The Oil-Drop Experiment

Here's a diagram of Millikan’s oil-drop apparatus. The main part of the apparatus is a big, hollow metal chamber, shown below:

An atomizer (“A” in the diagram) sprays oil drops into the chamber. Originally, Millikan used the atomizer from a perfume bottle (as in the photograph, I’m guessing), but over time he got fancier and starting using a pressure tank and passing the oil through glass wool before it got to the chamber, presumably to eliminate dust.

As the oil drops pass through the atomizer, they pick net electric charges due to friction, the same way that you and I pick up a net electric charge when we shuffle across the floor wearing wool socks. The charge oil drops then fall under the force of gravity until some of them pass through a small opening so that they are between the two metal plates labeled “M” and “N” in the diagram. Those plates are connected to a series of batteries. Those batteries set up an electric field across the plates. One can tweak the direction and magnitude of this electric field so that it exerts an upward force on a given drop that exactly balances the downward gravitational force on that drop. In this way, one can “capture” a drop and hold it suspended.

In an early version of the oil-drop experiment (actually, at that point it was a water-drop experiment), Millikan would simply balance the electric and gravitational forces on a drop until the drop became suspended, and then calculate the electric charge that the drop would have to have for the two forces to balance, based on an estimate of its mass. That procedure was a good start, but it wasn’t terribly precise and it didn’t allow for direct measurements of changes in charge on a single droplet, which would provide the most convincing evidence that electric charge comes in discrete multiples of a fundamental unit.

Millikan switched from water drops to oil drops because the latter evaporate much more slowly, allowing him to make a long series of observations on a single drop during which the mass of the drop remained essentially unchanged. After he had captured a drop, he would switch off the electric field and record the time it took for the drop to fall under an electric field alone. He would then switch on the electric field, with its magnitude adjusted so that it exerted an upward force of the drop that was stronger than the downward force of gravity. He would then record the time it took the drop to rise the same distance as before. From this data, he could calculate the charge on the drop.

Occasionally, during a series of observations an oil drop would pick up a positive or negative ion from the air around it, changing its charge. This change would manifest itself as a change in the speed of the drop during the rising phase of its movement. Millikan used a source of ionizing radiation to make these illuminating events more common. Sometimes a drop would pick up a charge in mid-rise, so that its speed changed suddenly and discretely while Millikan was watching it. Millikan says that it is particularly striking to see a rising drop suddenly stop moving—a phenomenon that the electron theory can explain easily as being due to a drop with one unit of charge picking up a unit charge of the opposite sign. One reason to re-do the experiment is to see these changes in the speed of a drop during its rising phase. Are they really as striking as Millikan says, or is he dramatizing the experiment for rhetorical effect?

Previous experiments had suggested that the fundamental unit of charge, if there is one, has a charge in the vicinity 3 x 10^-10 e.s.u. Thus, if there is such a fundamental unit, Millikan should find two things:

• First, all of his calculations of the total charge on a drop should yield values that are (approximately) integral multiples of a single number on the order of 10^-10 e.s.u.
• Second, the calculated changes in charge should occur in small integral multiples of the same number.

For the data Millikan reported, these conditions were satisfied beautifully. A few of his runs were anomalous, however, as I will explain in future posts. Millikan calculated a value of 4.774 +/- 0.009 x 10^-10 e.s.u. for the elementary electric charge, which is slightly smaller than a recent value of 4.80320420 +/- 0.00000019 e.s.u. (The discrepancy is due primarily to the fact that Millikan used an incorrect value for the viscosity of air.)

Next post: How we will re-create Millikan's experiment.