Electricity and Magnetism

Demonstration electroscope

Practical Activity for 14-16 PRACTICAL PHYISCS

Demonstration

This is a model of an historic instrument which can still be used to compare or measure charges.

Apparatus and Materials

  • Retort stand and boss
  • Perspex (insulating) rod
  • Brass or aluminium strip, about 20 cm x 1 cm
  • Metallised foil (e.g. Mylar or Melinex) strip, about 18 cm x 1 cm
  • Polythene tile
  • Cloth for rubbing
  • Electrophorus plate
  • Power supply, EHT, 0–5 kV (with internal safety resistor)

Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

  1. Clamp a Perspex rod in a boss on a retort stand.
  2. Bend the strip of brass or aluminium sheet as illustrated and fix it over the Perspex rod.
  3. To one side of the brass, attach the strip of foil with glue.
  4. To charge the foil to a high potential, use the electrophorus plate as follows. Rub the polythene tile to give it a negative charge. Holding the plate by the insulating handle, touch the plate on the tile. While it is still in contact with the tile, touch it momentarily with a finger, and then remove it from the tile.
  5. Bring the plate into contact with the top of the model electroscope. This charging process should be repeated several times.
  6. The plate can also be charged by connecting it to the EHT power supply in series with the 50 MΩ resistor incorporated in the supply. In this case, the other terminal of the supply should be connected to the retort stand base.

Teaching Notes

  • This is intended to be a very simple, large-scale introduction to the electroscope. It illustrates how charge can be transferred from one object to another, and how the presence of charge can be identified by the deflection of the metallized foil.
  • The gold leaf electroscope is a very important piece of apparatus for use in electrostatics experiments. The design is so simple that it is absolutely clear how it works, so there can be no mystery about how it provides the results you are looking for (unlike modern electronic meters).

This experiment was safety-tested in january 2007

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