Millikan Oil Drop Apparatus
Equipment
®
10
012-13093B
•
NOTE
: Remember that the droplets are being forced into the viewing area by the pressure of the atomizer.
Excessive pumping of the atomizer can cause too many droplets to be forced into the viewing area and, more
importantly, into the area between the chamber wall and the focal point of the viewing scope. Drops in this
area prevent observations of drops at the focal point of the scope.
•
NOTE
: If the entire viewing area becomes filled with droplets so that no one drop can be selected, either wait
three or four minutes until the droplets settle out of view, or turn off the DC power supply and disassemble the
droplet viewing chamber, thus removing the droplets.
•
NOTE
: When the amount of oil on the parts in the droplet viewing chamber becomes excessive, clean the
parts as detailed in the Maintenance section. Remember that the less oil that is sprayed into the chamber, the
few times the chamber must be cleaned.
Selection of the Droplet
1.
From the drops in view, select a droplet that both falls slowly (about 0.02 to 0.05 mm/s) when the plate charg-
ing switch is in the “Plates Grounded” position, and can also be driven up and down by turning the plate
charging switch to “TOP PLATE –” or “TOP PLATE +”.
•
HINT
: A droplet that requires about 15 seconds to fall the distance between the major reticle lines (0.5 mm)
of the viewing scope will rise the same distance, under the influence of an electric field (1000 V/cm) in the
following times with the following charges:
•
NOTE
: If too many droplets are in view, you can clear out many of them by turning the plate charging switch
to “TOP PLATE –” (connecting power to the capacitor plates) for several seconds.
•
NOTE
: If too few droplets have net charges to permit the selection of an appropriately sized and charged
drop, move the ionization source lever to the ON position for about five seconds.
2.
When you find an appropriately sized and charged oil droplet, fine tune the focus of the viewing scope.
•
NOTE
: The oil droplet is in best focus for accurate data collection when it appears as a pinpoint of bright
light.
3.
Turn the brightness adjustment knob on the light to optimize the contrast between the illuminated drop and the
dark background.
Collecting Data on the Rise and Fall of the Oil Droplet
1.
Measure the rise velocity (when the plates are charged) and the fall velocity (when the plates are not charged)
about 10 to 20 times. Maneuver the droplet up and down as needed using the plate charging switch.
•
NOTE
: The greatest accuracy of measurement is achieved if you time from the instant that the bright pinpoint
of light passes behind the first major reticle line to the instant that the pinpoint of light passes behind the sec-
ond major reticle line. (These reticle lines are 0.5 mm apart.)
2.
Calculate the charge on the droplet. If the result of this first determination for the charge on the drop is greater
than 5 excess electrons, you shown select slower moving droplets in subsequent measurements.
Table 1.1:
Time
Excess electron
15 s
1
7 s
2
3 s
3
