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Cathode Ray Tube: X-rays and the Electron
Our story begins in a laboratory in Würzburg, Germany, in 1895. It didn’t look much like the clean white spaces used by modern scientists; it had beautiful parquet floors and impressive high windows looking out over the park and vineyards opposite. The physicist Wilhelm Röntgen closed the shutters and turned to his work. On a long wooden table, he set up a glass tube the size of a small wine bottle, which had most of the air removed using a vacuum pump.1 Wires trailed off from metal electrodes, one in the end of the tube (the negative cathode) and one roughly halfway down the length (the positive anode). When high-voltage electricity was applied, a glow appeared inside—the so-called “cathode rays” that gave the tube its name. So far, everything was as he expected. Then, out of the corner of his eye, he noticed a small screen on the other side of his lab glowing.
He walked over to inspect it. The phosphor-coated screen was giving off a green-coloured light. When he turned the cathode ray tube off, the light disappeared. When he turned the tube back on, the light returned. Perhaps it was just a trick of the eye, a reflection of the light from the glowing cathode ray tube? He covered the tube with some black cardboard but found that the light on the screen persisted. He’d never seen anything like it before, but thought it could be important.
From this moment on, physics would never be the same. Beginning with this first serendipitous observation, experiments using cathode ray tubes would lead the field of physics into entirely new territory and start to overturn ideas about the natural world that had been accepted for millennia. In time, the cathode ray tube would lead to technologies which changed the way people live, work and communicate. It all started here, with this glowing screen, and the curiosity of an individual.
Wilhelm Röntgen, like most scientists around the world at the end of the nineteenth century, agreed that the subject of physics was almost complete. The Universe was made of matter that consisted of “atoms .” They’d figured out that there were different types of atoms, which corresponded to different chemical elements. From trees to metals, water to fur: all the complexity of the material world around them differed in terms of hardness, colour and texture because they were built of different atoms, which they viewed like tiny, spherical Lego pieces. If you had the right instructions, you could take a particular set of atoms and create anything you liked.
They also knew there were forces through which everything interacted. Gravity kept the stars in our galaxy, and our planet circling the Sun. Even the mysterious forces of electricity and magnetism had finally been brought together into just one force: electromagnetism. The Universe was predictable: if you had all the details of the inner workings and set things in motion, the movements of all matter could be predicted perfectly.
Now only the details were left to explore—details like how exactly the cathode ray tube worked, one of the few small things they couldn’t quite explain. There were theories of course, including the idea that the glow inside was related to ripples in the hypothetical aether, the medium through which light was thought to travel in much the same way as sound is transmitted by the air. Now, in his investigations of the details of the cathode ray tube, Röntgen seemed to have stumbled onto a complication. Not only was there something unexplained happening inside the tube, but he’d found a strange effect happening on the outside as well.
Röntgen had seemed ordinary as a child. The son of a cloth merchant, he loved exploring nature in the countryside and forests. The one thing he did show quite an aptitude for was making mechanical things and this early ability turned out to be useful to his experimental work later in life. As an adult, his dark hair stood up from his forehead “as if he were permanently electrified by his own enthusiasm.”
Röntgen was a shy man who gave lectures in an intolerably low voice, was strict with his students and was even slightly uncomfortable at the idea of having assistants in his lab. But he loved science, sometimes quoting the great engineer Werner von Siemens, who said, “The intellectual life gives us at times perhaps the purest and highest joy of which the human being is capable.”
Now he had found something that no one had seen before. When he saw the strange glowing screen, he assumed that he wasn’t looking at the same kind of “ray” which caused the cathode ray tube to glow, since that effect seemed contained inside the tube. Instead he’d found a new kind of invisible ray which seemed to be able to travel much further. He immediately dedicated himself to exploring more, channelling all his time and energy into the lab. When later asked what he thought at the time he said “I didn’t think, I investigated.” He had a number of similar tubes around his laboratory which he could now use with the phosphor screen, setting up each in a methodical and thorough way to figure out the nature of the new rays. He placed different materials between the tube and the screen, trying paper, wood and even hard rubber. The rays went through all of them, barely diminishing. When he pointed the rays through the thick wooden door to the adjoining lab, he found he could detect them on the other side. Only when he placed aluminium foil in front of the tube did the rays seem to have some difficulty getting through.
He spent seven intense weeks in his lab, occasionally being reminded to eat by his wife, Anna Bertha. Apart from those interactions, he was working almost entirely alone, and he remained silent about his research. He didn’t tell his assistants, let alone his international colleagues. He knew that if he didn’t announce his discovery first, hundreds of other scientists who had similar experiments sitting in their labs would beat him to it. The only report of him speaking about the work was to a good friend, to whom he simply said “I have discovered something interesting but I do not know whether or not my observations are correct.”
Next, he tried sticking his hand in the way of the rays and reported: “If the hand is 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 gave him an idea. He used the rays to make an image of Bertha’s hand on a photographic plate, which confirmed his understanding: the rays travelled easily through the skin and flesh but not so easily through bone or metal. The bones in her hand and her wedding ring showed up dark in contrast to the flesh that we normally see with the eye. The ability to block the new rays was related to the density of an object. According to legend, when Bertha saw the bones in her hand she exclaimed “I have seen my death!” and never set foot in her husband’s lab again.
Röntgen needed to give the new rays a name in his notebook. In science, we typically denote things which are unknown with a letter like “X,” and so Röntgen came up with possibly the best unintentional branding in the history of physics. He called his new discovery “X-rays.”
Once he was satisfied that he understood how X-rays behaved, Röntgen had a decision to make. Should he patent the idea, publish his findings, or do more work before he announced his discovery? There were many questions that he was still curious about, like how X-rays were related to light and matter, what they were made from and how they were formed. He determined that he couldn’t delay the announcement any longer; the chance of someone else finding X-rays was too high. If he published the discovery before applying for a patent, he would never make any money from it if it turned out to be useful in medicine. But Röntgen was a physicist, not a doctor, so he didn’t know if medics would be interested in his idea or not. He decided the best way to make it useful was to publish his discovery and communicate it to the medical community.
Overcoming his habitual shyness, on 23 January 1896, Röntgen set up a heavy table with his X-ray experiment in the Würzburg Physical Medical Society lecture theatre, just a short walk from his laboratory. The crowd had already caught wind of his discovery through newspaper articles and so many attended that there were men standing in the aisles. Röntgen presented the first ever lecture about what he’d discovered. He showed the audience how X-rays could go through wood and rubber, but not through metal. He showed them the photograph of Bertha’s hand and told them about his idea to use X-ray pictures to see inside the human body. To drive the point home, he decided he would demonstrate just how easy it was to create a similar image.
Standing in front of the hall, he invited the president of the society, a prominent anatomist, to place his hand in the path of the X-rays. Röntgen switched on the cathode ray tube and took an X-ray photograph of the president’s hand. The doctors in attendance were amazed. They immediately saw the value of his discovery and the president was so impressed that he led the crowd in giving Röntgen three cheers. They even proposed to name the new rays in his honour.
Word about this new phenomenon spread like wildfire, inspiring admiration, fear and even poetry across the world. At the same time that Jules Verne’s books about travelling to the centre of the Earth were capturing the public imagination, Röntgen had suddenly discovered the ability to see inside the human body. This led to some interesting misconceptions, like the idea that X-rays could see through a lady’s clothing (the idea of seeing through men’s clothing went unmentioned). The entrepreneurs of the time started selling X-ray-proof lead underwear, presumably only for women. “X-ray glasses” were banned in a number of opera houses, despite not actually existing. Philosophers feared that X-rays could reveal a person’s innermost self.
Hundreds of scientists around the world already had cathode ray tubes, a standard piece of equipment in physics labs. So, they first confirmed Röntgen’s discovery, and then set about putting the tubes to work, all in a matter of months. Within a year of his discovery, in 1896, X-rays were being used to find bone fractures and shrapnel in soldiers’ bodies on battlefields in the war between Italy and Abyssinia, and Glasgow Royal Infirmary had already set up the world’s first hospital-based X-ray imaging unit.
In other areas of society, business people capitalised on the capabilities of X-rays for other services. Popular at the time was the “pedoscope,” which made X-ray images of clients’ feet while they were trying on shoes, a practice later discontinued when evidence began to emerge that X-rays could sometimes cause damage to skin or tissue—an issue which we will return to later. Röntgen himself suggested another use by taking an image of metal weights inside an opaque box to show their potential use in industry. These early “radiographs” paved the way for modern security scanners found in airports.
As he had decided not to patent his discovery and potentially hinder its medical application, Röntgen didn’t see any income from all of this. He wisely left the responsibility of developing these techniques to the medical profession, claiming to be too busy with his other research, but continuing to offer his assistance where it was needed.
Röntgen might seem a strange character: a “lone genius” who made an “accidental discovery” out of nowhere. After all, anyone lucky enough to have a phosphor screen nearby could have stumbled on the same discovery. But if we look a little closer, there were other factors at play. He had access to a large network of experts around the world, had many years of experimental training and had cultivated a practice of patience and humility even in the midst of his excitement. When he noticed the glowing screen, he had the knowledge to realise its significance and the curiosity to dig deeper.
Despite all the hype, no one really knew what X-rays
were. Röntgen had shown that they didn’t have quite the same reflection or refraction properties as visible light, or the ultraviolet or infrared light beyond the usual visible spectrum. There was no clear idea of how X-rays were created from cathode rays, or how they interacted with other matter, like the phosphor screen. His discovery had raised a whole swath of new questions about what matter and light were made of and how they interact. Answering these questions required further experiments with the cathode ray tube, which continued to play a central role in the discoveries that came next.
In early 1897 in Cambridge, England, Joseph John (“J. J.”) Thomson, the founding director of the world’s pre-eminent physical laboratory, aimed to settle a twenty-year-old controversy. Instead of focusing on the X-rays outside the tube, he wanted to determine the composition of the glowing cathode rays inside the tube.
Thomson had an unpopular hypothesis. He believed that the cathode rays were some kind of corpuscle, or particle. This put him at odds with Röntgen who, with his German peers, thought that cathode rays were immaterial, a form of light.8 Thomson used the tubes available in his lab to study electricity in gases, but now he devised a new set of experiments designed to answer the question: what is the nature of cathode rays?
Thomson was the shy son of a Manchester bookseller, who announced at the age of eleven his intention to do original research. Where this precocious desire came from is unclear. His father passed away when Thomson was just sixteen, leaving no money for his education. Since no scholarships were available in physics, Thomson attended Trinity College, Cambridge, to study mathematics. There his quiet sense of humour—often expressed as a boyish grin—combined with his unshakeable intellectual self-confidence, frightened a number of his fellow students, who viewed him with almost a sense of awe.
Copyright © 2023 by Suzie Sheehy. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
