A Discovery With a Long Run-Up
The usual story is that Wilhelm Roentgen discovered X-rays in November 1895. That is the accepted date for the discovery as a recognized event, and it is well documented. But the conditions that produce X-rays had been created in laboratories for roughly a quarter of a century before that. Physicists were building devices that made X-rays whenever they were switched on, and the rays were passing through their benches, their cardboard and their photographic plates without being identified.
This article is about those earlier experiments. It does not claim that anyone other than Roentgen discovered the rays, and credit for scientific discoveries is often a matter of who first recognizes and explains an effect, not who first produces it.
The Tubes Behind the Story
The key apparatus was the cathode ray tube. By the middle of the 1800s, vacuum pumps had improved enough to remove most of the air from sealed glass vessels. When a high voltage was applied across electrodes at the ends of such a tube, strange glowing effects appeared. In the following decades, researchers studied these effects closely.
Some of the people most associated with this work:
- Julius Plucker and Johann Wilhelm Hittorf, German physicists who studied the glow and the paths of the discharge in the 1850s and 1860s. Hittorf is often credited with showing that something travels in straight lines from the negative electrode, the cathode, and casts shadows.
- Eugen Goldstein, who in 1876 gave the name “cathode rays” to the phenomenon.
- William Crookes, a British chemist and physicist, whose improved tubes in the 1870s became so widely known that tubes of this kind are still called Crookes tubes. He argued that the rays were a stream of charged particles.
- Heinrich Hertz and his assistant Philipp Lenard, who in the early 1890s showed that cathode rays could pass through thin metal foil. Lenard built a tube with a thin aluminum window so that the rays could be studied outside the glass.
All of them were studying cathode rays, not X-rays. But when a stream of fast electrons strikes the glass wall of a tube or a metal target, it produces X-rays as a by-product. So every working tube was an X-ray source, whether or not the experimenter knew it.
Clues That Were Missed
Looking back, historians have found several moments when X-rays left evidence that was misread or ignored.
Fogged photographic plates
Several experimenters reportedly noticed that photographic plates kept near their tubes were mysteriously fogged. A commonly told story is that Crookes had returned boxes of photographic plates to the maker as defective because they had been spoiled in his laboratory. The exact details vary from one retelling to another, and some versions may have grown in the telling, but the general point, that the plates were exposed by something invisible, matches what we now know. Photographic materials were a standard tool of early photography, and so an unexplained fogging could easily be blamed on poor manufacturing.
Accidental images
In February 1890, Arthur Goodspeed, a professor at the University of Pennsylvania, and a photographer working with him, William Jennings, are said to have produced images of coins on a plate while demonstrating a discharge tube. They did not understand what they had made. After Roentgen’s announcement, they recognized that their plate had been an X-ray image. This is one of the better-known cases of an accidental X-ray image preceding 1895.
Lenard’s window
Philipp Lenard’s experiments are particularly interesting because his window tubes allowed rays to leave the tube, and he used fluorescent materials to study them. He investigated the invisible radiation outside the tube but concluded that he was seeing cathode rays, which can travel a short distance in air. Later, when Roentgen’s work appeared, Lenard felt his own contribution had not been fairly recognized. He received the Nobel Prize in Physics in 1905 for his work on cathode rays. His attitude toward Roentgen’s credit was bitter, and in later life he held views that damaged his reputation.
Other claims
Over the years, other scientists, including some in Central and Eastern Europe and in the United States, have been named as having noticed or photographed X-rays before 1895. Claims of this type often came forward only after Roentgen’s fame, so they are hard to assess. Careful historians tend to treat them as evidence that the effect was present in many laboratories, but not as evidence that anyone had identified a new kind of ray.
Why Roentgen Gets the Credit
The reasoning usually given for Roentgen has several parts. He noticed an effect that the tube alone could not account for, and the screen was far from the tube and shielded from its light. He then followed it up with systematic experiments, showing how the rays passed through different materials and how they differed from cathode rays. And he wrote them up promptly and in a way that other laboratories could reproduce. His account is the reason the world learned about the rays within weeks.
Discovery in science is often like this: the effect may have been seen many times, but it becomes a discovery when someone recognizes that it is new, and shows others how to see it too.
What Followed From the Tubes
The cathode ray tube itself had a long future. Within two years, J. J. Thomson’s experiments with cathode rays led him to conclude that they consisted of very small negatively charged particles, later called electrons. In the twentieth century, tubes of this general kind became the basis of oscilloscopes and television picture tubes. The early X-ray machine was essentially a tube designed to make X-rays on purpose instead of accidentally.
The experiments of the 1800s also illustrate something about X-ray hazards. Researchers working with powerful tubes for years without protection were exposed to radiation, and the possible effects of that exposure were not recognized at the time.