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In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

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Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#31

Post by stevenwoo »

hn_throwaway_99 wrote: post #2

I love articles like this. I feel like too often in science education (at least my science education) that laws and theories are presented as just something that you need to memorize, when in my opinion the stories of how things were originally discovered and figured out is eminently more fascinating and inspiring. Like I remember having to learn all of these biochemical pathways, but I left school with nary a clue a…

Did you get your physics education in high school or university? I only had to take one physics class in the USA at college for my major, quantum electrodynamics for electrical engineering but my professor wrote the textbook and I recall he went over each experiment starting from the fundamentals of our understanding of the basics of the atom, Newton's understanding of light at the time, double slit experiment, to Maxwell's equations, the Michelson Morley ether experiment, to deriving then proving experimentally proving general relativity and decomposing GR into Newtonian physics/other laws of electromagnetism, I am still in awe at the people just figuring this stuff out from first principles.

Anyways, I haven't read this (have it on hold at my library) but someone recommended this book on reddit How to Make an Apple Pie from Scratch: In Search of the Recipe for Our Universe, from the Origins of Atoms to the Big Bang https://www.publishersweekly.com/9780385545655

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#32

Post by Terr_ »

n_plus_1_acc wrote: post #28
Terr_ wrote: post #15

That reminds me of the Millikan & Fletcher oil-drop experiment [0], which measured the charge of the electron. In short, microscopic atomized oil droplets had their fall-time through air measured to figure out their volume, and then a known electric field was used to levitate them. The calculated charge-per-molecule clustered around multiples of a smaller value, which would be the charge of an individual electron. [0…

How can you make sure you don't end up with 2e as a result? (Or any other multiple)

In 1909 the results results were couched in some "elementary electric charge" quantity, since the now-familiar subatomic particle model (and the "electron") was still gaining acceptance.

I expect that the greater the number of trials, it becomes easier it is to detect a distinction between closer-multiples, and if at some point more trials stops changing the answer then you've likely converged on e, unless there's some new principle like "X-ray exposure only affects charge in in multiples of e greater than one."

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#33

Post by dekhn »

n_plus_1_acc wrote: post #28
Terr_ wrote: post #15

That reminds me of the Millikan & Fletcher oil-drop experiment [0], which measured the charge of the electron. In short, microscopic atomized oil droplets had their fall-time through air measured to figure out their volume, and then a known electric field was used to levitate them. The calculated charge-per-molecule clustered around multiples of a smaller value, which would be the charge of an individual electron. [0…

How can you make sure you don't end up with 2e as a result? (Or any other multiple)

He did- he selected the lowest value, ignoring all the multiples.

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#34

Post by nkmnz »

n_plus_1_acc wrote: post #28
Terr_ wrote: post #15

That reminds me of the Millikan & Fletcher oil-drop experiment [0], which measured the charge of the electron. In short, microscopic atomized oil droplets had their fall-time through air measured to figure out their volume, and then a known electric field was used to levitate them. The calculated charge-per-molecule clustered around multiples of a smaller value, which would be the charge of an individual electron. [0…

How can you make sure you don't end up with 2e as a result? (Or any other multiple)

You do. Thae size of the steps between the results is the “quantum” of a single transferable charge.

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#35

Post by jjk166 »

bqmjjx0kac wrote: post #4

> Assuming that the oil formed a single layer of molecules — a monolayer — then the thickness of the oil film is the same thing as the length of one oil molecule. How did he know that the film of oil was one molecule thick? It feels like a huge assumption to me, but maybe this blog post left something out.

If there were multiple layers of molecules then the film would spread out over a wider area. With repeated experiments it would be clear that films are always an integer multiple of this thickness and never thinner.

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#36

Post by dekhn »

misnome wrote: post #27
dekhn wrote: post #20

Earlier quoted context omitted.

Some powder is added to the water, which covers the surface of the water but not the oil patch (which is circular). Then the oil patch diameter is measured.

This was how we did this when we replicated this experiment in high school. I guess from the other responses here that this wasn't common?

The original way was to cover the surface of a round bowl with oil. It certainly makes a lot more sense to me than trying to measure a floating disk of oil.

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#37

Post by ummonk »

bryanlarsen wrote: post #24
ummonk wrote: post #16

Earlier quoted context omitted.

Seems questionable that Rayleigh would have known that oil molecules are hydrophobic on one end and hydrophilic on the other.

He certainly knew oil was hydrophobic. I don't think the hydrophilic nature was necessary for the logic. If it was, I'm sure he knew that soap oils are both hydrophobic and hydrophilic and had ways of figuring out that soap oils consist of a single type of molecule and aren't a mixture.

He used olive oil though, not a soap oil.

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#39

Post by SyzygyRhythm »

n_plus_1_acc wrote: post #28
Terr_ wrote: post #15

That reminds me of the Millikan & Fletcher oil-drop experiment [0], which measured the charge of the electron. In short, microscopic atomized oil droplets had their fall-time through air measured to figure out their volume, and then a known electric field was used to levitate them. The calculated charge-per-molecule clustered around multiples of a smaller value, which would be the charge of an individual electron. [0…

How can you make sure you don't end up with 2e as a result? (Or any other multiple)

For that to happen, you would have to be very unlucky: all of your measurements would have to be 2e, 4e, 6e, etc. If a 3e or 5e sneaked in there, you'd realize that the charge was e, not 2e. With enough measurements, you can be confident that you've hit all the expected multiples of the quantum.

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#40

Post by jjk166 »

thirdhaf wrote: post #9
nick238 wrote: post #3

The page is timing out for me, but is it the inverse problem of the time when Steve Mould/Matt Parker measured the unknown quantity π, but already assuming a size of the molecules? Presumably Lord Rayleigh already had a at least a good order-of-magnitude approximation of pi... https://www.youtube.com/watch?v=lmgCgzjlWO4

By 1870 pi was known to several hundred decimal digits, for something like this calculation where you have other large sources of error Archimedes approximation from 2 millennia earlier would probably be fine. ( https://en.m.wikipedia.org/wiki/Chronology_of_computation_of...

Note that pi to 40 digits is sufficient to calculate the circumference of the observable universe to subatomic precision.
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