®
Model No. AP-8210A
Historical Information
012-13093B
15
Experiments with the balanced water drop produced the value of 3.422 x l0
-l0
e.s.u. for
e
. The most important
aspect of these experiments, however, was the observation by Millikan that a rising drop would suddenly change
its velocity. This phenomenon could easily be produced by placing a radioactive source near the drop. This dem-
onstrated that the drop had “captured” an ion, thus changing the charge of the drop and its respective velocity.
The Exact Evaluation of
e
In 1909 Millikan set about building a new piece of apparatus designed for the observation of single oil drops for
extended periods of time. Since water drops had proved inadequate for prolonged observation of this ion catching
phenomenon, Millikan used oil drops, which were not affected by evaporation. The apparatus consisted of two
parallel brass plates separated by a distance of 16 mm by ebonite blocks. Non-volatile oil was sprayed into the
chamber above the plates, and small drops slowly found their way into the area between the plates through a small
hole in the top plate. The drops were illuminated by a beam from a carbon arc lamp and were observed through a
measuring scope. The details of the construction of Millikan’s final apparatus built in 1914 (which was basically
similar to his earlier devices, and for the purposes of this discussion can be considered the same as the earlier
pieces of apparatus) attest to the effort expended in obtaining the most accurate evaluation of e possible. The fol-
lowing passage is part of Millikan’s description of the apparatus, including a diagram of the device.
“Accordingly, I built two years ago a new condenser having surfaces which were polished optically and made flat
to within two wave-lengths of sodium light. They were 22 cm. in diameter and were separated by three pieces of
echelon plates, 14.9174 mm. thick, and having optically perfect plate surfaces. The dimensions of the condenser,
therefore, no longer introduced an uncertainty of more than about 1 part in l0,000.”
5
“Complete stagnancy of the air between the condenser plates was attained, first, by absorbing all the heat rays
from the arc lamp by means of a water cell 80 cm. long, and a cupric chloride cell, and secondly, by immersing the
whole vessel in a constant temperature bath of gas-engine oil (40 liters), which permitted, in general, fluctuations
of not more than 0.02 °C during an observation.”
6
S
C
V
B
G
D
A
X
to pump
e
m
p
M
N
s
a
w
d
c
g
c
g
Figure 11: Diagram of Millikan’s Apparatus
7
A
, atomizer through which the oil spray is blown into the cylindrical vessel,
D
.
G
, oil tank to keep the
temperature constant.
M
and
N
, circular brass plates, electric field produced by throwing a 10,000-volt
battery
B
. Light from arc lamp
a
after heat rays are removed by passage through
w
and
d
, enters chamber
through glass window
g
and illuminates droplet
p
between plates
M
and
N
through the pinhole in
M
.
Additional ions are produced about
p
by X-rays from the bulb
X
.