With the ES-9070 Coulomb's Law Apparatus, students can accurately measure charge, force and distance between two conductive spheres. Its symmetrical design minimizes stray and mirror charges and built-in magnetic damping ensures quick, accurate measurements.

Electrostatic force is directly proportional to the angle of twist.

Typical Experiments

With Teacher’s Guide and Sample Data, determine or verify Coulomb’s Law. This can be performed as three separate experiments:

  • Verify the Inverse Square Law
  • Verify the Force/Charge Relationship
  • Determine the Coulomb Constant

Special Features

  • Built-in Balance Scale: A degree scale on the torsion balance provides accurate measurements of the torsion wire's twist angle.
  • Magnetic Damping: Allows measurements to be made quickly, minimizing the traditional difficulties with leakage currents.
  • Milligram Masses: Included so determining the torsion constant and verifying its linearity can be part of the experiment.
  • Insulated Track: The calibrated track is designed plastic to minimize mirror charges which can significantly affect results.
  • Built-in Track Scale: A millimeter scale on the linear track provides accurate measurements of the distance between the spheres.
  • Vary Charge Accurately: A third conductive sphere, identical to the experimental spheres, is included for reducing the charge on one or both spheres by fixed ratios. This method is quick and accurate.
  • Symmetric Construction: All conductive parts are symmetrical, so errors due to mirror charges are practically nonexistent.


  • Coulomb's Law Apparatus
  • Third Conductive Sphere
  • Charging Probe
  • Allen Wrench and Calibration Masses
  • Calibration Post
  • Manual
  • How It Works

    A conductive sphere is mounted on the end of an insulating, counterbalanced rod and suspended from a thin torsion wire. An identical sphere is mounted on a calibrated linear track. This second sphere can be positioned at various distances from the first.

    When the conductive spheres are charged, the force between them is proportional to the twist of the torsion wire that is required to bring the balance back to its equilibrium position. Beginning students can determine the Inverse Square Law in a simple experiment. Advanced students can perform a more sophisticated investigation into all the variables of electrostatic repulsion.


    Torsion Balance

    Torsion Assembly
    • 38 mm dia. conductive sphere on 12 cm rod with counterbalance vane
    Torsion Wire
    • Equals 10-6 Newtons/degree
    Degree Plate
    • 1° increments

    Calibrated Linear Track

    • 38 mm diameter conductive sphere
    Range of Movement
    • 350 mm in 1 mm increments
    • phenolic (to minimize charges)

    Miscellaneous Equipment

    Charging Probe
    • 17 cm long plus 1.5 m cable
    • banana plug connector
    • 200 Ω internal resistance
    Calibration Masses
    • 50 mg (1) and 20 mg (2)
    Conductive Sphere on Insulating Thread
    • Reduce charge by fixed ratio
    Spare Torsion Wire
    • 3 m

    Shipping Information

    • 28 x 38 x 61 cm (11 x 15 x 24 in.)
    • 9.5 kg (21 lbs)

    Buying Guide

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    Coulomb's Law Apparatus

    Coulomb's Law Apparatus
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