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More particle physics pages

A Little Nut: The Discovery of the Atomic Nucleus
Nuclear Twins: The Discovery of the Proton and the Neutron
Probing the Proton
Particle Physics Today

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Smaller than the Atom

The ladies and gentlemen could not believe what they were hearing. "He's pulling our legs," they thought.

The date was Friday, April 30, 1897. The place was the lecture theater of the Royal Institute of Great Britain. The ladies and gentlemen of London had gathered there to hear the latest news from the world of science.

By the desk in the center of the theater stood a man with a pince-nez, a straggling mustache, and a receding hairline. He was Joseph John Thomson, director of the Cavendish Laboratory at the University of Cambridge, and one of the most respected scientists in Great Britain.

Earlier that year, Thomson told his audience, he had made a surprising discovery. He had found a particle of matter a thousand times smaller than the atom.

No wonder the audience thought Thomson was joking. In 1897, most scientists believed that all matter was made of indivisible particles called atoms. Atoms, by definition, were the smallest bits of matter that could exist. The idea of a particle one thousand times smaller was absurd.

Thomson himself didn't want to believe in these new particles. For many years he had firmly believed that atoms were indivisible. But he could not deny the results of his experiment. There were indeed particles smaller than atoms.

Thomson called his particles corpuscles, meaning "small bodies." Today we call them electrons, from the Greek word for amber. More than 2000 years ago, the ancient Greeks discovered that when they rubbed amber with fur, it would attract small objects like feathers. This is the earliest recorded observation of what we now call electricity.

Electrons are elementary particles, which means that, as far as we can tell, they are not made of anything smaller. The electron was the first elementary particle to be discovered. Thomson's experiment was the beginning of a new field of research--particle physics.

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A Mathematician Turns to Experiment

It was by chance that Thomson became a professor of physics. As a boy, he planned to be engineer. Because his family was unable to pay for his apprenticeship, he went to college instead.

Thomson studied engineering, but found he was more interested in mathematics. His mathematics professor encouraged him to apply for a scholarship at Cambridge University. Cambridge was, and still is, one of the best universities in the world. Thomson had never dreamed of being a student there.

On his second attempt, Thomson won the scholarship. It was the first of two turning points in his life.

The second turning point came in 1884. In that year, Lord Rayleigh retired as the Cavendish Professor of Experimental Physics. Thomson, who had worked with Lord Rayleigh in the lab, applied for the job. To everyone's surprise, including his own, he got it.

After taking over as Cavendish Professor, Thomson began to study the passage of electricity through gases. His careful measurements made him the world expert on the subject. His experience enabled him to settle a 35-year-old argument about the nature of cathode rays.

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Bullets or waves?

The story of cathode rays begins in 1855. In that year, Heinrich Geissler invented the mercury vacuum pump. With the pump he could remove almost all of the air from a sealed glass tube.

Geissler's friend Julius Plucker used the pump to evacuate a special kind of tube. Inside the tube were two electrodes. Plucker attached one electrode, called the anode, to the positive terminal of a battery. He attached the other electrode, the cathode, to the negative terminal. He noticed that the glass near the cathode glowed with greenish light. When Plucker held a magnet near the tube, the glowing spot moved.

Plucker's student, Johann Wilhelm Hittorf, put solid objects inside the tube between the cathode and the glow. The objects cast shadows. Hittorf concluded that the cathode was emitting something that travelled in straight lines, like light rays. The German physicist Eugen Goldstein named them "cathode rays."

Today, a tube that produces cathode rays is called a cathode ray tube. Your television screen is the end of a cathode ray tube.

What were cathode rays?

The English scientist William Crookes thought cathode rays were streams of molecules that had picked up a negative electric charge. Crookes knew from the laws of electricity and magnetism that a charged particle in a magnetic field would move in a circle. Since a magnetic field caused cathode rays to move in a circle, Crookes reasoned, they must be made of charged particles.

If cathode rays were streams of charged particles, an electric field also should have deflected their path. The German physicist Heinrich Hertz tested this hypothesis. He set a cathode ray tube between two metal plates. One plate was positively charged and the other was negatively charged. Negatively charged molecules should have been attracted to the positive plate.

When Hertz connected his tube to the battery, the cathode rays kept going in a straight line. Hertz concluded that the cathode rays were a new kind of electromagnetic wave.

Hertz's student, Philipp Lenard, designed a cathode ray tube with a thin foil at one end. The cathode rays went right through the foil. Since molecules of gas could not go through the foil, Lenard knew that cathode rays could not be charged molecules. He agreed with his teacher that they must be electromagnetic waves.

The French physicist Jean-Baptiste Perrin believed that cathode rays were particles. To prove it, he designed a special cathode ray tube to catch the rays. Inside the tube was a metal cylinder with a slit in it. When the tube was turned on, the cathode rays passed into the cylinder.

Perrin connected the cylinder to an electroscope. An electroscope is an instrument that measures electric charge. The electroscope showed that the cylinder became negatively charged when cathode rays went into it. This convinced Perrin that the cathode rays were charged particles.

Not so, argued Lenard. There might indeed be charged particles coming from the cathode, but they had nothing to do with the glow. He insisted that cathode rays were waves.

Into this controversy stepped J. J. Thomson. Thomson knew that only an experiment could convince scientists to change their minds. He designed a series of experiments to find out exactly what cathode rays were.

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A Surprising Result

Thomson began with an experiment to show that the cathode rays were indeed carrying electric charge. He set up a tube with a metal cylinder inside it, similar to the one Perrin had used. But Thomson did not put the cylinder directly in the path of the rays. Instead he used a magnet to deflect the rays. When the rays were not aimed at the cylinder, he measured no electric charge. When they were aimed into the cylinder, he found it was charged. Thomson concluded that whatever carried the negative charge always followed the path of the rays.

Next Thomson showed that a pair of electrically charged plates did indeed deflect the path of the cathode rays. From his work with gases, he knew that when cathode rays passed through a gas, the gas molecules would become electrically charged. Positively charged molecules would drift toward the negative plate, and negatively charged molecules would drift toward the positive plate. The charge on the molecules would cancel the charge on the plate, and the cathode rays would not feel it.

Even though the cathode ray tube was evacuated, there were still many air molecules left inside it. Thomson thought that if he could pump more gas out of the tube, the cathode rays would be able to feel the electric force.

For his experiment, Thomson designed a special tube. The cathode was at one end. Beside it was an anode with a slit in it. In the middle of the tube was a pair of metal plates, one above the other, like the slices of bread on a sandwich. On the end of the tube opposite the cathode, Thomson pasted a ruler.

When Thomson turned the tube on, the rays passed through the slit in the anode, and between the plates. By looking at the ruler, Thomson could measure the position of the fluorescent spot.

Thomson pumped as much gas as possible out of his tube. Then he connected one metal plate to the positive terminal of a battery. He connected the other plate to the negative terminal. He looked at the end of the tube. The fluorescent spot had moved!

Thomson did his experiment over and over. He tried using different batteries. He changed the connections on the metal plates. Each time, he observed a deflection of the cathode rays.

From his experiments, Thomson concluded that the cathode rays were streams of negatively charged particles. But what particles were they? Were they atoms, molecules, or something else? If he knew their mass and electric charge, Thomson would be able to answer that question.

Your mass tells how hard someone would have to push you to change your speed and direction of travel. People often confuse mass with weight, but it is not the same thing. Your weight is the force of gravity on you at a particular place and a particular time. If you were on the moon, you would weigh only one-sixth as much as you weigh on earth. If you were in a spaceship, far from any planets or stars, you would weigh nothing at all. Your mass doesn't depend on where you are. It is the same whether you are on the earth, on the moon, or in a spaceship.

Thomson could not measure the mass or charge of an individual particle. What he could measure was the deflection of the cathode rays in electric and magnetic fields. From these he could calculate m/e, the ratio of mass to electric charge. Since different molecules had different values of m/e, the measurement would tell Thomson what sort of particles he had found.

Thomson's result was about one thousand times smaller than m/e for a hydrogen ion. At the time, the hydrogen ion had the smallest m/e ever measured. Thomson concluded that the cathode ray particles were neither atoms nor molecules. They were a new sort of matter altogether.

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Decade of Discovery

The discovery of the electron was just one of the experiments that changed the way physicists think about atoms. When Thomson began studying cathode rays, physicists believed that atoms were indivisible and unchangeable. Atoms could not be created or destroyed. Every atom in the universe had been there from the beginning, and would remain until the end.

The first hint that this might not be true came in 1896. The French physicist Henri Becquerel was trying to find a connection between X-rays and phosphorescence.

Only a year earlier, Wilhelm Konrad Roentgen had discovered invisible rays that could go through black paper. Since he didn't know what they were, he called them X-rays. While experimenting with his X-rays, he found they passed more easily through some materials than through others. In particular, they passed more easily through flesh than through bone. Doctors could use X-rays to take pictures of people's bones.

Within a few months, scientists throughout Europe were studying X-rays. Among them was Henri Becquerel. Becquerel was convinced that X-rays were some kind of phosphorescence.

You are probably familiar with phosphorescence in the form of glow-in-the-dark toys. You know that if you leave these toys in a dark place for a long enough time, they will stop glowing. Phosphorescent materials cannot produce light on their own. They store light from another source, such as the sun, and emit it for a short period after exposure.

Becquerel knew that uranium salts were phosphorescent. He wondered if they were emitting X-rays. To find out, he wrapped a photographic plate in black paper. No sunshine could get through the paper. He put a piece of uranium salt on top of the wrapped plate, and set it in the sun. When he developed the plate, it had a dark spot on it. Uranium did seem to be emitting X-rays.

February 26, 1896, was too cloudy for Becquerel's experiment. He prepared a wrapped plate with the uranium salt, and put it in a drawer while he waited for sunshine. A few days later, he developed the plate. To his surprise, he found a spot where the uranium had been.

Becquerel knew that visible phosphorescence in uranium salts lasted only one one-hundredth of a second. Perhaps, he thought, the X-ray phosphorescence lasted longer. He left a uranium salt in the dark for two months.

When Becquerel set the salt on a plate, he found its image was just as bright as it had been two months earlier. He tested other uranium salts. All of them left images. Pure uranium had an even stronger effect. Becquerel concluded that all uranium must be emitting these rays.

Here was a new mystery for physicists to study. Among those experimenting with uranium rays was a student named Marie Sklodowska Curie. It was she who gave the spontaneous emission of rays the name by which we know it today--radioactivity.

In the process of trying to purify uranium for her studies, Marie Curie discovered two new radioactive elements, polonium and radium. In 1898, she found that the element thorium, like uranium, was radioactive. G. C. Schmidt discovered thorium radioactivity at the same time.

In Montreal, Canada, physicist Ernest Rutherford and chemist Frederick Soddy studied radioactivity in thorium. Rutherford and Soddy concluded that whatever was producing the rays was changing one kind of atom into another.

In less than ten years, the notion of the indivisible, unchangeable atom was gone forever. Physicists knew that atoms were constantly being created and destroyed. They knew that particles far smaller than atoms existed. They still believed that matter was made of atoms, but they knew that atoms were not elementary particles. What, they wondered, could atoms be made of?

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Thomson's atom

To Thomson, the answer was obvious. Atoms were made of electrons. By 1905, he and other physicists had done more experiments. They seemed to find electrons everywhere.

Electrons came from hot metals, from light shining on metals, from burning salts, and from radioactive substances. Since they were found in all kinds of matter, electrons had to be elementary particles.

Electrons could not be the only subatomic particles. Electrons are negatively charged. Atoms are electrically neutral. There had to be a positive substance to balance the negative charge. But what was the positive substance, and how did it fit into the atom?

The Japanese physicist Hantaro Nagaoka imagined the atom to be like the planet Saturn. He thought it had a positively charged ball in the center, with hundreds of electrons in orbit around it.

There was a problem with Nagaoka's picture of the atom. When a charged particle, such as an electron, moves in a circle, it radiates electromagnetic waves. As it radiates, it loses energy. Unless it can get energy from some other source, it will slow down and follow a spiral path into the center of the circle. A Saturnian atom would have collapsed.

The English physicist Lord Kelvin pictured the atom as a sphere of positive matter, with the electrons stuck in it like raisins in a cake, or plums in a pudding. People called this the "plum-pudding" atom. There were stable electron orbits in the plum-pudding atom.

Thomson thought Kelvin's picture of the atom was correct. He calculated some of the possible electron orbits. The plum-pudding atom came to be known as the "Thomson atom. "

Physicists did not know which picture of the atom was correct. They needed an experiment to give them more information. It took a genius to think of the right experiment. That genius was Ernest Rutherford.

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Go on to: A Little Nut: The Discovery of the Atomic Nucleus


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Thanks to Mrs. Irene M. McCabe of the Royal Institute of Great Britain for giving me the correct date of Thomson's announcement.

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