12.1.5 Millikan’s Oil Drop Experiment

• This experiment was conducted by Millikan and Fletcher in 1909
• It determined the value of fundamental or elementary charge

Method for Millikan's Oil Drop Experiment

• A fine mist of atomised oil drops is sprayed into a chamber 
  • Oil is used instead of water because it does not evaporate quickly
  • This means the mass of the drops will remain constant
• As the drops pass out of the spray nozzle they are ionised by X-rays
  • This consequently changes their charge from neutral
  • They will become positively charged if they lose electrons 
  • They will become negatively charged if they gain electrons
• The drops pass into a region between two metal plates and are viewed using a microscope

Equipment Set Up for Millikan's Oil Drop Experiment

In Millikan's Oil Drop Experiment oil is sprayed into a chamber before passing between metal plates where the electric and gravitational forces are compared

Condition for Stationary Oil Drops

• The charged oil drops fall into a uniform electric field between plates separated by distance d  with potential difference V
  • Negative oil drops with magnitude of charge Q  experience an upward force from the uniform electric field 
  • The magnitude of this force F  is:

• The falling oil drops can be held stationary between the plates by increasing this upward force
  • For this to occur, the force F  has to be equal to the weight of the oil drop, mg, so there is no resultant vertical force on each drop
• Therefore, the condition under which oil drops are held stationary is:

• The aim of the experiment, however, was to determine the charge Q  of each oil drop
  • For that, Milikan needed to determine the mass of each oil drop, so he used Stokes' Law