In 1895, Wilhelm Röntgen was an honored and admired physics professor. He was viewed with admiration by his contemporaries. They thought of him as a careful experimenter, with a rich experience in laboratory work. But reputation aside, Röntgen was 50 years old — and at that age, it is rare for a scientist to make a significant contribution to his or her field.

[This article is the transcript of the episode: ‘The History of X-Rays & CT’, from the Curious Minds Podcast (CMPod). Subscribe to the podcast here.]

But, like your typical curious-minded scientist Röntgen kept plugging away on his Physics and laboratory experiments. He spent a lot of time in his lab, working alone. The focus of his curiosity was a peculiar device that, in spite of being very well known amongst physicists, was poorly understood. The device’s name was the “Crookes Tube”. It gave off an inexplicable, ghastly yellowish glow that was a mystery that puzzled many of the great minds of the late 19th century. No one could explain why this tube glowed the way it did.

Crooks Tube (Image: Wikipedia)

The solution to the Crookes Tube mystery would turn out to be one of the most important scientific discoveries ever made: a discovery that would affect millions of lives, and would make Wilhelm Röntgen a household name back in the day.

Like many of his colleagues in Physics, Wilhelm Röntgen was fascinated with the mysterious Crookes Tube. It was invented in 1870 by a British engineer named William Crookes. It’s a sealed glass cylinder, with no oxygen inside, containing 2 electrodes: an Anode and a Cathode. The Tube displayed an interesting phenomena: when a very high voltage difference was set between the positive Anode and the negative Cathode — a weird glow would appear inside the tube, and a greenish–yellowish spot of light appeared on the glass wall, behind the positively charged Anode.

This weird glow puzzled the scientists, who were left scratching their collective heads trying to figure the source of the glow. Some thought it was an “Ectoplasm”. If you’re thinking of Ghostbusters right now, you aren’t too far off. Ectoplasm is the mythical substance that ghosts are presumably made of. Others speculated that the glow had a connection with the Ether, a hypothetical, transparent material that was assumed to fill the emptiness between the stars in the heavens…

The only clue to figuring out the source of the e weird glow was that you could clearly see the Anode’s shadow on the glass wall of the tube — as if the cathode, on the other side of the tube, was shining a sort of an invisible light on the Anode, casting a shadow. The physicists called the invisible light emitted from the Cathode “Cathodic rays”. Wilhelm Röntgen spent a lot of time, working alone in his laboratory, trying to reveal the nature of these invisible Cathodic rays.

Wilhelm Röntgen

One day, in 1895, Röntgen was in his lab, working on his Crookes Tube. There are several different accounts of what happened that day, but most agree that while an electric current was flowing through the tube, Röntgen suddenly noticed that a board which was covered with Phosphorus and placed a few feet behind him — started to glow. Intrigued, Röntgen covered the tube with thick, black cardboard, blocking any trace of the faint glow emitted by the tube — but still, the phosphorus covered board continued to glow, even with the Crookes Tube completely covered up.

Rontgen and the first X-Ray picture (Image: Wikipedia)

Now, it is interesting to note that a few scientists had already came across this peculiar phenomena over the years: when near the Crookes Tube, phosphoric boards started glowing spontaneously, and photographic plates sitting in dark drawers became fogged, as if they’d been exposed to light. But none of these scientists investigated these phenomena: everyone was concerned with what was happening inside the tube — with the Cathodic Rays — rather than what was going on outside it. But Röntgen, thanks to his vast experience or maybe sharp intuition, immediately understood that he has stumbled onto something very important. Scientists had assumed that the Crookes Tube emitted only a single type of radiation — the mysterious Cathodic Rays. Röntgen realized that the tube also emitted a second and so far unknown type of radiation, one which could travel much farther outside the tube then the Cathodic Rays could. He named this unknown radiation “X-rays”.



Röntgen was likely excited about his discovery, but he was smart enough to realize that the clock was ticking — The Crookes Tube was sitting on top of hundreds of laboratory tables all across Europe and in the US — it was only a matter of time before other scientists came across the same discovery of these X-rays.

So Röntgen started secretly experimenting with X rays in his lab… He conducted as many experiments as he could come up, trying to figure out the characteristics of this unknown radiation. He wrote about his experiments in a later report:

“Paper is very transparent: behind a bound book of about one thousand pages I saw the fluorescent screen light up brightly, the printer’s’ ink offering scarcely a noticeable hindrance [to the unseen rays]. In the same way, the fluorescence appeared behind a double pack of cards: a single card held between the apparatus and the screen being almost unnoticeable to the eye. “A single sheet of tin-foil is also scarcely perceptible: it is only after several layers have been placed over one another that their shadow is distinctly seen on the screen. Thick blocks of wood are also transparent, pine boards one or two inches thick absorbing [the rays] only slightly. A plate of aluminium about [an inch] thick, though it enfeebled the action seriously, did not cause the fluorescence to disappear entirely. Sheets of hard rubber several inches thick still permit the rays to pass through them. If the hand be held between the discharge-tube and the screen, the darker shadow of the bones is seen within the slightly dark shadow-image of the hand itself..”

This last discovery — that X Rays could offer a glimpse inside the human body — gave Röntgen an idea….He asked his wife to place her hand in front of a photographic plate, and activated the Crookes tube. The dark shadows of her palm’s bones, including the distinct shadow of her wedding ring, were imprinted on the plate. Here was a definite proof that Röntgen found a means to look inside the Human Body.

The World Learns of X-Rays

When Röntgen published his findings, the world received the amazing news with wonder and immense enthusiasm. Within days scientists had reproduced Röntgen’s experiments, and barely one month later, physicians were already using X-rays to examine bone fractures and gunshot wounds. The medical world was ecstatic. One surgeon was quoted in a newspaper as saying:

“The surgical imagination can pleasurably lose itself in devising endless applications of this wonderful process.”

The general public was also overwhelmed by the discovery. Coin-operated X-ray photography booths were installed in some places, where loving couples took images of themselves holding skeletal hands… In newspapers, columnists argued whether “dignified” men were allowed to look at a woman’s x-ray photograph. Some ladies purchased lead underwear — just to be on the safe side. Shoe stores offered customers the possibility to see their toes while trying on shoes, using an X Ray “pedoscope”.

Wilhelm Röntgen became an international celebrity. Barely 2 months after his initial discovery, he received an honorable decoration from the Kaiser himself, and in 1901, he won the first ever Nobel Prize in Physics. In some places, X-rays are still called “Röntgen rays”, after him.

The Nature of X-Rays

But it wasn’t long before X rays revealed their darker side. At first, no one suspected that the new radiation could be dangerous, and some people had exposed themselves to X rays for minutes and even hours. In 1897, initial reports on the terrible effects of exposure to X rays began to appear: skin burns, hair loss, and cancer. We’ll never know how many people died due to overexposure to the radiation. A well known example is that of an assistant of the famed inventor, Thomas Edison. Edison’s engineers were working on X ray machines for medical applications, and the unfortunate research assistant would work in front of the emitting tube for hours with no protection. He suffered acute cancer and both of his hands were amputated — and sadly, he died a few months later. Many such tragic cases were reported in the media, and doctors started treating the new radiation with appropriate caution.

It took some time before scientists could decipher the nature of X rays. The first step towards understanding the nature of X-Rays was made by J.J. Thomson, of the prestigious Cavendish laboratory in Cambridge, England. Thomson proved that Cathodic rays are made of a new type of particle, not yet known to science. That particle was to be known as Electrons.

When electricity is passed through the Crookes Tube, the cathode emits high-velocity electrons. Electrons are negatively charged, and so are attracted to the positively charged Anode. But because of the high velocity of the electrons, they actually miss the Anode, and fly right by it, hitting the glass wall behind it. The collision — between the electrons and glass wall — transfers some of the electron’s energy to the atoms in the glass, causing the glass to shine in a greenish-yellowish light. So, the Cathodic Rays are physical particles emitted by the Cathode.

Later, when an accurate model of the atom was discovered, scientists could finally better explain X rays. It turns out that there are two distinct mechanisms for the production of X rays…both occur when the fast traveling electrons crash into the atoms in the glass wall of the tube.

The first way x-rays are produced is the result of the collision between the traveling electrons from the Cathode and the electrons in the glass atoms. When the fast moving Cathode electrons crash into the glass atom’s electrons, they can knock-off an electron — similar to the way a white billiard ball knocks-off a colored, stationary billiard ball. This collision causes the atoms to emit high-energy photons, which are the X rays.

The second way x-rays are produced is result of the moving Cathode electron penetrating deep inside the glass atom — and hitting the the atom’s tiny nucleus. The nucleus is a thousand times denser than the layers of electrons surrounding it, so this collision resembles a collision between something like a SmartCar or a Mini Cooper and a reinforced concrete wall. The electron crashes into the nucleus and is immediately slowed down: all the kinetic energy it has is transferred to the nucleus, which emits high-energy photons. The X rays emitted by this 2nd mechanism is called ‘Breaking X Ray radiation’.

In both cases, then, the result is not a radiation composed of physical particles — as in the case of Cathodic Rays — but an electromagnetic radiation, similar to visible light or Radio waves — except X-rays have a lot more energy.

This high energy is what allows X rays to penetrate deep inside solid matter. Visible light, for example, is blocked by the skin and the light’s energy does nothing but add some heat to the body cells. X rays, however, are not blocked by the skin or the soft tissues of the internal organs, and are only partially blocked by the high density of bones.

So, why are x-rays harmful? If an X ray encounters a molecule while it’s traveling inside the body — it can split the molecule open, shattering the chemical bonds that hold it together. These impacts, if they occur in big enough numbers, can cause real damage to the living tissue.

[This article is the transcript of the episode: ‘The History of X-Rays & CT’, from the Curious Minds Podcast (CMPod). Subscribe to the podcast here.]