See a recreation of the experiment that discovered the electron

Check out this awesome experiment that shows how J J Thomson began proving the existence of the electron. Thomson did this in 1897, despite the notable difficulty of electrons being much, much too small to see. (Sadly, that's still true today. And we say we've made progress.) We'll tell you how this simple demonstration indicates the presence of electrons, and how Thomson proved it once and for all.


J J Thomson received the Nobel Prize in 1906 for his discovery of what he had originally called "corpuscles." At the time he didn't have the opportunity to call them anything else, since all he knew about them is they were much, much smaller than an atom, and could apparently travel around independently of the atom itself. We see that tendency to travel in the above experiment. Thomson and his associates spent a lot of time in 1897 playing around with tubes like the one in the video. On one end is a cathode — a negatively charged electrode. On the other is an anode — a positively charged electrode. Thomson and his contemporaries saw a glowing beam emitted from the tube when a difference in electric potential was applied.

The tubes that Thomson was working with were not perfect vacuums, but did have most of the gas inside them pumped out. What gas remained was often in the line of fire for the electrons. The electrons, streaming toward the anode, slammed into the atoms of gas, knocking more electrons off of them. The electrons continued towards the anode. Thomson and his colleagues noted that a magnetic field applied to the beam would bend it, just as the magnet bends the beam in the video. By flipping the magnet and they would flip the direction of the bend.

To be fair, Thomson had to do more work than this to prove the existence of the electron. The vacuum in the tube is necessary in order to let the beam of electrons flow. The greater the vacuum, the more freely they move. Thompson and his contemporaries didn't have enough of a vacuum, and so the beam didn't seem to respond to an electric field placed across the stream of the beam. Thomson and his associates knew the beam should deform if it had an electromagnetic nature, and so assumed this was something different. It took more experimentation for Thomson to create a better vacuum and show the beam being deflected by an electric field. By experimenting with different strengths of electric fields, and observing the amount of deflection of the stream of electrons, Thomson was able to calculate the mass to charge ratio of the electron, and earned himself his Nobel.

Today we may not get a prize, but we do get a light show.

Via American Institute of Physics and DOE.



Nice article. This was a huge discovery. Prior to this most people thought that atoms were indivisible, the ultimate unit of matter. By showing that something could be knocked out of an atom Thomson showed this wasn't so. His first guess was that atoms were made entirely out of these "corpuscles", suspended in a uniform fog of positive charge. In 1911 New Zealand physicist Rutherford tested this theory by shooting alpha particles at a sheet of gold foil and measuring the scattering angles. Atoms made of the very light corpuscles would deflect the heavy alpha particles very little. Rutherford found, however, that some of the particles bounced back from the foil. He was dumfounded:

"It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

He concluded that the atom was composed of a very dense positively charged nucleus with a cloud of light negatively charged corpuscles around it. He then discovered that the nucleus is composed of smaller positive charges, later called protons, and the genie was out of the bottle. If the nucleus contains two or more positive charges it should fly apart, but it doesn't. He concluded there must be another force holding it together; this came to be called the Strong Nuclear Force. Thus, there must be a very large amount of energy stored in the nucleus. Rutherford thought that this energy was "bound" and could never be released:

"The energy produced by the breaking down of the atom is a very poor kind of thing. Anyone who expects a source of power from transformation of these atoms is talking moonshine."

In 1933 Hungarian physicist Leo Szilard conceived of the nuclear chain reaction to release this energy, and the rest is history. Thomson started it all.