Favourite examples of good experimental technique

[Strange]

I thought it would be interesting to find examples of really clever experimental techniques.

So not not necessarily experiments that have produced important (or difficult) results. And not just examples of really advanced technology. But experiments that used clever "tricks" to eliminate errors, cancel out other effects, or just to make the effect visible in the first place.

One example is Pound-Rebka: this used Doppler shift to cancel out gravitational time dilation of gamma rays from the top of a tower. They then did the experiment again with the source at the bottom of the tower. By subtracting the two results, they doubled the measured value (and probably cancelled out some noise).

Any other favourites?

[Carrock]

One of my favourites is what used to be called the Millikan oil drop experiment. (It seems Fletcher's contribution was not acknowledged.)

 

Measure the mass of a negatively charged oil drop by letting it fall at terminal velocity.

Then calculate the charge from the electric field required to provide exactly the upward force required to balance the gravitational force.

The calculated charge is always an integer multiple of the (now known) electron charge.

Never having investigated the experiment, I still have some of my original awe that such a conceptually simple (in hindsight) experiment could measure something as tiny as the electron charge.

 

 

[Sensei]

31 minutes ago, Carrock said:

Measure the mass of a negatively charged oil drop by letting it fall at terminal velocity.

If you shoot x-ray photon at oil drop to ionize it, oil drop will be positively charged afterward, and free electron will be ejected.

ps. Sorry for nitpicking..

 

Edited April 12, 2019 by Sensei

[studiot]

I think it was here we had a thread which had lots of good examples in the measurement of liquid volumes.

So burette measurement by reading difference.

Pipette measurement of a set volume.

 

Then there is four terminal measurement in electric circuitry.

 

And measurement by difference on several parts of a tape measure.

And the 'rise and fall' self checking method of levelling in surveying.

[MigL]

"Never having investigated the experiment", Carrock ?

I assumed your background was Physics.
Millikan oil drop is standard 1st year experiment for Physics.
( at least it used to be 4 decades ago )

[Sensei]

4 hours ago, Strange said:

I thought it would be interesting to find examples of really clever experimental techniques.

I like hard ionization mass-spectrometry.

Separation of isotopes with different m/z.

Separation of atoms with different m/z.

 

[Strange]

2 hours ago, Carrock said:

One of my favourites is what used to be called the Millikan oil drop experiment. (It seems Fletcher's contribution was not acknowledged.)

That is a good one.

It is similar conceptually to the way that the kilogram is being redefined in terms of fundamental units (there was a thread about this last year): https://www.nist.gov/si-redefinition/kilogram-kibble-balance

[Carrock]

1 hour ago, Sensei said:

If you shoot x-ray photon at oil drop to ionize it, oil drop will be positively charged afterward, and free electron will be ejected.

ps. Sorry for nitpicking..

 

Guess I'll denitpick by appealing to authority...

 

51 minutes ago, MigL said:

"Never having investigated the experiment", Carrock ?

I assumed your background was Physics.
Millikan oil drop is standard 1st year experiment for Physics.
( at least it used to be 4 decades ago )

I was told about the experiment; I don't recall doing it. And I didn't do the calculations so it still seems improbable.

Sometimes ignorance is worth preserving. I'll keep my vestigial sense of awe unless (improbably) I have to learn the details.

[donnieporko]

One of my favourites is Electrospray Ionization Mass Spectrometry

The success of the study of gas-phase ion chemistry and its application has been driven by the continuous advancement of the mass spectrometric technique since the studies were performed by Thomson

[John Cuthber]

On 4/12/2019 at 8:08 PM, Sensei said:

If you shoot x-ray photon at oil drop to ionize it, oil drop will be positively charged afterward, and free electron will be ejected.

ps. Sorry for nitpicking..

 

The x ray is more likely to hit an air molecule than an oil drop.

That will give rise to all sorts of bits and pieces.

Which one sticks to a drop will define the eventual charge.

You can, in fact, set up the experiment to look at drops with either charge.

However, Millikan was looking at the charge on electrons.

And it was not strictly safe for him to assume that the positively charged entities had the same charge as an electron.

So he looked at drops with negative charges.