Charge of the particle brigade

Today, cathode rays routinely illuminate our television screens, yet their discovery in the middle of the last century plunged physics into an intense scientific and intellectual struggle. It took 40 years of painstaking research and toil before they were finally understood, culminating in the discovery of the electron by J. J. Thomson exactly 100 years ago.

Per Dahl vividly captures the sense of this struggle in his new history of the discovery of electron, Flash of the Cathode Rays. This is an atmospheric book, which draws on a wide spectrum of original archives, bringing together a wealth of biographic and technical detail about the cast of characters and the discoveries they made.

Cathode rays were discovered at Bonn in 1858, by Julius Plucker, a mathematician-turned-physicist working in collaboration with a gifted instrument-maker and glass-blower, Johann Geissler. Electrical discharges in rarefied gases were already well known, but Geissler devised a new type of vacuum tube which attained lower pressures.

With this new set-up, Plucker observed a narrow beam of violet-coloured radiation from the negative electrode (cathode) which he could deflect using a magnetic field.

Plucker's discovery rapidly captured the imagination of two very different physics communities in England and Germany. Dahl brings out the stark contrast between these groups. It was the Germans, working in the world's first publicly-financed physics laboratories, who made the pioneering experiments on cathode rays. They showed that one can cast a shadow in the cathode rays, and coined the term Kathodstrahlen or "cathode ray". The German school believed the cathode rays were an electromagnetic wave that carried no charge. Around this time, the beautiful colours of gas discharges also caught the imagination of a clique of part-time Victorian physicists in Britain. The participation in these experiments of a retired stationery manufacturer, the head of a printing business and a prominent wine merchant makes an almost comical contrast with their professional counterparts on the continent. In 1870, Cromwell Varley a retired telegraph engineer and spiritualist, described cathode rays as: "composed of attenuated particles of matter, projected from the negative pole by electricity in all directions, but that the magnet controls their course''.

One cannot help noticing the admiration Dahl feels for this colourful cast of characters, who freed from the constraints of an overly rigorous training, were the first to arrive at the correct charged-particle interpretation of cathode rays.

Why did the German school fall so quickly under the spell of the electromagnetic wave interpretation of cathode rays? Dahl emphasises how this view is blatantly at odds with their experiments, but we are not offered any insight into why they preferred the "ray" interpretation. This is an unfortunate failing of the book, for it is clear that these early viewpoints set the tone for all subsequent investigations, placing the German school at a great disadvantage.

Thus when experiments by Heinrich Hertz in 1883 failed to observe a deflection of these rays by an electric field, this was viewed as further confirmation of their neutral character, when in fact it proved to be an artifact that disappeared at lower pressures.

Popular history ascribes 1897 as the year in which the electron was discovered by J. J. Thomson. This was the year in which it became clear that the particles inside the cathode ray have a ratio of mass to charge (m/e), that is thousands of times smaller than a hydrogen ion. Yet a long cast of characters whose contributions have been often understated, came remarkably close to the same discovery. Dahl provides an exciting description of the build-up to the critical events of 1897, and the near-miss discoveries that occurred in that year. We learn that in 1890, Arthur Schuster, professor at Manchester University and a long-time rival of Thomson, came very close to the discovery of the electron, making the first measurement of (m/e). Unfortunately, the inaccuracies of Shuster's results was too great for him to notice the anomalously small value of (m/e).

The use of the term "electron" to refer to the basic unit of charge actually predates its discovery as a particle, for this term was first coined in 1891, by an Irish physicist, Johnson Stoney. Six years later, in January 1897 it was not Thomson, but Emil Weichart in Konigsburg, who first noted the tiny value for m/e which he boldly interpreted in terms of particles he called "elektrons", with a mass 2,000 to 4,000 times less than that of the hydrogen atom. Thomson's approach was more cautious, but won by virtue of the unambiguous nature of his results. When he reported on a parallel set of results to the Royal Society on April 30 1897, he only tentatively suggested that the cathode-ray particles, which he called "corpuscles", might be much smaller than atoms. Thomson radically improved the measurements later that year by passing the cathode ray through a crossed electric and magnetic field, and it was from these experiments, described in his definitive paper of October 7, that he drew the conclusion that "The carrier, then, must be small compared with ordinary molecules."

During the following year Thomson finally removed all ambiguities in this interpretation, by experimentally confirming that the magnitude of the electron charge was indeed the same as a hydrogen ion. In the second part of his book, Dahl follows the wake of the discovery of the electron up until Rutherford's discovery of the structure of the atom. There is much additional information here that will appeal to the historian, but some may feel that too much of this material is peripheral to the main history. We learn how ambiguities in the early measurements of the electron charge lead to a raging controversy between Felix Ehrenhaft and Robert Millikan over the possibility of still smaller units of charge, or "sub-electrons", a controversy that was dispelled by the precision of Millikan's oil-drop experiment. Dahl also devotes a chapter to the French fiasco with "N-Rays", that purported to come from animate matter, but which were later found to be a figment of the imagination.

Dahl's effort to open the history of the electron to a broader audience deserves great admiration, yet it falls short of providing a definitive history by declining to analyse and discuss the parallel evolution of concepts and ideas during this period. Nevertheless, those seeking new biographical insight, particularly into the early stages of the cathode-ray history, will be richly rewarded; the large body of archival material referred to here makes this an incomparable source book for all those interested in this exciting era of discovery.

Piers Coleman is professor of physics, Rutgers University, New Jersey, United States.