In this installment on the history of atom theory, physics professor (and my dad) Dean Zollman discusses the discovery of the electron. Although one gifted scientist got the credit, he had help. – Kim
By Dean Zollman
In the last decade of the 19th century, discoveries began new directions in our thinking about the composition of matter. Two of these discoveries – radioactivity and x-rays – were somewhat accidental. We will look at each of them in future posts. Another — the identification of the electron as a component of matter – was the result of careful research and the development of improved technologies. In this post, I will discuss the electron, how it was discovered, and some of the recent views about whether this research was really a discovery.
The generally accepted year for the “discovery” of the electron is 1897. However, this discovery had its roots in research and development that date to the first half of the 19th century. Because research such as this is always built on previous work, I have a difficult time knowing how far back to go. I have chosen to start the story with Heinrich Geissler (1814-1879).
Geissler was an instrument maker who, in 1857, created electric discharge tubes. These tubes were long, sealed glass cylinders and had metal electrodes at each end. Geissler connected a high voltage across the two electrodes and used another of his inventions, the vacuum pump, to decrease the pressure inside the tube. He found that the gas inside the tube would glow, with the color depending on what gas was trapped inside. The picture below shows a drawing, published in 1869, of several different Geissler tubes. These instruments might remind you of neon lights and they should. The tubes used in neon lights are modern variations on Geissler tubes.
Geissler conducted research to improve the tubes. He provided many tubes to other researchers and sold them to nonscientists for entertainment and decorative purposes.
The Cathode Ray
One of the researchers was William Crookes (1832-1919). Crookes improved on the tube and conducted many experiments. One of his conclusions was that something was being emitted from the negative electrode and was moving in a straight line to the positive end of the tube. Whatever was moving seemed to behave somewhat like rays of light. The negative end of an electrical device was called the cathode, so these “things” became known as cathode rays and the vessels were called cathode-ray tubes. (If you wish to see several photographs of Geissler and Crookes tubes, you should visit the Cathode Ray Tube Site.)
Just like light, the cathode rays could cast a shadow. In a famous experiment. Crookes inserted a Maltese cross in a tube. He saw that a shadow of the cross was cast on the end of the tube. However, the cathode rays in some ways acted differently from light. For example, they could be deflected by a magnetic field.
Two different views of cathode rays developed. Most British physicists concluded that the experiments indicated that the “rays” were some type of particle. Crookes proposed that they were negatively charged molecules. On the European continent, primarily in Germany, the light-like behavior led physicists to the conclusion that the rays were disturbances in the ether.
Each side had some experimental evidence to support its view. At the time, light was “known” to be a wave that traversed the ether, and all waves and other disturbances in the ether were assumed to travel at the same speed as light. However, cathode rays moved at much slower speeds. So this fact was an indication that the rays were not disturbances in the ether but particles. Also, on the particle side, the rays were deflected by a magnetic field, indicating that they had an electrical charge.
Atoms Have ‘Corpuscles’
Many of the prominent scientists who were involved in this debate also conducted work related to the model of the atom as a vortex in the ether that we discussed last month. One was John Joseph Thomson (1856-1940). Last month, I mentioned that he had written a theoretical paper on vortices in the ether. In 1884, he became the Cavendish Professor of Physics at Cambridge University where he undertook many experimental studies. In 1895, x-rays were discovered to be coming from a Crookes tube (more on that discovery next time). This result piqued Thomson’s interest in cathode rays. He set about to measure the ratio of the mass of a cathode ray to its electrical charge. A drawing and a picture of his apparatus are shown below.
In the drawing the cathode is labeled C. That is where the cathode rays are emitted. The item marked A is the positive electrode (anode) so the cathode rays are attracted toward it. But there is a slit in the anode, so some of the cathode rays go through the slit and continue their journey. Object B narrows the beam of rays that pass into the next region. D and E are metal plates that can be connected to a battery. Not shown in the drawing but visible in the picture are two coils of wire that can be used to create a magnetic field. With all of this equipment Thomson and his assistants could deflect the cathode rays up or down. Connect the positive side of the battery to D and negative to E and the rays move up. Reverse it and they move down. The magnetic field is a little more complex, but up and down motion can be created by the direction of the electrical current in the coils.
Thomson’s plan was to balance the electrical and magnetic forces so that the cathode rays went straight through his apparatus even though they were subject to both electric and magnetic forces. From voltages and currents, he could determine the magnitude of these forces. Then, doing some algebra with equations that had been developed during the 19th century he could come up with a value for the ratio of the mass of a cathode ray to its charge. He could not determine either the charge by itself or the mass by itself. His measurements allowed only a determination of the ratio.
However, that ratio was enough to indicate that cathode rays were something quite different from any known object. First, they were particles. A disturbance in the ether could not have been deflected in this way. Second, the ratio that Thomson measured was about 1,000 times different than he would have expected if cathode rays were atoms. However, he could not with the experiment determine which was different. The mass could have been 1,000 times smaller or the electric charge could have been 1,000 times bigger. (Thomson measurements were not all that good. Today we know the electron is about 1,800 times less massive than the hydrogen nucleus. But nothing anywhere close to this small had ever been measured, so being off by a large amount did not matter.)
Thomson bet on the mass being smaller. On April 30, 1897, at a public lecture he announced the discovery of “corpuscles,” which he said were very small constituents of all atoms. Over the next few years, he completed several other experiments, including one that enabled him to determine the mass of the corpuscles. Eventually, he built a model of the atom which included the corpuscles. But, I will save that for a later post.
In an earlier post, I mentioned George Stoney (1826 –1911) who coined the word electron in 1891. Others began to use that label for the cathode ray corpuscles, but Thomson did not. In his Nobel Prize acceptance speech (1906), Thomson referred to “carriers of negative electricity” as “corpuscles.” Eventually however, “electron” became the commonly accepted name.
So, Who Deserves the Credit?
Historians and philosophers of science have lots of discussion about the discovery of the electron. Lots of experiments led up to Thomson’s. And others were doing similar experiments at about the same time. So, should Thomson deserve credit for the “discovery” when it was just one step in a many-step process? Further the impact of Thomson’s announcement was not immediate. It took a while to soak in.
Some philosophers will use this example to debate what it means to discover something new. I don’t want to go there. Clearly Thomson’s work was an important step in our understanding of the structure of matter. It built on other people’s work, and others built on it. Some people were doing similar work at the same time. That’s the way science happens.
Next time we will look at something which was clearly a discovery – x-rays.
- In addition to being an excellent scientist, Thomson was also a gifted mentor. Seven of his research assistants and his son received Noble Prizes.
- The cathode-ray tube may seem like an esoteric device. However, until very recently almost all of us had at least one in our homes. Before flat screen televisions, the picture tube on our TVs was an advanced version of a cathode-ray tube. At the back was a device that accelerated electrons. It was quite similar to parts A and C in Thomson’s diagram. Then magnetic coils similar to the coils of wire in Thomson’s apparatus applied forces to steer the electrons to various locations on the front of the screen. Of course, doing this is such a way that we saw a picture required some technology that was not available until the 20th century. But the basic principles are much the same as they were when J.J. Thomson identified cathode rays as corpuscles that eventually came to be called electrons.
- You can try a virtual version of Thomson’s experiment. This one shows a drawing of modern equipment such as students would use today. In this simulation, changing the current changes the magnetic force while changing the voltage changes the electric force. Another has the same experiment, but it is set in apparatus similar to that of Thomson. For this one, E is the electric field, and B is the magnetic field.
Images via Wikimedia Commons.
Dean Zollman is university distinguished professor of physics at Kansas State University where he has been a faculty member for more than 40 years. During his career he has received four major awards — the American Association of Physics Teachers’ Oersted Medal (2014), the National Science Foundation Director’s Award for Distinguished Teacher Scholars (2004), the Carnegie Foundation for the Advancement of Teaching Doctoral University Professor of the Year (1996), and AAPT’s Robert A. Millikan Medal (1995). His present research concentrates on the teaching and learning of physics and on science teacher preparation.