Re: Millikan Apparatus- best one ?

[stenton@phastf.phys.UVic.CA (Don Stenton)]

> 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.

I believe there is some misunderstanding regarding the direction the
experimenter should take in establishing an exact value for e. I would
agree that repeating the experiment for different size drops is valid
exercise in determining a most accurate value for e. But, the main effort
lies in retaining a drop for a sufficiently long time to establish all
possible multiples of the elementary charge.

Millikan explains this point in his book, Millikan, Robert A., The Electron
(The University of Chicago Press, 1963). Referring to one of his tables of
data, he states, "...It will be seen that during time of observation, about
four hours, this drop carried all possible multiples of the elementary
charge from 4 to 17, save only 15. No more exact or more consistent
multiple relationship is found in the data which chemists have amassed on
the combining of powers of the elements and on which the atomic theory of
matter rests than is found in the foregoing numbers."

Although Millikan performed the experiment with drops of other materials,
observations were all made upon the same drop, reducing the theoretical
uncertainty in the method to one of whether or not Stokes' Law applies to a
falling drop.

....Don

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Don Stenton                        voice: (604) 721-7709
Dept. of Physics and Astronomy     fax:  (604)  721-7715
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