Big Ideas

IN CONTEXT

BRANCH

Physics

BEFORE

1838 Michael Faraday passes an electrical current through a partially evacuated glass tube, producing a glowing electric arc.

1869 Cathode rays are observed by Johann Hittorf.

AFTER

1896 First clinical use of X-rays in diagnosis, producing an image of a bone fracture.

1896 First clinical use of X-rays in cancer treatment.

1897 J J Thomson discovers that cathode rays are in fact streams of electrons. X-rays are produced when a stream of electrons hits a metal target.

1953 Rosalind Franklin uses X-rays to help her to determine the structure of DNA.

Like many scientific discoveries, X-rays were first observed by scientists studying something else – in this case, electricity. An artificially produced electric arc (a glowing discharge jumping between two electrodes) was first observed in 1838 by Michael Faraday. He passed an electrical current through a glass tube that had been partially evacuated of air. The arc stretched from the negative electrode (the cathode) to the positive electrode (the anode).

Cathode rays

This arrangement of electrodes inside a sealed container is called a discharge tube. By the 1860s, British physicist William Crookes had developed discharge tubes with hardly any air in them. German physicist Johann Hittorf used these tubes to measure the electricity-carrying capacity of charged atoms and molecules. There was no glowing arc between the electrodes in Hittorf’s tubes, but the glass tubes themselves glowed. Hittorf concluded that the “rays” must have come from the cathode, or negative electrode. They were named cathode rays by Hittorf’s colleague Eugen Goldstein, but in 1897, British physicist J J Thomson showed that they are streams of electrons.

Discovering X-rays

During his experiments, Hittorf noticed that photographic plates in the same room were becoming fogged, but he did not investigate this effect any further. Others observed similar effects, but Wilhelm Röntgen was the first to investigate their cause – finding that it was a ray that could pass right through many opaque substances. At his request, his laboratory notes were burned after his death, so we cannot be sure exactly how he discovered these “X-rays”, but he may have first observed them when he noticed that a screen near his discharge tube was glowing even though the tube was covered in black card. Röntgen abandoned his original experiment and spent the next two months investigating the properties of these invisible rays, which are still called Röntgen rays in many countries. We now know that X-rays are a form of short-wavelength electromagnetic radiation. They have a wavelength ranging from 0.01–10 nanometres (billionths of a metre). In contrast, visible light falls between the range of 400–700 nanometres.

The first X-ray image was taken by Röntgen of his wife Anna’s hand. The dark circle is her wedding ring. On seeing the image, Anna is said to have exclaimed: “I have seen my own death”.

Using X-rays today

Today, X-rays are produced by firing a stream of electrons at a metal target. They pass through some materials better than others, and can be used to form images of the insides of the body or to detect metals in closed containers. In CT (computed tomography) scans, a computer combines a series of X-ray images to form a 3D image of the inside of the body.

X-rays can also be used to form images of very small objects, and X-ray microscopes were developed in the 1940s. The image resolution that is possible when using light microscopes is limited by the wavelengths of visible light. With their much shorter wavelengths, X-rays can be used to form images of much smaller objects. Diffraction of X-rays can be used to work out how atoms in crystals are arranged – a technique that proved crucial in elucidating the structure of DNA.

WILHELM RÖNTGEN

Wilhelm Röntgen was born in Germany, but lived in the Netherlands for part of his childhood. He studied mechanical engineering in Zurich before becoming a lecturer in physics at Strasbourg University in 1874, and a professor two years later. He took senior positions at several universities during his career.

Röntgen studied many different areas of physics, including gases, heat transfer, and light. However, he is best known for his investigations into X-rays, and in 1901 he was awarded the first Nobel Prize in Physics for this work. He refused to limit the potential uses of X-rays by taking out patents, saying that his discoveries belonged to humanity, and gave away his Nobel Prize money. Unlike many of his contemporaries, Röntgen used lead protective shields in his work with radiation. He died from an unrelated cancer at the age of 77.

Key works

1895 On a New Kind of Rays

1897 Additional Observations on the Properties of X-rays