What Is the World Made Of?
How was the “electrical atom”—the electron—discovered?
Further advances came not from electrolysis but from studies of gases. In the 1700s and early 1800s physicists used the vacuum pump, invented by Otto von Guericke in 1690, to reduce the pressure in glass tubes fitted with electrodes to allow electricity to pass through the tubes. In 1838 Michael Faraday passed an electric current through such a tube and noticed a strange arc-shaped light starting at the cathode (negative electrode) and ending almost at the anode (positive electrode).
When Heinrich Geissler was able to reduce the air pressure to about 1/1,000 of an atmosphere he found that the tube was filled with a glow, like the neon lamps used today. By the 1870s William Crookes was able to reduce the pressure to 1/1,000,000 of an atmosphere. As the pressure was reduced the glow gradually disappeared. Instead the glass near the anode began to glow. Without air to disrupt their passage, rays of some sort were able to travel from the cathode to the anode. At the anode end they were going so fast that they caused the glass to glow or fluoresce. By coating the glass with zinc sulfide the glow was made brighter. The mysterious invisible rays could be shown to travel in straight lines by placing metallic objects in the tube and finding that they cast sharp shadows at the anode. Because the rays came from the cathode they were called “cathode rays.”
Joseph John (J.J.) Thomson (1856-1940) conducted three experiments with Crookes tubes that showed the nature of cathode rays. His first experiment used an electrode at one side of a tube, out of the direct path, which was connected to an electrometer that could detect electric charge. Thomson could deflect the path of the rays using a magnet and follow their path by observing the fluorescent glow on the tube’s surface. He found that the electrometer showed a negative charge, but only when the rays were deflected on to its terminal. Thus he showed that the rays consisted of a beam of negative particles.
Using a tube with the best possible vacuum, Thomson next explored the effect of an electric field on the rays. He added two parallel metal plates to the tube, connected a battery across the plates, and found that the rays were attracted toward the positive plate and away from the negative one.
In his third, and most important experiment, done in 1897, he combined the deflection of an electric field with one by a magnetic field. In doing so he could calculate the ratio of the mass to the charge of the particles. He found that this ratio was 1,800 times lower than that of a positively charged hydrogen ion. Thus the particles must be either very light or very strongly charged. He later showed that they had the same charge as the hydrogen ion, and so they were very light. Further experiments showed that the particles had the same properties no matter what metal was used for the cathode or whether the cathode was cold or incandescent. For his work Thomson was awarded the Nobel Prize in 1906.
The charge of the electron was measured in 1909 by American physicist Robert Andrews Millikan (1868-1953). Before Millikan’s experiments some physicists claimed that Thomson’s results could imply that electrons had an average mass-to-charge ratio given by his experiments, but that they could have a variety of masses and charges. But Millikan showed that all electrons had the same charge, and thus the same mass.