southern Germany. He was a professor of physics and had been appointed rector of the university, the equivalent to a president of an American university. However, in those days the university leader had time for research. So, Roentgen had a laboratory where he was studying the properties of cathode rays.

As we discussed last time, the location of cathode rays could be determined because light was emitted when the rays struck a fluorescent material. This process is the same as the way a pre-flat-screen TV creates a picture on it screen. So, Roentgen had some fluorescent material in his lab. A small amount of this material was placed a rather long distance from the cathode ray tube. Yet, Roentgen noticed that the fluorescent material glowed when the tube was operating and the cathode rays were hitting a metal. By this time, cathode rays were known to travel only a few centimeters in air, but the distance between the tube and the glowing material was much greater than that.

Roentgen was motivated to undertake a series of careful experiments. On November 8, 1895, he made sure that no extraneous light could reach the fluorescent material. The lab was dark; the cathode ray tube was covered with black cardboard. Even in this environment Roentgen saw a faint glow on the fluorescent material. Something was traveling through the cardboard and causing light to be emitted from material. Roentgen gave the name x-rays to these new type of radiation.

Early Crookes x-ray tube

An early Crookes x-ray tube from a museum dedicated to Wilhelm Conrad Roentgen in Würzburg, Germany (image used under the terms of GNU Free Documentation License

Why the Professor Didn’t Notice His Dinner

For the next six weeks, Roentgen neglected his students and his duties as university rector to investigate his discovery. (A luxury university administrators do not have today.) He kept his research to himself and talked to very few people about it. The intensity of his work even put some strain on his marriage. This situation was described by a story related on the 35th anniversary of the discovery.

“One of the few persons who knew about the discovery before the announcement was made [on December 28, 1895] was Roentgen’s wife, Bertha. One evening in November 1895, she became very angry with her absent-minded husband because he did not comment upon the excellent dinner she had prepared for him, and he did not even notice that she was angry until she asked him what was the matter. He finally took her downstairs to his laboratory, which was in the same building, and for the first time presented to her astonished eyes the wonders of the x-ray.”

This story, which sounds like it could be an outline for an episode of The Big Bang Theory, is contained in a paper by Otto Glasser and appeared in the American Journal of Roentgenology in 1931. It is verified by a letter which Frau Roentgen wrote to her husband’s cousin.

Bertha Roentgen’s hand

Bertha Roentgen’s hand x-rayed by Wilhelm Roentgen

In his six-week marathon research, Roentgen discovered that x-rays could penetrate materials such as wood, flesh, and a 2,000-page book. However, more dense materials such as metals and bone allowed much less, if any, penetration. He also discovered that photographic plates could be exposed by the x-rays. Three days before Christmas 1895, Roentgen took his wife into his lab and recorded the first (and perhaps most famous) x-ray of part of a person. The bones in Bertha Roentgen’s hand are clearly visible, as is the ring on her hand. Both of the Roentgens’ exposure to x-rays while this picture was being taken must have been tremendous. Today, x-ray machines carefully aim the rays so that only the area of interest is exposed. Over the years, special film and digital detectors have been developed so that only a very small amount of exposure produces the desired result. Roentgen had none of that. He had, at best, a very crude point and shoot x-ray device.

See Your Bones Everywhere, Even in Shoe Shops

On December 28, Roentgen submitted a paper, “Ueber eine neue Art von Strahlung” (On a New Type of Rays) to the Proceedings of the Würzburg Physical-Medical Society. It included the picture of Frau Roentgen’s hand and quickly became a sensation worldwide. The medical implications were immediately recognized, and by May 1896, a handbook Practical Radiography had been published. However, it took a while for the value of x-rays to be fully understood. Tragically, it also took quite a while for the dangers of x-rays to be understood. Many people suffered damage and even death because their exposure to x-rays.

X-raying hands

X-raying hands in the late 19th century (from William J. Morton and Edwin W. Hammer, 1896, The X-ray, or Photography of the Invisible and Its Value in Surgery)

The picture above from an early handbook on using x-rays shows the cavalier ways in which they were treated. Here, two images are being created. The sitting man has his hand on a photographic plate. He will obtain a picture much like the one of Frau Roentgen’s hand. The standing man is holding his hand in front of some fluorescent material. He will be able to see bones move as he changes the position of his hand. The x-rays for both of images are coming from one Crooke’s (cathode ray) tube which is the glass bulb above the sitting man’s hand and in front of the standing man’s hand. X-rays are being emitted in all directions, and neither person has any protection from them. A far cry from your dental hygienist leaving the room when he/she take an x-ray of your tooth (with a machine that focus the x-rays very narrowly on the tooth.)

To show how slowly some of the dangers were addressed, consider buying shoes in the 1950s. When I got new shoes both the salesperson and my parents wanted to be sure that the shoe fit correctly. I, of course, just wanted them to look cool. So, the shoe store had x-ray machines. I would stand on a platform with my feet in this machine. X-rays were directed up from below my feet. There were viewing places with fluorescent material for me, my mother, and the salesperson. We could see how well the shoes fit. Even better, I could wiggle my toes and watch the bones move. So, I had “motivation” to stay in the machine longer than necessary. This “innovation” did not last long, but it did expose us to unnecessary radiation. Maybe that explains why the 1960s were so weird.

So, What Causes X-rays?

Roentgen’s scientific contribution was quickly recognized. The first Nobel prizes were awarded in 1901. The first physics prize went to Wilhelm C. Roentgen “in recognition of the extraordinary services he has rendered by the discovery of the remarkable rays subsequently named after him.” (Roentgen called his discovery x-rays. But in many parts of the world, x-rays are called Roentgen Rays.)

Explanations for what x-rays were and how they were produced took further research. Earlier in the 19th century, James Clerk Maxwell had shown that whenever an electrically charged particle was accelerated, it emitted or absorbed electromagnetic radiation. (For a physicist, acceleration means increasing or decreasing speed or changing direction. The change in direction will haunt us in a few blogs.) To create the x-rays, Roentgen had directed the cathode rays (electrons) into pieces of metal. As the electrically charged electrons slammed into the metal, they slowed and thus lost energy. This energy was emitted as x-rays.

Further research also showed that x-rays had the properties of light. They were much shorter wavelength and higher frequency than visible light. Maxwell had also concluded the light was a form of electromagnetic radiation. So the overall conclusion was that x-rays were the same physical phenomenon as visible light and radio waves. They were much higher energy and thus could pass through some materials that other forms of electromagnetic radiation could not.

Of course, the overall effect of x-rays on society has been much more positive than negative. And eventually they could be explained by models of the atom. In the meantime, other discoveries of the late 19th century were adding to the mystery of the structure of matter. We will look at a really big one – radioactivity – next time.

All images are from Wikimedia Commons. They are in the public domain unless otherwise noted.

Dean Zollman is university distinguished professor of physics at Kansas State University where he has been a faculty member for more than 40 years. During his career he has received four major awards — the American Association of Physics Teachers’ Oersted Medal (2014), the National Science Foundation Director’s Award for Distinguished Teacher Scholars (2004), the Carnegie Foundation for the Advancement of Teaching Doctoral University Professor of the Year (1996), and AAPT’s Robert A. Millikan Medal (1995). His present research concentrates on the teaching and learning of physics and on science teacher preparation.


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