Happy centenary, electron!

It is one hundred years since a Cambridge scientist discovered the first sub-atomic particle, ending 2,000 years of belief in the indivisible atom. By Marcus Chown

A hundred years ago this month, the Cambridge physicist Joseph John Thomson discovered the electron. It was a dramatic moment in the history of science because the electron was the very first "sub-atomic" particle discovered, and its advent dealt a death blow to the 2,000-year- old idea that atoms were the smallest things.

The story of the discovery of the electron begins with attempts to understand the mystery of electricity. These attempts were severely hampered by the fact that electricity was always mixed up with the material through which it flowed - for instance, the copper in a copper wire. What was desperately needed was a way to observe electricity in its naked state.

A solution to the problem was inadvertently provided by a mechanic and glass-blower from Bonn in Germany, called Johann Geissler. In the middle of the 19th century he invented a "mercury air pump", which enabled him to suck the air out of a glass vessel until what remained was virtually indistinguishable from empty space. It was absolutely perfect for studying electricity in the raw, a fact immediately recognised by Julius Plucker, a physicist whose daily walk to the University of Bonn happened to take him past Geissler's workshop.

In 1858, Plucker commissioned Geissler to make him a long, thin glass tube with two metal plates, or "electrodes", sealed inside. The electrodes, one at either end, stuck through the glass wall so that they could be connected to a source of electricity on the outside. What Plucker saw when he made a current leap from electrode to electrode along the tube was an eerie greenish glow on the glass near one of the electrodes. Plucker's student Johann Hittorf demonstrated that the glow was caused by invisible "rays" which stabbed outwards from the other electrode. The rays cast sharp shadows, which indicated that they travelled in straight lines; and they could be bent by a magnet, which indicated that they were electrically charged.

The greenish light evidently marked the spot where the rays slammed into the wall of the tube. Hittorf christened the rays "glow rays", but the name which stuck was "cathode rays", since the rays came from the electrode universally known as the cathode.

Nowadays everyone has direct experience of cathode rays, since they are responsible for painting the moving picture on our television screens. But as the 19th century drew to a close, cathode rays were one of the outstanding mysteries of science.

Two theories were put forward to explain them. One, promoted by scientists in Germany, was that cathode rays were some kind of "wave in space" not unlike light. The other view, championed by scientists in Britain, was that they were streams of tiny negatively charged particles.

It was with the aim of settling the question once and for all that "J J" Thomson, a scientist reputedly so ham-fisted that his students never let him near their apparatus, embarked on the experiment that would make his name in April 1897.

The details of that experiment are not very interesting. Suffice to say that Thomson demonstrated beyond any doubt that cathode rays, and hence electricity, did indeed consist of tiny bullet-like particles. The particles were soon christened "electrons". Torn from atoms in their countless billions, they streamed through space as cathode ray or drifted along a copper wire as an electric current.

The nature of electricity was finally understood. But the discovery of the electron had ramifications far beyond solving this mystery. For when Thomson measured the mass of his new particle, he found that it was thousands of times smaller than even the smallest atom. The existence of the electron therefore dealt a devastating blow to the 2000-year-old idea that atoms were the smallest things.

The idea of atoms had actually been invented by the Greek philosopher Democritus. In 440 BC, he had held up a clay tablet, or it might have been a stick or a stone, no one quite knows, and asked himself the question: "What if I cut this in half and half again - can I go on forever?" Democritus's answer had been an emphatic No. It was absolutely inconceivable to him that matter could be subdivided forever. Sooner or later, he reasoned, the cleaving process would result in a grain of matter that could not be made any smaller.

Since the Greek phrase for "uncuttable" was a-tomos, Democritus christened the indestructible grains out of which everything was made "atoms".

Thomson, by showing that the atom was made of smaller things, had cut the uncuttable. It spelt the beginning of the end for the indestructible atom.

In the 20th century, the atom has come apart repeatedly. First it split into a nucleus and electrons, then the nucleus itself came apart into protons and neutrons, then the protons and neutrons disintegrated into quarks.

Some scientists are convinced that with quarks we have reached the end of the road and that we have really found the ultimate building blocks of all matter.

Others are not so sure. They fear that quarks too will be found to be made of smaller things and that as we probe deeper and deeper into the atom, it will continue to come apart into smaller and smaller constituents like an infinite series of Russian dolls.

The truth is that nobody can honestly say where it will all end. But where it all began is absolutely certain: 100 years ago this month with Thomson's discovery of the electron.