In April 1897, J.J. Thomson told the Royal Institution that he had found something smaller than an atom. The reaction in the room was polite scepticism. One senior colleague told him afterwards that he had assumed the whole talk was an elaborate joke.
The reaction was reasonable. The atom had been defined as indivisible since the Greeks, and Dalton’s chemistry had been built on that assumption for ninety years. Thomson was claiming that every atom of every element contains identical particles roughly a thousand times lighter than the lightest atom known. It took about two years for the evidence to become undeniable.
The Cathode Ray Problem
The story begins with a piece of laboratory equipment that had become common in the second half of the nineteenth century. Take a sealed glass tube, pump most of the air out, and apply a high voltage between two electrodes inside it. Something travels from the negative electrode, the cathode, to the positive one, and makes the glass at the far end glow.
These cathode rays had been studied for decades. They travel in straight lines, they cast sharp shadows of objects placed in their path, they can turn a small paddle wheel, and they are deflected by a magnet.
What they were was disputed, and the dispute had a national character. Most German physicists, including Goldstein, Hertz, and Lenard, held that they were some kind of disturbance in the ether, comparable to light. Most British physicists, including Crookes and Schuster, held that they were streams of charged particles.
The strongest evidence for the wave view was an experiment by Heinrich Hertz in 1883. Charged particles should be deflected by an electric field. Hertz passed cathode rays between charged metal plates and observed no deflection at all. If they carried charge, they should have bent, and they did not.
Thomson’s Three Experiments
Thomson had become director of the Cavendish Laboratory at Cambridge in 1884, at twenty seven, to general surprise. In 1897 he attacked the cathode ray question with a sequence of three experiments, each answering a specific objection.
First, he tested whether the charge could be separated from the rays. He used a magnet to bend the beam into a metal cylinder connected to an electrometer. Whenever the beam entered the cylinder, negative charge registered. When the beam was bent away, the charge stopped. Whatever carried the charge and whatever produced the glow were travelling together and could not be separated.
Second, he returned to Hertz’s failed deflection. Thomson suspected the problem was residual gas in the tube: the rays ionise it, the resulting charges collect on the plates, and the field inside is cancelled out. He worked at getting a much better vacuum, and at low enough pressure the beam bent towards the positive plate exactly as a stream of negative particles should. Hertz had not been wrong about what he saw. His pump had not been good enough.
Third, and most importantly, he measured. By balancing the electric and magnetic deflections against each other he could calculate the ratio of mass to charge for whatever the rays were made of.
The Number That Mattered
The result was startling. The mass to charge ratio came out roughly a thousand times smaller than that of the hydrogen ion, the smallest value known in electrochemistry. Later refinement put the factor at about 1,836.
Two interpretations were available: the particles carry an enormous charge, or they are extraordinarily light. Thomson argued for the second, and subsequent measurements of the charge itself, culminating in Millikan’s oil drop experiment, confirmed it.
The second finding was equally significant and easier to establish. Thomson varied the gas in the tube and the metal of the electrodes. He used air, hydrogen, carbon dioxide, aluminium electrodes, iron electrodes, platinum electrodes. The ratio never changed.
Whatever these particles were, every material produced identical ones. They were not a property of any particular element. They were a component of all matter.
Corpuscles
Thomson called them corpuscles and continued to do so for years. The word electron had been coined by George Johnstone Stoney in 1891 for the fundamental unit of electric charge, and physicists gradually attached it to Thomson’s particle instead. He resisted the change, without success.
The conclusion he drew in his 1897 paper is stated plainly and is worth quoting in substance: matter in different states is of one and the same kind, and this substance is the material from which all chemical elements are built. Having established that atoms have parts, he had also opened the possibility that the elements are not fundamentally different substances but different arrangements of the same components.
The Plum Pudding Model
An atom is electrically neutral, so if it contains negative corpuscles it must contain positive charge as well. In 1904 Thomson proposed a structure: the electrons are embedded in a diffuse sphere of positive charge, distributed in rings whose arrangement would determine the chemical properties of the element.
The nickname refers to a traditional English pudding with fruit suspended in it. The model was a serious attempt, not a placeholder. Thomson worked out stable ring configurations mathematically and tried to connect them to periodicity in the elements.
It was disproved by his own former student. In 1909, working in Manchester, Ernest Rutherford’s assistants Geiger and Marsden fired alpha particles at gold foil and found that a tiny fraction bounced almost straight back. In a diffuse pudding of positive charge, nothing could produce such a violent deflection. The positive charge had to be concentrated in a minute nucleus, and Rutherford said so in 1911.
From there the path runs to Bohr’s quantised orbits in 1913 and to quantum mechanics proper in the following decade.
The Cavendish Under Thomson
Thomson’s second career was as a builder of physicists. Under his direction the Cavendish became the most productive physics laboratory in the world. Seven of his research assistants went on to win Nobel Prizes, as did his son.
That son, George Paget Thomson, received the prize in 1937 for demonstrating that a beam of electrons produces diffraction patterns when passed through a thin crystal, which is behaviour characteristic of waves. The father won the Nobel Prize in 1906 for showing that the electron is a particle. The son won it thirty one years later for showing that it is a wave.
Both results are correct, which is the central strangeness of quantum mechanics rather than a contradiction. Louis de Broglie had proposed in 1924 that all matter has an associated wavelength, and the Thomson experiments bracket the transition from classical particles to quantum objects almost perfectly.
The Discovery in Context
1895 to 1897 was an extraordinary period. Röntgen found X-rays in November 1895. Becquerel found radioactivity in early 1896. Thomson found the electron in 1897. Within thirty months, physics acquired three phenomena that classical theory could not accommodate.
All three came from the same corner of the laboratory. X-rays were discovered while working with a cathode ray tube. Radioactivity was discovered while trying to connect X-rays to phosphorescence. The electron was discovered by asking what cathode rays actually are. A single piece of glassware, developed to study electrical discharge in gases, produced the raw material for modern physics. It is one of a small number of instruments that changed what humans could see.
The theoretical consequences arrived in parallel, most importantly in Planck’s quantum hypothesis of 1900. Our edition of Planck’s three key publications gathers the works in which the classical picture was formally abandoned, written by a man who spent years trying to avoid that conclusion.
What Thomson Established
The specific claim, that cathode rays are streams of light negatively charged particles, was settled quickly once the measurements were repeated.
The general claim mattered more. Atoms have internal structure. They contain identical components regardless of which element they belong to. The chemical elements are therefore not irreducibly distinct kinds of matter, and the divisions in the periodic table must arise from differences in arrangement and number.
Every subsequent development in atomic physics depends on that. It began with a tube of rarefied gas, a magnet, two charged plates, and a physicist who suspected that a famous negative result had been caused by a leaky pump.
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