Re: Millikan Apparatus- best one ?

[sample@lyon.edu (John D. Sample)]

At 5:28 PM on 10/11/96, <phys-l@mailer.uwf.edu> wrote:

> > I think I had the same experience as you.  As a student we measured only 5
> > drops, but one had very nearly twice the charge of another.  The other
> > three had approximately 3/4 the charge of the smaller of the first two, so
> > we guessed the last three each had three units of charge apiece, and then
> > one had four and the other 8.  Using least squares we calculated
> > e=1.675E(-19) C.  Unfortunately we weren't required to calculate the
> > uncertainty in this, only the percent error.
> >
> > Even using so few drops there is little possibility that all of them might
> > have an even # of charges and so would yield 2e instead of e.
>
> I had hoped someone else would join the interrogation, but no one did.
> Did it occur to you to wonder why Millikan had to measure many more
> than five drops? (I think he measured thousands.) If your uncertainty
> in measurement of the absolute charge on that 8e drop was +/-5% then
> you could not even claim that the number of charges was 8; it could
> easily have been 7 or 9!
>
> Five drops is certainly not sufficient to measure the quantum of
> charge. The whole point of the measurement is missed if students are
> instructed to measure five charges.
>
> Leigh

I tend to agree with you.  (Although it looks more like 11 or 12% to me).
I went to my undergrad lab notebook to look at my work when the thread
started, and I have to admit I was kind of disappointed at it, although I
apparently did what was expected.  I would liked to have seen explicit
error analysis to put an uncertainty on the charge and to see whether the
quantization assumption is even justified.

Just guessing here, but I'll bet that the professor knew we could measure
the charges accurately enough with the equipment at hand to get decent
results (it was not a "student" apparatus), and because we had only slide
rules to do the computations, we probably weren't required to do anymore
than the minimum to get an answer we could compare with the accepted value.
In that sense, the experiment became a test of the carefulness of reading
and handling the data, much like what you might be expected to do in
chemistry lab.  It was a tedious procedure:  each drop was timed during 5
descents and 5 ascents in the chamber.  Included in the calculations were
compensations for drag and buoyancy of air.  (Is this standard in an
undergraduate lab?)

With computers today, one can automate the computations and data from more
drops could have been collected and analyzed in the same amount of time.
Then students can actually do "science"; analyze data and demonstrate
whether, within the uncertainty of the results, they support or contradict
a particular hypothesis.  I much prefer this to measuring things with the
purpose of comparing with an accepted value.

Chip