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The Physics Teacher -- February 2012 -- Volume 50, Issue 2, pp. 98

Millikan's Oil-Drop Experiment: A Centennial Setup Revisited in Virtual World

Michel Gagnon

Université de Saint-Boniface, Winnipeg, MB Canada

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Early in the last century, Robert Millikan developed a precise method of determining the electric charge carried by oil droplets.1–3 Using a microscope and a small incandescent lamp, he observed the fall of charged droplets under the influence of an electric field inside a small observation chamber. In so doing, Millikan demonstrated the existence of a fundamental unit of electric charge, and established its quantization. Now renowned as one of the most famous experiments of 20th-century physics, Millikan's oil-drop experiment has been reproduced with more or less success in most, if not all, high school and university physics classes. This has encouraged many improvements of the apparatus, now making this experiment much more accurate and easier to realize for advanced students. However, the required apparatus remains rather expensive, and for introductory college or high school students the experiment is still quite difficult to conduct. As an alternative to the traditional setup, a realistic computer-based simulator to replicate the Millikan oil-drop experiment has been developed. Using this software, students are able to undertake a complete experiment, obtain an accurate set of results, and thus gain a better understanding of the original experiment and its historical importance.

© 2012 American Association of Physics Teachers

KEYWORDS and PACS

Keywords

drops, electric charge, filament lamps, further education, oils, physics education, student experiments

PACS

  • 01.50.Pa

    Laboratory experiments and apparatus

History
Online Jan 2012

PUBLICATION DATA

ISSN

0031-921X (print)  

Publisher

$abstract.society.value is a Member of CrossRef American Association of Physics Teachers

ARTICLE DATA

Digital Object Identifier
http://dx.doi.org/10.1119/1.3677285

  1. R. A. Millikan, “The isolation of an ion, aprecision measurement of its charge, and the correction of Stokes's law,” Phys. Rev. (Series 1) 32, 349–397 (April 1911).
  2. R. A. Millikan, “On the elementary electric charge and the Avogadro constant,” Phys. Rev. (Series II) 2, 109–143 (Aug. 1913).
  3. A. Franklin, “Millikan's oil-drop experiments,” The Chem. Educ. 2(1), 1–14(1997).
  4. Anthony Papirio, Jr., Claude M. Penchina, and Hajime Sakai, “Novel approach to the oil-drop experiment,” Phys. Teach. 38, 50–51 (Jan. 2000)PHTEAH000038000001000050000001.
  5. Steve Brehmer, “Millikan without the eyestrain,” Phys. Teach. 29, 310 (May 1991)PHTEAH000029000005000310000001.
  6. Ray C. Jones, “The Millikan oil-drop experiment: Making it worthwhile,” Am. J. Phys. 63, 970–977 (Nov. 1995)AJPIAS000063000011000970000001. [ISI]
  7. Lowell I. McCann and Earl D. Blodgett, “The “nut-drop” experiment — Bringing Millikans challenge to introductory students,” Phys. Teach. 47, 374–379 (Sept. 2009)PHTEAH000047000006000374000001.
  8. Dan MacIsaac, “Websites for teaching high school and introductory college modern physics topics,” Phys. Teach. 45, 124 (Feb. 2007).
  9. R. A. Millikan, The Electron: Its Isolation and Measurement and the Determination of Some of its Properties (University of Chicago Press, 1917).
  10. N. D. Finkelstein et al., “When learning about the real world is better done virtually: A study of substituting computer simulations for laboratory equipment,” Phys. Rev. ST-Phys. Educ. Res. 1, 010103 (2005).


Figures (7) Tables (2)

Figures (click on thumbnails to view enlargements)

FIG.1
The main window of the Millikan oil-drop experiment software shows the observation area in black surrounded by the Millikan apparatus on the left, the power supply on the bottom, and virtual tools on the right.

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FIG.2
The heart of the software is the Millikan apparatus with its observation chamber, microscope, sprayer, lamp, and bubble-level.

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FIG.3
The power supply has two sliders for adjusting the voltage and a switch to reverse the polarity.

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FIG.4
The central part of the screen replicates what can be seen in the microscope. Tools are available to accurately measure the flight time and the distance traveled.

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FIG.5
Data can be entered on a grid and saved in a file.

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FIG.6
Data set obtained with the first method, organized in ascending order and showing a stair pattern.

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FIG.7
Data set obtained with the second method and organized in ascending order. It still exhibits a stair pattern and moreover the lower value equates the fundamental unit of electric charge.

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Tables

Table I. Raising velocities of beads under the influence of an electric field and the corresponding computed electric charge they carried. Data have been reorganized in ascending values of the electric charges. Prior to these measurements, the falling velocity of beads in the absence of electric field has been measured as 8.94×10−4 m/s, and after this the voltage has been set to 40 V for the remaining experiment.

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Table II. Voltages used to immobilize beads and the corresponding electric charge carried by the beads. Data are reorganized in ascending values of the electric charges.

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