Classic Physics Experiments

Mike Hewson
Mike Hewson
Joined: 1 Dec 05
Posts: 6,123
Credit: 121,462,588
RAC: 44,694
Topic 209492

I found on YouTube a great demonstration of a classic experiment performed by Robert Millikan over one hundred years ago. It is true brilliance. With straightforward apparatus ( not a multi-billion dollar device ) and a moderate amount of care one can establish a marvelous fact about our universe : one can only measure electric charge in integer multiples of a basic unit. One can measure that amount* but regardless of the precision of that estimation one is still left with the inescapable conclusion of charge quantisation. Specifically integer could mean zero, and of course plus or minus whole numbers.

{ Note that I said measure. Allegedly more basic particles eg. quarks are assigned fractions of this charge however they are not able to be separated apart from one another ( skip complex explanation ) in practice to directly measure on their own. Though these fractions are indirectly established as true by the consistency of predictions with respect to experimental outcomes. In any event these fractions have only ever been found to be integer multiples ( yet again ) of one-third of the electron charge. }

There are many variations on exactly how to go about this process. I did one of those during undergraduate physics. I recall having to think carefully for a very long time as to why Millikan's technique guaranteed the final deduction of the experimental process. Fortunately I used professionally prepared apparatus in a laboratory session. You could probably buy some online kits, but beware it is not hard to spend a 4+ figure sum on this. 

Bear in mind Millikan's circumstances ( ~1909 IIRC ) : the actuality of an electron was still contended, atoms too for that matter. His work is a lovely example of hard data forcing a sea change in thinking which some two decades later gave us most of the modern formulation of quantum mechanics.

Cheers, Mike.

* Strictly speaking, the mass to charge ratio of the electron and then use other determinations to yield that charge as a physical quantity.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal