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Determining the fundamental electric charge
Kit Peters∗
Dept. of Physics and Astronomy
College of Charleston
(Dated: May 2, 2005)
By measuring the terminal velocity of droplets of mineral oil sprayed into a hollow chamber,
and subsequently measuring the upward velocity of those droplets when an electric field of known
magnitude was applied, we were able to determine the charge on eachdroplet. We used an ionizing
source to change the charge on some of the droplets, then measured their charge again. We did this
a number of times for each droplet. Visual inspection of our data showed evidence of quantization,
so we grouped the individual charge values into eight bins of varying sizes. The mean difference
between the mean values of each bin was (1.72 ± 0.19) × 10−19 C. This is inagreement with the
accepted value for the fundamental electric charge, 1.602 × 10−19 C.

I.

INTRODUCTION

In 1897, J.J. Thomson discovered the electron. He was
able to experimentally determine the ratio of its charge
to its mass, but neither were known separately. In 1909,
Robert Millikan was able to determine the charge on the
electron.
FIG. 1: Top view of apparatus

Millikan’smethod was to spray tiny oil droplets into
the air above two charged plates separated by an insulating ring. The top plate had a small hole in it, and
it was through this hole that a few droplets of oil would
go through. By shining a bright light source into the
chamber, the oil drops were illuminated, and could be
observed through a microscope. Millikan first observed
these droplets with noelectric field present. The droplets
quickly reached terminal velocity and this could be used
to calculate the droplets’ mass. When the electric field
was then turned on, Millikan could vary the strength of
the field until the drops were stationary. This allowed
him to determine the charge on the drops.
Our apparatus[1] is similar to that of Millikan. It has
two metal plates separated by a clearplastic spacer. This
spacer is hollow in the center, creating a chamber that oil
droplets can enter through a small hole in the top plate.
The entire chamber is protected by a removable cylindrical shield that contains two circular windows. When
the shield is properly aligned, one of the windows allows
a light, attached to the apparatus near the chamber, to
shine into the chamber andilluminate its contents. This
window is dichroic in order to minimize heating of the
chamber. The chamber is viewed with an attached microscope. There are two parallel lines etched on the reticle
of this miscroscope. On the side of the chamber opposite
the light is a switch allowing one to allow radiation from
the built-in ionizing source to enter the chamber.

∗ URL:http://www.cs.cofc.edu/~cdpeters

FIG. 2: Closeup view of chamber

II.

PROCEDURE

We began by determining values for ρ, η , P , and d, all
of which remained constant througout the experiment.
s
We found ρ = 870 kg . We found η = 1.8365 × 10−5 N ·2 [2]
m
m
5
We found P = 1.016 × 10 P a[3]. We found d = 5.60 ×
10−3 m.
After the constant values had been determined, we began to spray drops into the chamber. Weplaced the
provided atomizer, which was filled with mineral oil[4],
above the entry hole for the chamber and sprayed until
we were able to see a droplet through the microscope.
We determined the droplet’s terminal velocity by applying an electric field to the drop sufficient in intensity to
cause it to rise. When the drop had risen above the top
line marked on the reticle of the microscope, weremoved
the electric field and allowed the drop to fall until it had
passed below the bottom line. We timed this fall. We
then applied the electric field again, causing the drop to
rise at constant velocity. We timed this rise, then removed the electric field and allowed the drop to fall once
again. After we had repeated the sequence of causing the
drop to rise and allowing it to fall a number...