Free Radicals - Michael Brooks [22]
According to modern accounts of the history of science, it shouldn’t have been possible to hold this view in the twentieth century. In 1887, two American physicists, Abraham Michelson and Edward Morley, had performed an experiment which showed that the aether did not exist. They had been trying to measure how fast the Earth’s motion through space was moving us through the aether by finding the direction in which light moved fastest. The direction of motion of a point on the Earth’s surface is constantly changing as the Earth rotates and moves around the Sun. In the same way that you feel a wind when you move through the air, there should be an ‘aether wind’ as the Earth moves through the aether. And because the aether carries light, the Earth’s motion should produce a measurable difference in the speed of light coming from different directions. To their surprise, Michelson and Morley found no such difference: light had no ‘preferred’ direction. The only explanation was that there is no aether.
Michelson and Morley’s work is now cited as a classic experiment, but it has been given something of a positive spin. Because there is no aether, the plan to measure the speed of the Earth’s movement through it was doomed to failure. In science, a null result is not always reported, let alone shouted from the rooftops, and this experiment was very easy to ignore. It took decades for this null result to come to the attention of the international community. No one is sure whether, ten years on, the German physicists had even heard of Michelson and Morley’s experiment.
Millikan, though, would have been well aware of it: Abraham Michelson was Millikan’s boss. It doesn’t take great insight into human nature to surmise that Millikan knew his career would gain an immediate (and desperately needed) boost if he could measure e. If he could find the charge on an individual electron, he would cast more shadow on the aether theory, validating Michelson as well as Thomson. To a struggling junior researcher nearing the middle years of his career, the prospect must have been irresistible.
Millikan’s idea was simple. A droplet of water that had been given an electric charge would be attracted to a metal plate which carried an opposite charge. He arranged his apparatus so that the electrical attraction pulled the droplet up, while gravity pulled it down. This gave him a way to measure e. First he would find the mass of the droplet by measuring its size. Then he would measure how much voltage is needed for the attraction to the metal plate to cancel out the downward pull of gravity. From those two pieces of information he could get a measure of the charge on the droplet. Millikan guessed – correctly – that whatever the charge on the droplet, it would always be some integer (whole-number) multiple of the same number. That number would be e, the all-important charge on Thomson’s electron.
The experiment was far from simple to carry out, however. Millikan found that the water droplets tended to evaporate before any measurements could be made, so he set his graduate student Harvey Fletcher the task of trying the same trick with oil droplets. And that is where the real anarchy began.
When Millikan and Fletcher had refined the technique to the point where it looked as if it would work, Millikan elbowed his student off the project, promising him full credit for some other work. Even Millikan’s champion, the Caltech physics professor David Goodstein, admits that this was an act of enormous selfishness: ‘Millikan understood that the measurement of e would establish his reputation, and he wanted the credit for himself.’ Fletcher resigned himself to the shortcomings of junior status. ‘I did not like this, but I could see no other way out, so I agreed,’ he wrote in a memoir that he published only after Millikan’s death.
His unfortunate student safely sidelined, Millikan set about using a perfume atomiser and a can of watch oil to create the oil droplets. Some of the droplets would have electrons knocked off as