# Millikan's Oil Drop Experiment

1. Sep 4, 2007

### mbrmbrg

My chemistry professor was discussing Milikan's oil drop experiment today, and she said that he determined the mass of the oil drop by measuring its speed as it fell. (I found this further description online) If that's the case, why doesn't air resistence appear in the eqation Eq=mg from which e was derived?
I guess what's really bugging me is why, in the name of all that phyisicists hold dear, was this experiment not conducted in a vacuum?

~Malka

2. Sep 4, 2007

### mbrmbrg

Oh ho! This site purports that Millikan used the approach I assumed would have been taken--get the volume of a drop and compute mass by using density. Is the terminal velocity thing an urban myth?

3. Sep 5, 2007

### mgb_phys

He used the terminal velocity to calculate the mass of the drop - in fact the reason he got the wrong value was that he had the wrong viscosity of air.
The reason for not doing it in vacuum is simply that in vacuum the speed of the drop is independant of it's mass and so there is no way to measure the mass. The terminal velocity technique is a very clever method of measuring the mass of a very small object.

ps. Don't ever try and do this experiment as a lab project, it is even worse than Cavendish's G measurement to get right!

4. Sep 5, 2007

### AlephZero

That sounds like the voice of experience speaking, and I agree (about both experiments).

The idea of measuring the diameter using the telescope seems far fetched. From my memory of doing it, the only drops that were "catchable" were so small they appeared as points in the telescope. And anyway it's impossible to get the drop perfectly stationary to measure it visually.

It would be impossible to do this is a vacuum. You have to "catch" a drop by adjusting the field as it drifts through your field of view. If you lose sight of it, you never get to see that particular drop again. Ideally having caught a drop, you can do repeated experiments on it before you lose it. You don't set the field to zero to measure the terminal velocity, just change its value by a small amount so the terminal velocity is slow enough to measure.

As amazon.com would say, "People who enjoyed pushing pieces of string uphill will also like repeating Millikan's experiment"

5. Sep 5, 2007

### mbrmbrg

whoa... scary. Thanks!