On J.J. Thompson Essay, Research Paper

J.J. Thomson

Science lecturers who traveled from town to town in the middle nineteenth century delighted audiences by showing them the ancestor of the neon sign. They took a glass tube with wires embedded in opposite ends . . . put a high voltage across . . . pumped out most of the air . . . and the interior of the tube would glow in lovely patterns. In 1859 a German physicist sucked out still more air with an improved pump and saw that where this light from the cathode reached the glass it produced a fluorescent glow. Evidently the cathode emitted some kind of ray that was illuminating the glass.

What could these rays be? One possibility was that they were waves traveling in a hypothetical invisible fluid called the ether (similar to the quintessence of Aristotle). At that time, many physicists thought that this ether was needed to carry light waves through apparently empty space. Maybe cathode rays were similar to light waves? Another possibility was that cathode rays were some kind of material particle. Yet many physicists, including J.J. Thomson, thought that all material particles themselves might be some kind of structure built out of ether, so these views were not so far apart.

Experiments were needed to resolve the uncertainties. When physicists moved a magnet near the glass, they found they could push the rays about. Nevertheless, when the German physicist Heinrich Hertz passed the rays through an electric field created by metal plates inside a cathode ray tube, the rays were not deflected in the way that would be expected of electrically charged particles. Hertz and his student Philipp Lenard also placed a thin metal foil in the path of the rays and saw that the glass still glowed, as though the rays slipped through the foil. Did that not prove that cathode rays were some kind of waves?

Other experiments cast doubt on the idea that these were ordinary particles of matter, for example gas molecules as some suggested. In France, Jean Perrin had found that cathode rays carried a negative charge. In Germany, in January 1897 Emil Wiechert made a puzzling measurement indicating that the ratio of their mass to their charge was over a thousand times smaller than the ratio for the smallest charged atom. When Lenard passed cathode rays through a metal foil and measured how far they traveled through various gases, he concluded that if these were particles, they had to be very small.

Drawing on work by his colleagues, J.J. Thomson refined some previous experiments, designed some new ones, carefully gathered data, and then . . . made a bold speculative leap. Cathode rays are not only material particles, he suggested, but in fact the building blocks of the atom: they are the long-sought basic unit of all matter in the universe.

One hundred years ago, amidst glowing glass tubes and the hum of electricity, the British physicist J.J. Thomson was venturing into the interior of the atom. At the Cavendish Laboratory at Cambridge University, Thomson was experimenting with currents of electricity inside empty glass tubes. He was investigating a long-standing puzzle known as “cathode rays.” His experiments prompted him to make a bold proposal: these mysterious rays are streams of particles much smaller than atoms, they are in fact minuscule pieces of atoms. He called these particles corpuscles, and suggested that they might make up all of the matter in atoms. It was startling to imagine a particle residing inside the atom — most people thought that the atom was indivisible, the most fundamental unit of matter.

Thomson’s speculation was not unambiguously supported by his experiments. It took more experimental work by Thomson and others to sort out the confusion. The atom is now known to contain other particles as well. Yet Thomson’s bold suggestion that cathode rays were material constituents of atoms turned out to be correct. The rays are made up of electrons: very small, negatively charged particles that are indeed fundamental parts of every atom.

But, do atoms have parts? J.J. Thomson suggested that they do. He advanced the idea that cathode rays are really streams of very small pieces of atoms. Three experiments led him to this:

First, in a variation of an 1895 experiment by Jean Perrin, Thomson built a cathode ray tube ending in a pair of metal cylinders with a slit in them. These cylinders were in turn connected to an electrometer, a device for catching and measuring electrical charge. Perrin had found that cathode rays deposited an electric charge. Thomson wanted to see if, by bending the rays with a magnet, he could separate the charge from the rays. He found that when the rays entered the slit in the cylinders, the electrometer measured a large amount of negative charge. The electrometer did not register much electric charge if the rays were bent so they would not enter the slit. As Thomson saw it, the negative charge and the cathode rays must somehow be stuck together: you cannot separate the charge from the rays.

All attempts had failed when physicists tried to bend cathode rays with an electric field. Now Thomson thought of a new approach. A charged particle will normally curve as it moves through an electric field, but not if it is surrounded by a conductor (a sheath of copper, for example). Thomson suspected that the traces of gas remaining in the tube were being turned into an electrical conductor by the cathode rays themselves. To test this idea, he took great pains to extract nearly all of the gas from a tube, and found that now the cathode rays did bend in an electric field after all.

Thomson concluded from these two experiments that he could do nothing but conclude that cathode rays were particles of matter carrying a charge of negative electricity. The question still remained, however: are they atoms, or molecules, or matter in a still finer state of subdivision?

Thomson’s third experiment sought to determine the basic properties of the particles. Although he could not measure directly the mass or the electric charge of such a particle, he could measure how much the rays were bent by a magnetic field, and how much energy they carried. From this data he could calculate the ratio of the mass of a particle to its electric charge. He collected data using a variety of tubes and using different gases.

The results were astounding. Just as Emil Wiechert had reported earlier that year, the mass-to-charge ratio for cathode rays turned out to be over one thousand times smaller than that of a charged hydrogen atom. Either the cathode rays carried an enormous charge (as compared with a charged atom), or else they were amazingly light relative to their charge.

Philipp Lenard settled the choice between these possibilities. Experimenting on how cathode rays penetrate gases, he showed that if cathode rays were particles they had to have a very small mass — far smaller than the mass of any atom. The proof was far from conclusive. But experiments by others in the next two years yielded an independent measurement of the value of the charge and confirmed this remarkable conclusion.

Thomson boldly announced his hypothesis that. He said that in the cathode rays, we have found a new state of matter, a state in which the subdivision of matter is carried beyond the ordinary gaseous state: a state in which all matter is of the same kind. This new form of matter being the substance from which all the chemical elements are built up.

Works Cited

1) “Thomson, Sir Joseph John.” Microsoft Encarta 97 Encyclopedia. 1993-1996 Microsoft Corporation.

2) “Thomson, Joseph (1856-1940)”. Compton’s Interactive Encyclopedia. 1993, 1994 Compton’s NewMedia, Inc.

3) Brazil, Georgia L, and Moore, Dan. “History in Chemistry”. Volume Library, Volume 1. Southwestern/ Great American Inc., Nashville, Tennessee, 1998. p. 472-473.