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M o d e l N o . A P - 8 2 1 0 A
3.
Introduce more oil droplets into the chamber using the procedure previously described and select another
droplet.
4.
Measure the rise and fall velocities of the selected droplet about 10 to 20 times or until the charge changes
spontaneously or the droplet moves out of view.
5.
Bring the droplet to the top of the field of view and move the ionization lever to the ON position for a few sec-
onds as the droplet falls.
6.
If the rising velocity of the droplet changes, make as many measurements of the new rising velocity as you
can (10 to 20 measurements).
7.
If the droplet is still in view, attempt to change the charge on the droplet by introducing more alpha particles,
as described previously, and measure the new rising velocity 10 to 20 times, if possible.
8.
Repeat step (7) as many times as you can.
9.
Record the plate potential, the oil density, the viscosity of the air at the temperature of the droplet viewing
chamber (see Appendix A), and the barometric pressure for each set of velocity measurements.
•
NOTE: It is desirable to observe as many different charges on a single drop as possible.
Computation of the Charge of an Electron
•
The following formula is derived from equation ( 10 ) in the Introduction. Use it to calculate the charge of an
electron.*
*The formula is derived from equation ( 10 ) using SI units as defined below.
The definitions of the symbols used and SI units:
q – charge carried by the droplet [C]
d – separation of the plates in the droplet viewing chamber [m]
r – density of oil [kg/m
g – acceleration of gravity [m/s
h – viscosity of air in [N• s / m
b – constant, equal to 8.20 x 10
p – barometric pressure [Pa]
a – radius of the drop [m]
v
– velocity of fall [m/s]
f
v
– velocity of rise [m/s]
r
V – potential difference across the plates [V]
Note: The accepted value for e is 1.60 x 10
®
2
4
b
-- - g
----- -
q
=
3
2p
3
]
2
]
2
] (See Appendix A)
-3
Pa • m
-19
C
0 1 2 - 1 3 0 9 3 B
3
9v
v
+
v
b
f
f
r
---------- -
----- -
-------------- -
+
–
2g
2p
Ev
f
E q u i p m e n t
11
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