Robert Milliken (born: 1868)
Robert Millikan was the physicist behind Millikan's oil drop experiment.  He was born in 1868 and raised in a small town in Iowa.  He Excelled in physics as a university student and was even asked to teach physics before he graduated.  After graduating, Millikan became a professor of physics and busied himself with various research projects. 

Millikan made two major contributions to the body of knowledge we call modern physics.  He contributed to the development of Planck's constant, a number which is vitally important to quantum mechanics.  He also performed his "oil drop experiment" which is the subject of this web site!  This experiment, performed between 1909 and 1913 demonstrated the quantization of charge.

       

In 1923 Millikan received the prestigious Nobel Prize for Physics for his work with the quantization of charge and Plank's constant.

Millikan's experiment showed that electrical charge is quantized, meaning that charge exists only in little "pieces" of definite size.  The size of any electric charge is an integer multiple of this smallest possible "elementary charge."  Consider the following analogy; an electric charge is like a brick wall - the smallest possible size for a wall is one brick, a single brick is like the elementary charge.  Just as a wall can not exist that is smaller than one brick, an electric charge can not exist that is smaller than the "elementary charge."  Any large brick wall is made up of an integer number of bricks, just as any large electric charge is made up of an integer number of elementary charges.

        Before the oil drop experiment no one knew if charge existed in "pieces" of a definite size, there was no reason to think charge could not exist in an infinitely small amount.  Today many high school physics students know that the elementary charge Millikan observed is the size of the positive or negative charge carried by a single proton or electron  but back in the early 1900's when Millikan performed his experiment even the top physicists in the world were still unaware of the quantization of charge.

As the name implies, Millikan's oil drop experiment studied the physical interaction between a system of surroundings and a drop of oil.  A diagram of Millikan's apparatus has been included to help explain how the experiment was carried out.

Millikan began his experiment by using the atomizer to spray a fine mist (a bunch of drops) of oil into the region above both charged plates.  (Originally the experiment was performed with drops of water, but the water would often evaporate before the experiment was through, so Millikan decided to use oil, which has a lower evaporation rate than water under standard laboratory conditions.)   Some of the oil drops would enter the region between the two charged plates, which I shall refer to as the lower chamber, through the small hole in the top plate.  Millikan and his colleges were able to view these drops by shining a light, which is not shown in the diagram, onto the drops and viewing them through the lense of the microscope.

As the oil drops fell through the air, they would become electrically charged one of two ways.  Possibly, when the drops fell through the air electrons (which carry a negative charge) would "rub off" air molecules and "stick to" the oil droplets due to friction.  However, this method of charging proved to be somewhat unreliable as it resulted in very few oil drops becoming electrically charged.  Most often Millikan would radiate the lower chamber with x-rays which caused the air molecules inside to ionize so that some free electrons were present in the lower chamber.  As oil drops fell through the ionized air these free electrons became attached to the oil drops.  This method dramatically increased the probability that an oil drop would become charged.

The force due to gravity acted on the drops, tending to move them downward.  The force due to gravity is described by

Fg = mg

Where Fg is the force due to gravity on the drop, g is the gravitational field, and m is the mass of the drop.

When the power supply attached to the parallel plates was turned on the region between the plates contained a uniform electric field.  Millikan could easily calculate the magnitude of this electric field because he could measure the distance between the plates and use a voltmeter to measure the difference in potential between the top and bottom plates.  By dividing the applied voltage by the distance between the plates Millikan would obtain the electric field created by a specific voltage.

     A charged drop of oil in the lower chamber would experience a force, according to:

Fe = Eq

Where Fe is the force on the drop, E is the electric field between the plates, and q is the electric charge on the drop.  Millikan arranged the power source so that the electric field was orientated downwards, and the negatively charged oil drops would experience a force in the upwards directions, since a negatively charge object always experiences a force in the opposite direction of any electric field it is placed in.  Millikan could adjust the voltage of the power supply, thereby adjusting the electric field and electric force.

Since the electric force opposed the force due to gravity, when the magnitudes of the two forces were equal the drop would reach a state of equilibrium, the total force acting on it would be zero because the equal-but-opposite electric and gravitational forces would cancel each other out.  When the drop stood still in the lower chamber Millikan knew that the electric force and gravitational forces were equal in magnitude.

  Fg = mg and Fe = Eq

  Fe = Fg

 mg = Eq

   q = mg/E

Using the formula derived above Millikan could calculate the magnitude of the electric charge on an oil drop.  Once he did this he sprayed in a new oil drop and did it over again.... about a thousand times!  (He calculated the value of m by allowing drops to fall in the lower chamber with the electric field turned off so no electric force was experienced.  The drops would reach a maximum or "terminal" velocity when the gravitational force downward was equal to the viscous force upward caused by the drop colliding with air molecules.  By measuring this terminal velocity Millikan could calculate mass using concepts from fluid mechanics.)

Since randomly selected drops had different number of free electrons attached to them, they had different charges as well.  Millikan would change the voltage between the plates slightly before starting a run of the experiment so a drop with a different charge would be suspended in equilibrium.  This allowed Millikan to obtain many different values for the charge on a single oil drop.