Ep. 377: Thomson finds Electron

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At the end of the 19th century, physicists were finally beginning to understand the nature of matter itself, including the discovery of electrons – tiny particles of negative charge that surround the nucleus. Here’s how J.J. Thompson separated the electrons from their atoms and uncovered their nature.

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This episode is sponsored by:  Casper, Swinburne Astronomy Online, 8th Light

Show Notes

J.J. Thompson
Review: Elements of the Mathematical Theory of Electricity and Magnetism by J. J. Thomson
Thomson on the Nature of Matter: Corpuscles and the Continuum
On the Structure of the Atom


Transcription services provided by: GMR Transcription

Female Speaker: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest-running astronomy degree program. Visit astronomy.swin.edu.au for more information.
Fraser Cane: Astronomy Cast episode 377: Thomson Finds the Electron. Welcome to Astronomy Cast, your weekly facts-based journey through the cosmos. We help you understand not only what we know, but how we know what we know. My name is Fraser Cane; I’m the publisher of Universe Today. And with me is Dr. Pamela Gay, a Professor at Southern Illinois University Edwardsville and the Director of CosmoQuest. Hey Pamela, how are you doing?
Dr. Pamela Gay: I’m doing well; how are you doing, Fraser?
Fraser Cane: Good. Happy Victoria Day.
Dr. Pamela Gay: Happy Victoria Day to you.
Fraser Cane: You don’t even know what that is, but that’s okay. It’s a Canadian thing. All the Canadians will be nodding sagely, and for the first time they’ll realize that I’m not an American, but actually a Canadian.
Dr. Pamela Gay: Hey, my husband’s Canadian, and now I know I need to go tell him Happy Canadian Day. I feel guilty that I didn’t know earlier.
Fraser Cane: Yeah, have we got to anything – can people still donate for the Hangout-a-thon?
Dr. Pamela Gay: Yes, they can. I have not yet taken the page down, and so if you want to add your glorious donations to our yearly massive capital campaign, we’d greatly appreciate it. It’s cosmoquest.org/hangoutathon.
Fraser Cane: Yeah, and we really appreciate your support. It was a great, very successful Hangout-a-thon this year, but we definitely need to raise a little more to kind of just smooth out the rough patches as all those grants come in.
Dr. Pamela Gay: Yeah. Donations are pretty much all we have to get us through until October, because we’re in an unfunded extension because sequestration.
Fraser Cane: Okay. So let’s get rolling with the show.
Female Speaker: This episode of Astronomy Cast is brought to you by 8th Light, Inc. 8th Light is an agile software development company. They craft beautiful applications that are durable and reliable. 8th Light provides disciplined software leadership on demand and shares its expertise to make your project better. For more information, visit them online at www.8thlight.com. Just remember, that’s www dot, the digit 8, T-H, L-I-G-H-T, dot com. Drop them a note. 8th Light: software is their craft.
Fraser Cane: So at the end of the 19th century, physicists were finally beginning to understand the nature of matter itself, including the discovery of electrons, tiny particles of negative charge that surround the nucleus. So here’s how J.J. Thomson separated the electrons for their atoms, and uncovered their nature. So J.J. Thomson. Who was this guy? And what led up to this series of experiments to search for the electron?
Dr. Pamela Gay: He was a Professor. He was at the Cavendish Laboratory at Cambridge University, which is kind of one of those places where all the early let’s do things with particle physics, and what would later go on to be called physical chemistry started out.
Fraser Cane: About what time are we talking here?
Dr. Pamela Gay: So he got there in 1884, or he got the position in 1884, and the work that we’re talking about today pretty much culminated in 1897. So we’re looking at work that was done in the 1800s.
Fraser Cane: And so what was Thomson attempting to understand? What was the setup that he was doing, and why – what was the hunch?
Dr. Pamela Gay: Well, it wasn’t so much a hunch as a huh, how does that work, that was going on. It had been realized that if you take a tube – a cathode ray tube, is what the call them – remove as much of the gasses inside as possible, try and create a vacuum, and then add positive and negative hookups, basically cathode and anode, and send electricity through the tube, you’re going to end up with this weird fluorescence, where whatever it is that’s going on hits the side of the tube.
And there was this debate the time on, what is the fundamental unit of matter? Is it the atom? There was a debate on, is this thing that’s going on with cathode ray tubes – which were not just scientifically interesting things, but they were kind of like the “it” demo of the time that you did to amaze audiences. No one quite knew, is this a particle phenomena? Is this a wave phenomena? What’s going on? And Thomson was basically trying to get to the bottom of what’s happening with these cathode ray tubes.
Fraser Cane: Yeah, I think people don’t realize that back 100 years ago, scientists would go on tour and do demonstrations of scientific experiments, and they would fill audience halls of people just there to watch them perform experiments. Which is pretty awesome.
Dr. Pamela Gay: What Mythbusters do today.
Fraser Cane: Yeah, but it was just like, people would go see the theater, or they would go see a scientist perform some experiments
Dr. Pamela Gay: And these cathode ray tubes were one of the ones that were particularly annoying, because we didn’t have really good vacuum technology yet. And so depending on how much gas was left in, they glowed in different ways, and it was just frustrating. Other researchers, Hertz for instance, had tried to see what would happen if you took a cathode ray tube and exposed it to magnets, to electromagnetic fields, and they weren’t coming up with anything really definitive. Which was Thomson was like, okay, let’s just step back. Let’s do this completely right. And he did a very systematic experiment where he looked at both electromagnetic field – and electric field and a magnetic field would affect the path of the cathode rays.
Fraser Cane: And did they know what the cathode rays were?
Dr. Pamela Gay: No, not a clue.
Fraser Cane: Right, so they knew that they were hooking it up to a form of electricity, and this was generating something. Which – cathode rays that were coming out, and they were actually being detected on a surface on either side.
Dr. Pamela Gay: And one thing I’d like to point out at this point that just kind of amazed me is, like, those old, bulky TVs and monitors that some of us still have randomly hiding in corners of our house, those were called CRTs because they were cathode ray tubes, just large and much fancier. So the technology that we’re talking about, in the 1890s, is technology that, at the time, they didn’t know had any practical uses whatsoever. Thomson didn’t think cathode rays could really be used for anything beyond entertainment. And so when you fund basic research, you never quite know what you’re gonna get out of it.
So his basic research is, what the heck is going on with this glowing tube? And what he did was he took a glass tube and worked really hard to get as low a gas pressure inside the tube as possible. This way, there was less gas inside that might potentially get excited by whatever was going on. They knew that whatever the phenomena was, it carried charge. Now, the problem is that an electric field can get blocked by a variety of different things. This is why cell phones don’t work real well on airplanes, for instance. The gas surrounding the cathode rays inside the tube, it would get excited, it would generate its own electric field, all of this would serve to block an external electromagnetic field.
So the only way for him to really see what was going on was to get rid of all that gas. Royal pain when you’re working in the 1890s, but he did it. And he ended up being the first person to be able to bend the cathode ray with both a magnetic field and an electric field. He was able to figure out from the magnetic field that this was something that was negatively charged, based on how they understood how positive ions, negative ions reacted in negative fields. So he started out with the, well, now I know it’s something that has a negative electric charge. And the question is, what the heck is it?
Fraser Cane: But how did they know that they were dealing with a particle and not, like, light? You know what I mean?
Dr. Pamela Gay: Well, light isn’t charged. So first of all there was the light doesn’t carry charge, light doesn’t have mass. Whatever this is, they were able to first get at the it has charge.
Fraser Cane: Right, and different amounts of charge, and different amounts of –
Dr. Pamela Gay: Well, no. It always has the exact same amount of charge. That was one of the particularly annoying thing, is it had a constant mass to charge ratio. Because all electrons have the exact same charge.
Fraser Cane: But if you crank the electricity up higher, you make it deflect more, right?
Dr. Pamela Gay: No, because each electron has the exact same amount of charge.
Fraser Cane: Okay.
Dr. Pamela Gay: It’s one of those things that completely non-intuitive. And so what mattered here is what is the speed of the electron? He was able to go ahead and measure the velocity of the electron by looking at the difference in how the electric field and the magnetic field were able to bend that cathode ray. And by getting at that velocity, he was able to figure out, this is something that’s moving at a significant fraction of the speed of light. So now we have a charged something that has velocity. We’re gonna call that a particle.
Fraser Cane: And I love the name – the original name that he came up with. Do you know that?
Dr. Pamela Gay: I don’t know what that was.
Fraser Cane: He called them corpuscles?
Dr. Pamela Gay: No, that’s true. It always makes me think of blood cells, which I think is what he was thinking of as well. And so he was trying to figure out, okay, so now what does this mean? So there’s a whole series of systematic experiments. So he was able to first get at the velocity, where he found that it was one-third the velocity of light, which is 60,000 miles per second, or 100,000 kilometers per second. So these are fast moving, small, small things.
Now, this raises the question of, how the heck do you get something moving that fast? And low mass is part of that, but it took a while to get to the low mass, and what he actually got to instead was the charge-to-mass ratio, and then the realization that these things have about the same charge as an ionized hydrogen atom, which is a fancy word for saying proton. And it was between those two that he figured out, this is something that has a mass that is 6,000 times smaller than the mass of a normal hydrogen atom.
Fraser Cane: Right, but at this point, like, when Thomson was doing this work, they weren’t even sure that if matter was just – atoms themselves were just, like, little balls. Little marbles that made up a larger block of matter. And they weren’t really sure if there was any sub – any particles that made up the atom themselves.
Dr. Pamela Gay: Right, and there was the weirdness of understanding that there were these different charge amounts, different – so you could ionize something to different levels. There was starting to be the realization that you have different isotopes, so things that behave slightly differently, but are really the same atom. So there was a whole lot of confusion going on. This was really the point in time where physics and physical chemistry started to split apart. Nowadays, a lot of those problems we throw into the physical chemistry way of dealing with research on these things.
So with the understanding of now I have something that is 6,000 times smaller than your hydrogen atom. Now with the understanding of, huh, this thing that I have is also the same charge in the opposite sense. So negatively charged versus positively charged, but the same amount of charge as that ionized hydrogen, which is a proton. What does this mean?
Fraser Cane: What does this mean?
Dr. Pamela Gay: Well, he got it wrong. But this is one of those awesome turning points in science where they’re trying to understand things. So one of the things that had him completely perplexed is with the cathode ray tubes, while they were trying to figure out what the heck was going on, one of the tests they did was to put a piece of foil inside the cathode ray tube to see if that piece of foil – they used gold foil, because it’s really easy to get gold foil that’s only about an atom thick. They used basically atom-ish thick gold foil, shot the cathode rays at the foil, and as near as they could tell within the lab conditions of 1897, the cathode rays quite happily when straight through the foil, which was perplexing. And so –
Fraser Cane: Were they – sorry, why was it perplexing? Were they expecting them to bounce off the atoms of gold, or were they expecting them to get deflected by the gold?
Dr. Pamela Gay: Well, it’s basically the idea of if you have a solid object, why is a particle not going to get stopped by the solid object? So the foil was perceived as a solid. It was figured if – this was still when we were debating whether photons were waves or particles, and so it was thought that waves were able to go through solids like particles would get stopped by the solids.
Fraser Cane: Right. And it would be like shooting bullets at a brick wall, and them going right through the brick wall and there being no damage to the brick wall, right?
Dr. Pamela Gay: Exactly. And when, instead, they found that they had something that they would shoot at the brick wall and it would happily go through, that had velocity, mass, charge, all these other things, it made them start to rethink, just how solid are atoms? And that was a completely new way of thinking. Now, while Thomson was trying to figure all of this out, what he came up with is what we now refer to as the plum pudding model of the atom. The idea here – have you ever had plum pudding? Is that a Canadian thing?
Fraser Cane: No. No, I just remember learning that model.
Dr. Pamela Gay: Okay, so basically, the idea is you have this gelatinous mass, basically, the plum pudding. That’s kind of cruel for the plum pudding, I do like it, I just have no clue how to explain it. You have this region that is the atom that has within it, this consistent mixing of the electrons and whatever it is that’s giving the atom its atom characteristics. Hadn’t gotten to the neutron really, hadn’t gotten to the proton as completely. Still figuring these things out. It’s all kinda fuzzy.
So working on the idea that you have this mixture of electrons and stuff – protons, neutrons – he saw all of this as mixed together, like your grandma would do in the kitchen with a bunch of ingredients. And that mixture, with the electrons moving inside the mixture – it wasn’t like they were baked in like actual plum pudding. With them baked in – sorry, with them orbiting inside the plum pudding, you end up with this atom.
Fraser Cane: Right, and if I understand, he thought there was, like a lot, like, thousands of electrons all – because of the amount of mass required, after he detected the amount of mass, he realized that, wow, there needs to be – they’re very light, so there needs to be a lot of them to make up the mass of this.
Dr. Pamela Gay: Or the protons just have to be very massive. They were fairly quick to start figuring out that the protons have this one charged mass ratio, therefore they’re heavy. In this case, proton being ionized hydrogen. Whereas the electrons have this really radically differently charged mass ratio, leading to really low mass. And where they started to wonder is, does this mean that the atom is mostly empty? So even though he has this plum pudding model, where everything is mixed together, that mixed together still had a lot of empty space in it. And it was that empty space in the plum pudding model that allowed the cathode rays to go through the gold foil.
Fraser Cane: Right. And so sort of what happened next? Because as you said, he guessed wrong. But I mean, his student, right – we did – we talked about the Millikan experiment that helped determine the charge of the electron. And his students were people like Rutherford, Bohr, etc., right? And they went on to really get to the bottom of this.
Dr. Pamela Gay: And it was Lord Rutherford – to be fair, Thomson went on to get knighted after he got his Nobel Laureate, but Rutherford kind of started there a little bit earlier. It was Lord Rutherford who, in 1899, set up experiments to start to get to the bottom of this. And so what he did was he set it up and it took him a long time to get all the way there. His results were finally published in 1911.
He set up experiments where he had the gold foil, and shot the electrons at the gold foil, and had around that a bunch of different detectors. Photographic plates, different ways of detecting what was going on. And was able to realize that, while the bulk of the electrons happily went straight through, those that didn’t ended up deflecting in a completely random way that seemed to indicate that you had a dense sphere that was making up this – well, a whole series of dense spheres and empty space that was making up this gold foil.
Fraser Cane: Right. And so I mean just to use that analogy again, right, it’s like you’ve got the brick wall, and you’re firing the bullets and most of the bullets pass right through, and every now and then, one of those bullets ricochets off by 45 degrees. And this is where they realized, yeah, as you said, right? That the atom was really composed at the very center, of something really dense. And then there was empty space around it that nothing was being – interacting with.
Dr. Pamela Gay: And another way to look at it might be imagine firing rubber bullets that bounce quite happily. Not the normal ones, but, like, happy clown rubber bullets. I don’t know if such a thing exists.
Fraser Cane: You just invented them. Little kick-starter.
Dr. Pamela Gay: I just invented them. Let’s say that we’re firing super balls. So we have a kids’ toy gun, we’re firing super balls at a wall made up of basketballs. And most of the time, the super balls pass between the spaces in the basketballs. But periodically, they either hit head-on and come back and attack you, hit at a weird angle and bounce off, and that distribution of how they bounce off allows you to track what is the shape of the basketball. You could, for instance, replace the basketballs with some sort of weird geodesic shape that was still a bouncy substance of some sort, and you could map out the shape of those. Again, based purely on how the bounces occur.
Fraser Cane: If they were cubes, you would expect a lot of them to bounce straight back, while some of them would bounce at a little bit of an angle. But in this case you can really – so they were able to really tell, no, it’s spheres at the middle of these atoms.
Dr. Pamela Gay: Exactly. And so this was one of those really neat points where you have an experiment that started as a, huh, we have no clue what’s going on. And I’d love to know why someone got the idea to run electricity through a glass container of mostly vacuum. This is the type of thing that started way back with Benjamin Franklin working with various basically jars of electricity.
People continued to play with their glass vacuum shapes, sending electricity through them in different ways. Got to the question of what’s going on, the discovery of the electric field, the magnetic field. Let’s see what we can do with this, with both of these. Came up with a model of the atom, ran experiments to see if it was right. Realized it was completely wrong, fixed that, and systematically, over time, we got a deeper and deeper fundamental understanding of how atoms work.
Now at the same time, while they’re doing all of these experiments, they have to start perfecting the vacuum pump, so they can get lower and lower amounts of gas inside these gas tubes. We’re getting higher and higher precision control of the electric fields and the magnetic fields. Eventually, these technologies, as we start to master electricity, would allow us to precisely pick where is that electron getting sent to? Allowing us to first sweep it back and forth, and sweep it up and down and back and forth, allowing us to eventually produce –
Fraser Cane: Television!
Dr. Pamela Gay: Exactly. And this is the story of scientists trying to figure something out, fully saying, this has absolutely no purpose, being dead wrong in the no purpose, and creating something that we’ve all wasted great segments of our life in front of.
Fraser Cane: Right. So truly, that was the runaway experimentation with cathode ray tube gave us television.
Dr. Pamela Gay: Exactly.
Fraser Cane: Yeah. Thanks, Thomson and friends. So I mean, Thomson sort of thought it was the plum pudding. Everything was sort of evenly distributed. Other people, like Rutherford and such, came along afterwards and really helped come up with the other ideas. That it was – that you had this hard core with the electrons around it. But what did they sort of think the electrons were doing around the central nucleus?
Dr. Pamela Gay: Well, the idea of orbits didn’t take too long for them to get to. They figured they weren’t just hanging out. They know that there was some sort of an attraction going on. But they clearly weren’t merging together. So this is where you start getting at the idea of something with velocity in motion, around whatever, leads you to the idea of orbits, motion. The motion prevents things from colliding, collapsing together.
But it would still take a little bit of time before we started to get to the idea of discreet energy levels. It would take a lot more work. Planck needed to start to join the field; we need to start to figure out, what the heck is the ultraviolet catastrophe, black body radiation. But all of this, again, was happening in this brief period from the 1880s to the 19-teens that we’ve been focusing on for the past couple of months now, I think.
Fraser Cane: Yeah. Awesome. So I think that wraps up this week. We’ll talk to you next week, Pamela.
Dr. Pamela Gay: That sounds good, and it will be my turn to be having a holiday. It’ll be Memorial Day here in the United States, but I’ll be here to record.
Fraser Cane: Awesome. As will I. All right, thanks Pamela.
Dr. Pamela Gay: Okay, bye-bye.
Male Speaker: Thanks for listening to Astronomy Cast. A nonprofit resource provided by Astrosphere New Media Association, Fraser Cane, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at astronomycast.com. You can email us at info@astronomycast.com, tweet us @astronomycast, like us on Facebook, or circle us on Google+. We record our show live on Google+ every Monday at 12:00 p.m. Pacific, 3:00 p.m. Eastern, or 2000 Greenwich Mean Time. If you miss the live event, you can always catch up over at cosmoquest.org.
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