Ep. 372: The Millikan Oil Drop

In 1909 Robert Millikan devised an ingenious experiment to figure out the charge of an electron using a drop of oil. Let’s talk about this Nobel Prize winning experiment.

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

Show Notes

The Millikan Oil Drop Experiment slideshow
Millikan Oil Drop animations
The Millikan Oil Drop Experiment explanation
Millikan Oil Drop Experiment video
Millikan’s Oil Drop Experiment video explanation
The Millikan oil drop experiment lecture from Cornell University


Transcription services provided by: GMR Transcription

Announcer: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program. Visit Astronomy.Swin.edu.au for more information.
Fraser Cain: [inaudible][00:00:20] cast episode 372 The Millikan Oil Drop Experiment. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos where we help you understand not only what we know but how we know what we know.
My name is Frasier Cain. I’m the publisher at the university and with me is Dr. Pamela Gay, a professor at Southern Illinois University, Edwardsville and the director of CosmoQuest. Hey Pamela, how you doing?
Dr. Pamela Gay: I’m doing well. How are you doing, Fraser?
Fraser Cain: I am doing awesome. Yeah, I can’t complain. I can’t complain about the terrible, horrible rainy weather that just never seems to end.
Dr. Pamela Gay: You live –
Fraser Cain: As a Canadian I know I’m one of the lucky ones so –
Dr. Pamela Gay: I was gonna say, you’re kind of on the west coast in tropical, what is it, mid latitude tropics?
Fraser Cain: Yeah, exactly.
Dr. Pamela Gay: Rain is the thing.
Fraser Cain: It’s pineapple express all the way here. So let’s just remind people that there’s gonna be a live event where we get to watch Pamela try to help raise money for research.
Dr. Pamela Gay: Yes. So on April 25, 26 we are going to be holding this year’s CosmoQuest Hangoutathon. You can find out everything you wanna know by going to CosmoQuest.org/hangoutathon, all one word. And we’re still in the process of pulling things together but what I can tell you right now is we are partnered with Astronomers Without Borders and we’ll be bringing you star parties around the world as the night passes from nation to nation.
We’re also going to have scientists coming on to help us understand the entire process of asteroid and meteorite understanding, folks talking about the moon, Mars. And we’re gonna be actually racing to map out other worlds with data visualizations to see you as you compete to get Vesta, Mercury, the Moon. And we’ll have Mars for the hangoutathon all mapped out in real time.
And so it’s all about science and hopefully funding science. The only reason I am here is because folks like you have donated. CosmoQuest needs your help so, yeah, CosmoQuest.org/donate if you wanna get in ahead of the game.
Fraser Cain: And that date again?
Dr. Pamela Gay: It is April 25, 26. We’re running from 8:00 a.m. Pacific to 8:00 p.m. on Sunday Pacific time, 36 straight hours.
Fraser Cain: Sweet. All right. Let’s get cranking.
Announcer: 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 Cain: In 1909 Robert Millikan devised an ingenious experiment to figure out the charge of a single electron using a drop of oil. So let’s talk about this Nobel Prize winning experiment. This is one of my favorite experiments and I remember being in physics class in high school. We were, what, grade 11 I think, and they explained the oil drop experiment.
And it blew my mind in its simplicity and sorta out-of-the-box thinking to figure out a way to come up with a fundamental calculation of the universe. And it’s still just an amazing experiment. So set the stage. What is the – what was the unknown at the time?
Dr. Pamela Gay: Well, at that point we understood that there were positive and negative charges out there. What we didn’t know is if they were associated with quantized individual particles that came with set charge or if this was a continuum or what basically. We just didn’t know. And so what Millikan and Fletcher, who never gets mentioned but was his PhD student at the time and who did the experiment and designed the apparatus and then was left off of the paper, what we know is the work that Fletcher did actually allowed them to find a way to look to see if charge only appeared in certain multiples that would allow us to say, well, every time we see charge it’s a multiple of this unit which means there is a fundamental amount of charge that you don’t get smaller than.
Fraser Cain: And so the possibility that was figured out was that it was, I guess, the difference between it being binary and it just was like a field that increased with [inaudible][00:05:23] strength, that you could have the charge be .001, .002 or anywhere in between. There was no distinct jumps that the charges went through. And so they were trying to figure out whether it was just this distinct charge or whether it was – I’m looking for a word and it’s not coming to me.
Dr. Pamela Gay: They were trying to figure out if it was a continuum or if it was quantized. So this is basically the difference between at the macroscopic scale the amount of applesauce that you have in your bowl is a continuous number. You can have a little bit, a little bit more, a lot more. Applesauce doesn’t come in quanta but apples do. You can either have an apple, you can have two apples. It’s an apple is a quanta of apple that you are limited on what you can put in your bowl.
Fraser Cain: Digital or analog?
Dr. Pamela Gay: Yeah, sort of.
Fraser Cain: Is it digital or analog? So you imagine some old school stereo where you stand in front of your stereo and you’re cranking the volume and you’re just turning it up and turning it down and it’s analog, right? And as opposed to when you take your iPhone and you turn up the volume and it goes in discreet volume levels that’s –
Dr. Pamela Gay: — that are never good volume levels.
Fraser Cain: They’re never good levels, no. And it complains and won’t let you go past a certain volume even though you tried. You change the settings, it never holds the settings. And every time you turn your phone back on again, it forgets the settings and anyway.
Dr. Pamela Gay: Are you a bit annoyed with your iPhone, Fraser?
Fraser Cain: Yeah, a little. So – but that’s the question. So the question is, is it – and so quantum theory which was just being developed at this time, was coming up with sort of a whole series of theories about what this meant, about what quantized energy states meant.
Dr. Pamela Gay: And so to paint a picture of where we were, this was all happening in the 19 otts. The paper came out in 1913. The research started around 1908. And this was a point in time where we were starting – not we, I wasn’t born, my grandparents weren’t born – the field of science was working on trying to understand how to play with all of their new electro mechanic toys.
We’d known about electricity for a while. Benjamin Franklin is the person that we all think about who’s actually one of the first people to start figuring out things like using capacitors to store charge.
One of the things that I learned in preparing for this particular Astronomy Cast was that batteries were figured out in the late 1700s because in 1780 Luigi Galvani was dissecting a frog. And when he touched his iron scalpel to the brass hook the frog was attached to, the frog’s leg jumped. And he assumes that the dead frog had some sort of intrinsic energy still in it, which is a really horrifying thing to imagine.
But what was actually discovered, as explained by Alessandro Volta was if you have a voltaic substance or electrolytic substance, in this case the guts of the frog –
Fraser Cain: Guts of the frog, yeah.
Dr. Pamela Gay: — and you combine a couple of different metals, the brass in the iron, this generates charge.
So fast forward 200 years. And they have this ability now to build batteries often by taking briney water and soaking cardboard in it – briney just is a fancy word for salty. You use it to make pickles. Take cardboard, soak it in brine and then layer various metals with this briney cardboard and this will actually output electricity. And they knew how to wire this up. They knew how to create electric and magnetic fields.
And so Millikan and Fletcher realized, if you have something that’s charged and you have an electric field between two plates, so you take one plate of metal, you take another plate of metal, separate them with an insulator to keep them at a known separation, wire them up to a battery they will collect charge on the two surfaces to create a constant electric field between the two surfaces.
If you put something that’s charged between those two surfaces, you can delightfully make it float. This is actually something that if you are daring and experimental you can play with using just simple things you have around your house.
If you have a powerful enough battery and little plastic balls, you can actually grab a cat and use some non-conducting thing to rub the plastic balls on the cat. Charge will build up on the plastic ball because of – this is my favorite word in science – the tribal electric difference in value between the cat and the plastic ball. You now have a charged plastic ball. Stick it between two metal plates that you have safely attached to a battery. And if you get the charge-to-weight ratio just right you can hover that little ball using the electric field.
Fraser Cain: My favorite, although it’s not exactly the same but it’s just a great experiment, is take a comb, brush your hair and then hold the comb right next to a stream of water that’s coming out of the tap, and it will divert the stream. And then as the static electricity runs out of your comb then you can start again, run it through your hair, not that I can anymore but one could.
Dr. Pamela Gay: You could do this to a cat as well. Cats are a great source of electrostatic charge.
Fraser Cain: Exactly. Right. But it’s a great thing to do with your kids is to give them a comb, have them generate static charge and then see what things are – which are pushed or pulled by the static charge. By anyway, we – let’s get back to the show.
Dr. Pamela Gay: Okay. So your happy little charged ball is actually not that different from the oil drops that Milligan was using. Anything that can hold a charge can be used in this experiment. So the reason he used oil drops is they didn’t exactly have the greatest of batteries back in 1908. So when you have an electric field you needed little tiny things to put charge on to try and make hover. And oil drops were a perfectly reasonable thing.
The other reason was they need things small enough that they’d hit their terminal velocity, that they’d accelerate through the air until they reached a point where the force of gravity trying to accelerate them downward versus the buoyant force of air acting as a friction sort of kind of analog force. Basically the buoyant force pushing up balanced so that the object is no longer accelerating.
So in order to get something tiny enough to suspend the electric field and that would hit terminal velocity quickly enough that it would hit terminal velocity within the experimental apparatus, they opted – Fletcher and Millikan opted to use oil drops. They used a small sprayer, not too different from what you’d use with perfume, to create little tiny droplets. And just the action of shooting through the nozzle is enough to transfer some charge onto the oil drops.
They sprayed this above a set of electric plates that had a hole in them to allow some of the oil drops to just happen to fall through the hole. And while looking between the oil plates – the electric plates rather through a magnifying apparatus, they were able to adjust the strength of the electric field to, well, select out oil drops that had just the right amount of charge and amount of mass to be able to be suspended within this field.
Fraser Cain: Right. So I can just imagine they’ve got this oil sprayer, they’ve got these two charged plates. In between there’s a gap. They’ve got a knob and as the oil drops are sort of dropping through into the charged plates, they’re falling really quickly. And then they turn the knob up to increase the charge and the speed starts to slow down as, I guess, the upward force of the charge on the particles on the charge of the oil drops is countering the gravity that’s pulling them downward. And you hit this equilibrium where the particles just sorta stop, hovering in between being pulled upward and being pulled downward by the gravity. And then fancy math?
Dr. Pamela Gay: Sort of. So the math wasn’t actually all that fancy. It was just kind of annoying because you had to get all the variables to cancel out. So the problem that Millikan and Fletcher, Fletcher and Millikan were left with was you have these little tiny oil drops and you can’t really accurately hold a ruler up to an oil drop and go, okay, what’s your radius. But they did know the density of oil really accurately so they could get it their – the mass of the oil drops if only they could get at the radius accurately.
So to get accurately at the radius, what they did was they figured out, well, what is the terminal velocity of these little tiny oil drops. And the terminal velocity was related to a whole bunch of different parameters that they could calculate. And once they got at the terminal velocity they could also get at the weight of it by taking – they knew the density. They could get at the radius via the terminal velocity.
So if you know the density and you know the radius you can then calculate the weight by going, here’s the radius. I know four-thirds pi r cubed is the volume of a sphere. Multiply the density by that. That gets you at its mass. Multiply that by the acceleration of gravity. You now have the downward force of gravity.
Lots of math that’s not difficult. It’s just monotonous. She threw the monotonous math. And what you’re left with is an understanding of the radius, the downward gravitational force, the upward buoyant force. And then you’re in a position where you can calculate, well, what’s the electric force?
Now, the electric force is a function strictly of how much charge is on the object. So if you measured this whole rigmarole, go through all of the same crazy math over and over for multiple oil drops, you can start to see, well, this one has this much charge on it. I can tell because of the electric force it must have. And this one over here has this much electric force on it.
You can start to realize, okay, are all of these numbers multiples of each other and ask, does that mean that the charge is always a multiple of some core value?
Fraser Cain: Right. So you might notice that just – you might notice that say, this one’s got 5, that one’s got 10, this one’s got 35, that one’s got 100. And you might say, hum, maybe these are a multiple of 5.
Dr. Pamela Gay: Exactly. So it – unfortunately the math wasn’t that clean. And what gets me is they were able to go through all of their maths and they came up with a value of a single electron having 1.592 times 10 to the minus 19th [inaudible][00:17:28]. Their published value was within about half a percent of the actual value of the charge on an electron. So this was actually some really impressive work that they did but it wasn’t without controversy.
Fraser Cain: What? Right. It took a little while before the Nobel Prize was awarded.
Dr. Pamela Gay: Well, that actually – so that wasn’t so much the problem. The problem came much later when people started looking at the results a little more closely. So the first thing to come up was, wait, you seem to have thrown out a lot of outliers, dear Millikan. And the truth was that Millikan did throw out a lot of data saying, well, these things simply weren’t fully analyzed that related to charged particles that may not have had as close to the center of values as some of the other experiments.
So had he included all of the data he took in his initial publication, his error would’ve been higher and he still would’ve been within 2 percent of the actual value – sorry, he still would’ve had about 2 percent error in his results. But he didn’t want to leave that much wiggle room for detractors so he only published the best –
Fraser Cain: Oh, oh.
Dr. Pamela Gay: — or at least only published the stuff that told the story he was hoping to tell.
Fraser Cain: So he cherry-picked the evidence.
Dr. Pamela Gay: He did. Now, the other thing that was pointed out by [inaudible][00:19:04] was not only did he cherry pick the data but then there was this problem over the following years that as people recreated his experiment did similar – did experiments that yielded different ways of getting at the charge of the electron – people kept coming up with higher values but no one was willing to step forward and say, hey, my value’s like 2 percent higher than his value or half a percent higher, as the case may have been.
Fraser Cain: Like Millikan is a god and how dare we – right.
Dr. Pamela Gay: So everyone was like, we’re getting a different number. What’s wrong with what I did? And so there was never this question of what was wrong with what he did. There was a whole lot of self doubt among all of these powerful physicists all over the world working to recreate an experiment. And I find it fascinating that instead of using the scientific theory, as you’re supposed to, to test your hypothesis, to verify results, all of these people did the scientific process up until they got results that weren’t identical to what some other famous, powerful, Nobel Prize-carrying dude got.
And then they suddenly became very worried and looked for ways to get their value closer to his value. And it actually took several tens of years to get a current value of the electron.
Fraser Cain: Right. But people had come up with other methods of calculating the charge of an electron, as you said, not just this one genius experiment with cherry-picked data but other methodologies, right?
Dr. Pamela Gay: But even with these other methodologies, they were very cautious about saying, no, the electron’s value is actually significantly higher. So if you look at the published value of the charge of an electron, it just ever so slowly crept up until it started basically hovering at the value that we now very precisely know.
Fraser Cain: But I think the implication of Millikan’s data and Millikan’s experiment was to provide evidence to this idea of the quantized state of the electron. And that –
Dr. Pamela Gay: He did that.
Fraser Cain: — that created a very fruitful level of research after that.
Dr. Pamela Gay: It did. And the sadness that I have is even as we do this, we’re referring to this as Millikan’s work and the –
Fraser Cain: — and Fletcher.
Dr. Pamela Gay: Yes, and Fletcher. Let’s keep saying Fletcher. Let’s rewrite history a little bit to be more accurate. Now that Fletcher’s dead people have had access to his letters and seen the documents where Millikan basically said, look dude, if I leave you off of the paper I’ll make sure you get a cherry job at Bell. It was phrased a little bit different but that was the gist of what was said.
So this all told was one of those projects where Millikan knew he was doing something amazing. He knew, well, actually that Fletcher was doing something pretty amazing, that this had a lot of potential to change how we – not change but to focus how we do everything. Let’s stop looking for the continuum idea of charge on the electron and start admitting electrons are discreet particles that have discreet amounts of charge. And that suddenly focused science greatly. And, yeah, that was a good thing. It’s just sad that it came in such a let’s leave the grad student off the paper kind of format.
Fraser Cain: That seems to be a theme in your research into these historical topics.
Dr. Pamela Gay: It’s not something I purposely look for. I am not –
Fraser Cain: Well, maybe it just happens all the time.
Dr. Pamela Gay: That’s my concern. And I know I’ve gone out of my way to consistently not do that. And the thing is, when you choose not to do that there’s always the, well, you don’t publish enough. You don’t do single author papers enough. Old guard complaints. So we need to change the entire field so that the people doing the work, which are often the graduate students in the lab, get the credit for their amazing contributions that allow our field to progress while us senior and mid career people – I’m a mid career person – well, we’re stuck writing grants 90 percent of the time while they’re in the lab. And let’s acknowledge that.
Fraser Cain: Right. But I think you’ve made a case – I mean, we don’t wanna destroy this episode of Astronomy Cast and go into this, but I think you’ve made a pretty good – you’ve made a case both ways, right, that it is the person – it is the senior researcher who’s putting their name and saying, I stand for this research, all this research that’s getting done. And they helped raise the money and they convinced the powers to be that this experiment should be done and that the money should be spent. And the results are high quality. They are the leader of the research team that gets this work done.
And if various team members drop out at the end of the day it’s still that researcher’s name on the paper. And there’s value to that. And of course you wanna have the names of all the people who are involved on the process. And I guess different research teams will, depending on who they are and how magnanimous they are, will have their name first and then everybody else’s name as well. And others will just put – only put their name on it. And I think that’s just like credits. You can make sure that everyone’s role is done.
But I think as the Nobel Prizes are awarded, I think there is something to be said for the person taking a stand for the research, for organizing it to be done, to make sure the money is set aside. When people can work uninterrupted and the discovery can be made. So it’s tough to decide which way it goes.
Dr. Pamela Gay: Well, and there’s a difference between whose on the Nobel and whose on the paper. It’s getting left off the paper where you have infinite space – well, not infinite, you have page charges. But it’s a tricky issue. And to see it come up over and over throughout history is kind of heartbreaking. You have to wonder how many careers got lost simply because the person doing the work in the lab was just a student, even though they were the ones who were devising the apparatus saying, hey, I have this idea. Let’s try it. It’s an interesting thing to think about and to consider as we’re trying to find how to move forward.
Fraser Cain: It sounds like a book, Pamela.
Dr. Pamela Gay: Yeah, after I write these grants. I have to pay my bills first.
Fraser Cain: Yep, then write a book.
Dr. Pamela Gay: Donate CosmoQuest.org/donate. Free me from grant writing to actually do content creation in science.
Fraser Cain: That sounds great. Cool. Well, thanks Pamela. Thanks Millikan and Fletcher. And we will see you next week.
Dr. Pamela Gay: Sounds good.
Announcer: Thanks for listening to Astronomy Cast, a nonprofit resource provided by Astro Speer New Media Association, Fraser Fraser Cain: 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 at AstronomyCast, like us on Facebook or circle us on Google Plus.
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