Electrifying history

How did we discover the electron?
06 August 2019

Interview with 

Katy Duncan, University of Cambridge


Atom cartoon


The word electron comes from the ancient Greek word for amber. If you rub a piece of amber, you can generate static electricity, a bit like rubbing a balloon on your head. Some Greek physicians even suggested putting electric eels on your head to cure migraines - an early version of electrotherapy. But although we have known about electricity for a while, the electron itself, wasn’t discovered until 1897. Ankita Anirban took a trip to the Cavendish Laboratory in Cambridge to speak with Katy Duncan, from the Department of History and Philosophy of Science in the University of Cambridge, to find out how this came about...

Ankita - We've come to the Cavendish Lab which is where J. J. Thompson discovered the electron. And we’re walking down this corridor, and there's lots of portraits of people in the Cavendish, and we’re slowly going back in time. Lots of men in suits in the 20s. And the first one is 1919 with Professor Sir J. J. Thompson right in the middle.

We walked through the museum until we came to a case which held a glass tube. It was about the length of my arm and maybe twice as wide, with large bubbles along it. This was the famous cathode ray tube used in the discovery of the electron. You might remember old TVs with huge boxes at the back. These held cathode ray tubes that were used to generate the images on the screen. And in fact even today we still use cathode ray tube to produce X-rays. I spoke to Katy Duncan from the Department of the History and Philosophy of Science at Cambridge about how they work.

Katy - So if you've ever seen a neon light this is very similar to what these gas discharge tubes are. They tend to be long glass tubes with an electrode at either end, one of them is positive and one of them is negative, and you connect them up to a battery. When you put a voltage across it, whatever is in that tube, they found that it would glow. And that's essentially what a cathode ray tube is.

Ankita - So how did cathode ray tubes come about? J. J. Thompson made his famous discovery of the electron in 1897. What did we know about electrons - or rather, I should say, electricity - before that?

Katy - The 1800s were a period where we see understandings of what we can do with electricity massively increase. So we could develop formulas and ways to manipulate it, and we could use it, such as the electric telegraph. But what electricity actually was? People didn't really know. There were understandings that there were two kinds of charge, but how that manifested wasn't really known. Whether that was an electrical fluid, two electrical fluids... it was all really quite up in the air. At the time lots of physicists were interested in finding out about the electrical conductivity of gases at low pressure, because usually in air electricity doesn't travel anywhere - otherwise we'd be electrocuting ourselves all the time as we walked through the atmosphere. If you had a low pressure tube that you filled with gas you could actually put a current across it. So this is what they were investigating; the electron just happened to be something, almost as an accident. 

Ankita - How do you discover the electron by accident?

Katy - When he was experimenting with this tube he took it to a very, very, very low pressure, which is something that other physicists at the time had been trying to attempt. And when he did this he found that there was a shadow that was being cast behind the anodes at the positive end, which indicated that something was travelling in a straight line from the negative end to the positive end. And the bits that weren’t hitting the anode were hitting the surrounding glass and glowing. So he investigated what this ray was by putting an electric field around it and a magnetic field around it, and also examining how much heat that this ray itself was generating. And he used a Lorentz force law to calculate a mass-to-charge ratio.

Ankita - Electricity and magnetism are two sides of the same coin. And the Lorentz force is a law which links the two. It states that if you place an electric particle in a magnetic field it will be deflected at an angle; the amount of deflection depends on the charge and the mass of the particle. By measuring this deflection, J. J. Thompson could measure the charge-to-mass ratio of the electron.

Katy - And in doing so he found that this mass was a thousand times smaller than the size of the smallest known atom at the time, which was hydrogen. So this started to make some interesting questions about whether there was something smaller than the atomic, whether there was something sub-atomic.

Ankita - So he effectively discovered that the atom - which we had previously thought of as the fundamental unit of matter, something that couldn't be broken down any further - was in fact made up of even smaller particles. How did that fit into the existing theories at the time?

Katy - Well there were lots of different theories at the time. J. J. Thompson didn't actually call what he found the electron, he called it a corpuscle. And he believed that this corpuscle was in fact the only sub-constituent of an atom. And he devised what we know now as a plum pudding model. So these were small, hard electrons that existed in a sea of positive charge, and they could move around freely, and then they could be made to leave this plum pudding model under the presence of something like a high electric charge. And that's what we saw in the cathode rays.

Ankita - But the usual picture of the electron today is not like a plum pudding. Most of our science textbooks now describe electrons as orbiting the nucleus of an atom, a bit like planets around the sun. So where did this theory come from?

Katy - That was only a few years after Thompson came up with his plum pudding model. Thompson had a student called Ernest Rutherford and he undertook a few groundbreaking experiments, the results of which didn't actually fit with any theory that the physicists at the time had. Rutherford demonstrated that atoms are actually the mostly empty space model that we know today.

Ankita - Rutherford did this by firing tiny positively-charged particles at a gold foil. He found that the vast majority of the particles made it through, and only a handful bounced back. And this led him to believe that most of the atom was in fact empty space with a small positive nucleus in the middle and a cloud of negative electrons circling it. And this is a model that's still taught in schools today.

Science is rarely simple and not everyone at the time accepted this model.

Katy - It took a good number of years for the scientists to come around to the idea that it was indeed a particle. So it went through this very fluid definition in the early 20th century before it became what we recognise as the electron today.


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