Sir Joseph John “J. J.” Thomson (1856-1940) was born in Manchester, England. Showing early interest in technical matters, he studied engineer­ing at the University of Manchester in 1870 and then moved to Trinity Col­lege, Cambridge, in 1876 to study mathematics. In 1880, he earned a B. A. degree (Second Wrangler) and an M. A. in 1883. In 1884, he became Caven­dish Professor of Physics, in 1890, he married the daughter of the Regius Professor of Physics at Cambridge, and in 1897, he analyzed the atom into component parts, sending atomic science bounding in new directions.

Thomson was interested, as were many of his fellow physicists, in the mystery of the cathode rays. He built more sophisticated, more compli­cated glass tubes, in which he electrically accelerated the ray from the tube’s negative electrode through holes drilled in positive electrodes, sending the beam gliding through the deep vacuum beyond the electrodes and to the far end of the tube, where it would hit a fluorescent screen and cause a small spot to glow. He found that he could deflect the thin cathode ray streaming through the hole in the positive electrode using a magnet at the side of the tube.

To investigate the nature of the cathode rays, Thomson devised three, sequential experiments. The cathode rays obviously involved a negative charge, as they originated at the negative electrode and vanished into the positive electrode, and for his first experiment Thomson wanted to know whether the negative charge could be separated from the rays. He built a special variant of his tube, blowing a thin, wide beam of cathode rays through a slit in the positive electrode. This beam would traverse the tube, unencumbered by air molecules, and hit a third electrode at the end of the tube. He connected an electrometer to the electrode to measure the charge from the cathode rays and confirmed that there was an elec­trical current flowing between the negative electrode origin of the rays and his target electrode. The target electrode had a slit cut in it, off the straight axis of the beam. With the tube operating at full power, Thomson adjusted a horseshoe magnet across the length of the ray’s flight path,

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throwing the beam into a downward turn. He aimed it for the hole in the third electrode. The rays missed his electrode and hit the wall of the tube, causing fluorescence. At that point, the electrical current stopped register­ing on his electrometer. Thomson concluded that the electrical charge and the rays were one and the same, and that one could not be separated from the other.

Thomson suspected that he knew the nature of the mysterious rays, but he set up experiment number two for a stronger case. If the rays were purely electrical charge in motion, then he should be able to bend the rays with a stationary electrical charge. He set up another tube, this time with a thin beam established at one end of the tube and shot through a couple of parallel metal plates, against a fluorescent screen at the far end of the tube. This experiment had been tried several times by others with no results, but Thomson thought he knew why. The ray must have been crashing into gas particles left in the tubes, because of imperfect vacuums. Thomson made

certain that his tube was pumped all the way down. He put the 30,000 volts on the negative and positive electrodes, turned down the lights, and observed his bright spot on the end of the tube, as he charged the parallel plates from a battery. Just as he had thought, the beam deflected away from a negative electrical charge and toward a positively charged plate. He could not see the beam itself, but he could watch as the spot of light changed position on the end of the tube. Knowing the angle of deflection of the beam and the voltage required to do it, he was able to calculate the ratio of charge to mass of the particle he suspected made up the beam.

There was one more experiment needed. Thomson repeated the beam- deflection measurement using a magnetic field instead of an electrical field to bend the beam, and again he calculated the ratio. He was then prepared to make a bold, sweeping conclusion: The cathode rays were composed of tiny negatively charged particles he called “corpuscles,” which were stripped-off atoms in the negative electrode and thrown down the length of the tube. He went further, to propose that, because matter was naturally without electrical charge, the rest of the atom, with the electrons stripped off, had to be positively charged, so that it would cancel the negative charge of his corpuscles. He imagined that the tiny lightweight electrons were stuck in a relatively large, soft ball of positive charge. It was called the “plum pudding” model of the atom, and it would do nicely for the time being.

J. J. Thomson’s corpuscles would later be named electrons, and he would be awarded the Nobel Prize in physics in 1906 for this important discovery.