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The Millikan Oil Drop Experiment
An experiment performed by Robert Millikan in 1909 determined
the size of the charge
on an electron. He also determined that there was a smallest
!nit charge" or that charge is#!antized. He recei$ed the %obel
&rize for his work. 'ere going to explain that
experiment here" and show how Millikan was able to determine the
size of a charge on asingle electron.
'hat Millikan did was to p!t a charge on a tiny drop of oil" and
meas!re how strong an
applied electric field had to be in order to stop the oil drop
from falling. (ince he was
able to work o!t the mass of the oil drop" and he co!ld
calc!late the force of gra$ity onone drop" he co!ld then determine
the electric charge that the drop m!st ha$e. )y $arying
the charge on different drops" he noticed that the charge was
always a m!ltiple of *1.+ x
10 *19 ," the charge on a single electron. -his meant that it
was electrons carrying this!nit charge.
Heres how it worked. Ha$e a look at the apparat!s he !sed
An atomizer sprayed a fine mist of oil droplets into the
chamber. (ome of these tiny
droplets fell thro!gh a hole in the !pper floor. Millikan first
let them fall !ntil theyreached terminal $elocity. /sing the
microscope" he meas!red their terminal $elocity" and
by !se of a form!la" calc!lated the mass of each oil drop.
%ext" Millikan applied a charge to the falling drops by
ill!minating the bottom chamber
with x*rays. -his ca!sed the air to become ionized" and
electrons to attach themsel$es tothe oil drops.
)y attaching a battery to the plates abo$e and below this bottom
chamber" he was able toapply an electric $oltage. -he electric
field prod!ced in the bottom chamber by this
$oltage wo!ld act on the charged oil drops if the $oltage was
!st right" the
electromagnetic force wo!ld !st balance the force of gra$ity on
a drop" and the drop
wo!ld hang s!spended in mid*air.
%ow yo! try it. ,lick here to open a sim!lation of Millikans
chamber. 2irst" allow thedrops to fall. %otice how they accelerate
at first" d!e to gra$ity. )!t #!ickly" air
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resistance ca!ses them to reach terminal $elocity.
%ow foc!s on a single falling drop" and ad!st the electric field
!pwards !ntil the drop
remains s!spended in mid*air. At that instant" for that drop"
the electric force on it exactlye#!als the force of gra$ity on it.
(ome drops ha$e more electrons than others" so will
re#!ire a higher $oltage to stop.
'hen yo!$e finished playing with the apparat!s" close the window
and well contin!e.
3.4." lets look at the calc!lation Millikan was now able to
do.
'hen a drop is s!spended" its weight m 5 g is exactly e#!al to
the electric force applied #
5 6
-he $al!es of 6" the applied electric field" m the mass of a
drop" and g" the acceleration
d!e to gra$ity" are all known $al!es. Millikan determined the
charge on a drop. -hen he
redid the experiment n!mero!s times" each time $arying the
strength of the x*rays
ionizing the air" so that differing n!mbers of electrons wo!ld
!mp onto the oil molec!leseach time. He obtained $ario!s $al!es for
#.
-he charge # on a drop was always a m!ltiple of *1.+ x 10 *19 ,"
the charge on a single
electron.
-his n!mber was the one Millikan was looking for" and it also
showed that the $al!e was#!antized the smallest !nit of charge was
this amo!nt" and it was the charge on a single
electron.
