Millikan Oil Drop Charge Measurement 

        Introduction

The electric charge carried by any object such as an elementary particle cannot take arbitrary values, but in fact is quantized: it can take on only an integer multiple of a fundamental value e = 1.602 ´ 10^-19 C . This fact was first discovered experimentally by Robert Millikan in 1909, using an apparatus of his devising conceptually very similar to the one you will use in this experiment. His technique utilizes tiny oil droplets ejected from an “atomizer”, which also ionizes the oil droplets in the process (electrons are ejected from some of the atoms, providing a net electric charge). The droplets would normally fall down under the influence of gravity, but by providing an external upward electrostatic force via the application of an electric field, the forces can be equalized so that the net force on the droplets is zero. Note that the effects of air viscosity also have to be taken into account! When the zero net force condition is achieved, the small net charge carried by the droplets can be determined, and found to take on discrete values. For his fundamental discovery, Millikan was awarded the Nobel prize in Physics in 1923.

Equipment

    Pasco AP-8210 Millikan Oil Drop Apparatus, which includes:

•  Instruction manual and experiment guide

• Platform with droplet chamber, E field system, light source, viewing scope

• Atomizer

• Oil

• High voltage power supply

• Digital multimeter

• Stopwatch

Procedure

               Read carefully the Pasco manual accompanying the set up. In particular, make sure you understand the theoretical motivation and method, and that you know what parameters are known and which need to be measured. Follow the detailed instructions in the manual for setting up the equipment and making the charge measurements.

               The success of this experiment lies in taking great care in following the procedures painstakingly. Make sure you understand each step before you attempt it, and be gentle in your tweaking of any variable, while monitoring the system closely.

               As described in the manual, select various droplets in turn. Watch them drop under the influence of gravity and air viscosity alone to determine their terminal velocity. Turn on the electric field to cause the droplets to rise or fall, and measure their velocity at the voltage applied. For each droplet, make at least 10 – 20 velocity measurements to get an accurate average. You can change the charge on the droplet by exposing it to the ionizing radiation from an embedded radioactive source. Do this to one individual droplet, and watch it rise or fall faster with increased ionization. Ionize it multiple times, each time measuring the velocity of the same droplet. This will yield the most accurate elementary charge determination.

               The equation used to determine the final charge on the droplet is a complex one, that requires multiple parameters. Make sure you estimate the uncertainties on all parameters, and that you carry out a careful error analysis of your measurements!

History

               Read the interesting story of Harvey Fletcher, the person who actually did the experiment and first suggested using oil--and why he never got any credit.