Coulomb’s Law Apparatus

Product Code: ES-9070


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.


  • 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.

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.

What’s Included

  • 1x Coulomb’s Law Apparatus
  • 1x Third Conductive Sphere
  • 1x Charging Probe
  • 1x Allen Wrench and Calibration Masses
  • 1x Calibration Post
  • 1x Manual

Product Specifications

Torsion Balance Assembly 38 mm dia. conductive sphere on 12 cm rod with counterbalance vane
Torsion Balance Wire equals 10-6 Newtons/degree
Torsion Balance Degree Plate 1° increments
Torsion Balance Magnetic Damping dampens oscillations for quick measurements
Linear Track Sphere 38 mm dia. conductive sphere
Linear Track Range of Movement 350 mm in 1 mm increments
Linear Track Material phenolic (to minimize mirror charges)
Charging Probe 17 cm long plus 1.5 m cable; banana plug connector; 200 μΩ internal resistance
Calibration Masses 50 mg (1), 20 mg (2)
Conductive Sphere on Insulating Thread for reducing charge by fixed ratios
Spare Torsion Wire 3 m
Shipping Size 28 x 38 x 61 cm (11 x 15 x 24 in.)
Shipping Weight 9.5 kg, 21 lbs

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