Cathode Rays Lead to Thomson's Model of the Atom

So today, I'm at the University of Sydney with Doctor Phil Dooley, and we're talking about how our idea of the atom changed from a tiny little hard sphere to something more complicated. And this apparatus has something to do with that.

Phil: Exactly, exactly. It was quite a landmark historical experiment. It's just made of a tube of glass connected to a vacuum pump, and in the tube of glass, there are two pieces of metal connected to a high-voltage power supply, so we can suck the air out of here.

Phil: That's right. And then apply a high voltage to the metal plates.

Phil: Yes. Normally with high voltage in air, we don't get anything except maybe a big spark. But what Geissler found, who did this in 1857, was something quite different. So shall we have a look?

Derek: Let's do it.

Phil: Ok, let's turn on the pump and the high volts. Turn off the lights so we can see what's going on. As you can see, something's happening now.

Derek: Yes, I see a bit of a pink glow in there.

Phil: That's right. So imagine Geissler, the first person to ever see this. It's pretty surprising. That started people thinking, "What is the stuff that's coming from the cathode?" There seems to be a ray emitted from this negatively charged one, which they called the cathode. So therefore, it's called a cathode ray, and it became the topic for research for the next forty years. Since all the big wigs of physics and science basically tried to work at what was going on.

So what did they figure out? I mean, what is going on? What's producing this glow?

There were two main camps: one thought that it was waves, and the second thought that the electricity was being carried by a stream of charged particles. Over the next forty years, they basically came to the conclusion—well, it was JJ Thomson in eighteen ninety-seven who came up with the idea that there is a stream of very small charged particles—negatively charged. They're smaller than an atom; so the smallest atom, a hydrogen atom, they're about one thousandth of the size of that. And it doesn't matter what the material is; the metal is always the same particle. So this particle appears to be in everything, and this electron, as it was known, is what's carrying the electricity.

So Thomson came to the conclusion that all atoms have this particle as one of their constituents.

Phil: That's right, that's right. So it wasn't a billiard ball; there had to be something inside the billiard ball. So how do I picture a billiard ball with bits inside it?

Phil: Well, you should probably think about a pudding.

Derek: A pudding?

Phil: Yeah, well that's what his idea was—that it was a plum pudding, very dear to every British person's heart.

Derek: Right.

Phil: And inside the pudding, you have plums, and the plums can come out of that. So when you apply high voltage, then a negatively charged electron, which Thomson pictured as a plum, comes out and leaves behind a positively charged pudding.

Derek: Are you a fan of plum pudding?

Phil: For research purposes, definitely.

Derek: Alright, let's see if we can research this a little further. Sounds like fun.