THE MILLIKAN-BEGEMAN-LEE OIL DROP EXPERIMENT

Most electric charges that affect our lives come about from the presence or absence of electrons, which are so tiny that scientists are not even sure they have any size. Luckily, we can show the quantization of charge -- one electron, one vote -- by looking at objects we can see (with some help). Robert Andrews Millikan gets credit for discovering the charge of an electron. J. J. Thomson, of course, had earlier measured the charge-to-mass ratio for an electron, so with Millikan's work, both charge and mass were now both known. Millikan received the Nobel Prize in 1923 for this work and for experimental work which complemented Einstein's theory of the photoelectric effect. Actually, a graduate student, Louis Begeman (Ph.D. 1910) did the measurments, and another graduate student, Jhon Yiu-Bong Lee (Ph.D. 1914) imporved the method by replacing liquid spheres with solid ones.

LIBRARY RESEARCH. Locate some references on this experiment. Read them. Identify what quantities you would want to measure, what variables there might be to control. If you can find one or more detailed descriptions of the experimental setup, compare them and see if you can come up with any suggestions for improving this lab. You will be expected to explain this experiment in your lab writeup, so don't skimp on this step.

POWER SUPPLY. We will try to balance some charged objects with an electric field. In equilibrium, qE=mg. The electric field is formed by supplying a potential difference, DV, across two capacitor plates. We have some vintage vacuum-tube power supplies just for this purpose. Be sure to wait a minute or two after turning it on for the tubes to warm up.

THE 'OIL DROPS'. The objects suspended in the original experiment were oil drops which had picked up a slight charge by being squeezed through an atomizer. (Before that, Millikan had tried water, but the drops changed mass as they evaporated.) The problem with using actual oil drops is that they come in different sizes, and that gives you one more thing to measure. We will use a suspension of latex spheres. Check the bottle for the diameter and mass density of the spheres.

THE ATOMIZER. Find a plastic atomizer bottle for spritzing latex spheres. Make sure it has some fresh liquid suspension of latex spheres. Connect a hose to the top of the bottle, and make sure there is a needle-like outlet at the other end of the hose. Also make sure that there is a thin tube inside the sprayer so that you are actually spraying spheres, not just air.

THE CHAMBER AND STAND. Place an "oil-drop" chamber on top of the stand. The wires leading to the two internal plates should be connected to the side of the stand. Attach the banana plugs on the side of the stand to the power supply. (0-300VDC) Make sure the telescope is working properly. It should be focused only by sliding it inside the clip that holds it to the stand.

LIGHT SOURCE. We have found a laser to be a good light source for this experiment. It is terribly important that you do not look directly into the laser itself. You will want to mount the laser and then aim it at the center of the chamber. Put a bent paper clip (there should be one near the apparatus) into the chamber through the hole in the side. Now position the laser so that it illuminates this paper clip brightly. We recommend that the laser light come into the chamber at an angle (when viewed from above), but that it be level with the region between plates in the chamber. Once you have your lighting sorted out, see if you can spray some drops into the chamber and locate them in the telescope. Take appropriate precautions to eliminate the danger of looking directly into the laser source!

THE EXPERIMENT.
Spray some latex spheres into the chamber. Make sure the tube inside the atomizer is well into the level of liquid when you spritz. Drops seem to fall upward, because of the optics of this setup. If you don't see any drops, wait about a minute and spray again. When you see them, turn on the potential to the plates to see whether you can get the drop to stay motionless. If the drop falls upward even faster, reverse the polarity of the voltage. Record the voltage (and any uncertainty) once you have managed to keep the drop motionless for a minute or so. Repeat for about a dozen drops. Since you are looking to show that charge is quantized, the best data points are those that corresond to the smallest charges. Try to get data points for voltages that correspond to 1 or 2 or 3 electrons' charge.

ANALYSIS.
If a drop (having a charge, Q) is motionless, the electric force on it equals its weight: QE=mg. The electric field is given by E=DV/d. Measure the distance, d, between the plates. Calculate the mass of the latex spheres, and then calculate the charge, Q for each of your drops.

Now, each drop has to have an integer number, N=Q/e, of elementary charges. You have a slight advantage over Millikan and his students in that you already know the charge of an electron, e. Calculate the integer number, N of electrons of charge on each of your data points. If Q/e is not close to being an integer, do not include it in your analysis. Calculate a table of N vs Q from your raw data. If you plot Q vs N, the slope should equal the charge of a single electron, e. Do this. Compare your result (and uncertainty!) to the accepted value for e.