Millikan’s Oil Drop

Purpose: The purpose of this lab was to determine the charge, or number of excess electrons, carried by an oil droplet. By allowing oil droplets to enter an electric field with known potential difference across the plates and distance between the plates, the fall and rise velocity could be determined experimentally. From here the radius, mass, and charge of the oil droplet could all be determined mathematically. In result, the number of excess electrons on the oil droplet would be verified.
Sketch:

Equipment: * Pasco Millikan Oil Drop Apparatus Model AP-8210 Platform and plate charging switch * 12 volt DC transformer * Non-volatile oil * Atomizer * High voltage DC power supply * Digital multimeter * Patch cords with banana plug connectors * Stopwatch

Theory: q – Charge carried by droplet – Separation of the plates in the condenser in meters - Density of oil () m – Mass of droplet g – Acceleration of gravity in t – Time in seconds
E – Electric field - Viscosity of air in b – Constant,
P – Barometric pressure in Pascal a – Radius of the drop in meters - Velocity of fall in m/s - Velocity of rise in m/s
V – Potential difference across the plates in volts

Procedure:
Equipment Setup: 1. Ensure the room is as dark as possible as to were one can still read the multimeter, stopwatch, and recorded data. 2. Place the equipment on a flat surface free of drafts and movement. 3. Level the apparatus utilizing the bubble level and leveling screws. 4. Take apart the droplet viewing chamber and gauge the thickness of the plastic spacer with accuracy to determine the distance between the plates. Record this value. 5. Put everything back together with care and unscrew the focusing wire from its storage place. Insert it into the hole found on the upper capacitor. 6. Connect the 12 V DC transformer into the halogen lamp housing to power it on. 7. Bring the reticule into focus by turning the reticule-focusing ring. 8. Bring the wire into precise focus by turning the droplet-focusing ring. 9. Adjust the horizontal filament adjustment knob so the light is brightest on the right side of the wire. 10. Return the focusing wire back to its storage place and place the droplet hole cover back on. 11. Connect the high voltage DC power supply to the plate voltage connection utilizing banana plugs. Adjust the power supply to deliver about 500V. 12. Use a multimater to determine the voltage delivered. Record this value. 13. Connect the multimeter to the thermistor connectors and measure the resistance of the thermistor. Utilize the Thermistor Resistance Table found in the theory to determine the temperature of the lower brass plate. This value should match the temperature within the droplet-viewing chamber. Record this value.

Experimental Procedure: 1. Put non-volatile oil of known density into the atomizer and prepare the atomizer by quickly squeezing the bulb until oil is spraying through the tip. 2. Move the ionization source lever to the Spray Droplet Position. This will allow air to escape from the chamber as droplets are placed into the chamber. 3. Place the nozzle of the atomizer into the hole on the lid of the droplet-viewing chamber and squeeze the atomizer bulb with a quick squeeze while observing through the scope. The objective is to get a small amount of drops into the chamber. If it is too cluttered, simply wait a few minutes for them to disperse. 4. When you see a shower of drops through the viewing scope, move the ionization source lever to the OFF position. 5. Select a drop in view that falls between 0.02-0.05 mm/s when the plate charging switch is on the grounded position, but can also be driven up and down when voltage is applied. 6. Fine tune the focus onto this droplet, and keep it in view. 7. By turning the charging switch on and off over, measure the rise and