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\subsubsection*{PHSX 222L\hfill  Millikan Oil Drop\hfill Name:\underline{\hspace*{2in}}}

\underline{Object:} 1) Repeat Millikan's famous oil drop experiment to
determine the charge $e$ on the electron (and that it is quantized). 
2) Find out how hard physics experiments can be.

\underline{Theory:} Millikan's oil drop experiment is one of the
classic physics experiments of the past century; it is also one of the
hardest to duplicate in a student laboratory.  For his work on this
and the photoelectric effect Millikan received the Nobel prize in 1923
(though his graduate student, Harvey Fletcher, deserves more of the
credit than is widely known).

Small drops of oil are injected into the space between two horizonal 
metal plates.  They normally begin to fall due to gravity, but if a potential 
difference is applied to the plates the electric field can exert a 
force on the drops if they carry a net charge; by controlling the 
voltage between the plates the experimenter can make the drops move 
up or down and control their speed.

For example, if the voltage is adjusted to bring the drop of mass 
$m$ and charge $q$ to rest (so 
the electrical force just balances the gravitational force), then 
\begin{eqnarray}
F_{g} &=& F_{e}\\
mg&=&(V/d)q
\end{eqnarray}
where $d$ is the distance between the plates and $V$ is the potential 
difference between them.  Then we can solve for $q$ in terms of 
quantities that can be ascertained:
\begin{equation}
	q = m g d / V.
\end{equation}
Of course Millikan and Fletcher did not know the mass of their oil 
drops beforehand, so some figuring using Stokes' Law was necessary.

\goodbreak
\underline{Pre-lab:} 
Before you come to lab, watch {\em The Mechanical Universe}
video (episode 12) about the Millikan Oil Drop experiment and read the
accompanying section from {\em The Mechanical Universe} textbook. 
Also read the \emph{Physics Today} article entitled ``My Work with
Millikan on the Oil-Drop Experiment'' by Harvey Fletcher.  The 
readings can be found on the class website.

\goodbreak
\underline{Procedure:} 
\vspace{-.25in}
\begin{enumerate}

\item Run the experiment as a computer simulation on a DOS computer. 
See the instructions that came with the software.  The software has
three versions of the experiment ranging from simple to hard (the hard
version is almost as hard as the real original experiment).  You will
have time for only one version.  Take and analyze data as if it were
real data, and compute $e$, the charge on the electron.  Use a
spreadsheet to tabulate and analyze your data.  Compare your value of
$e$ with the accepted value of $e= 1.602177 \times 10^{-19}$~C.
Especially note the quantization of your data (by plotting it in a
histogram).

\item Try the experiment on the real apparatus.  At least try to see 
the oil drops (they are extremely small points of light).

\end{enumerate}

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