Gold Leaf Electroscope

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a typical Gold Leaf electroscope

Introduction

Time for some bad news: The "Gold leaf" is actually more likely to be a form of bronze called "dutch metal". A Gold leaf electroscope is used in electrostatics to show the magnitude and polarity of charged entities. These are best stored in a warm dry environment as the charge is easily dissipated in moist conditions. It is also used with a zinc plate to illustrate the photo-electric effect.See Photoelectric effect

Description

A gold leaf electroscope demonstrates potential difference between the leaf and the case (or earth). The leaf rises because it is repelled by the central electrode or stem. The leaf and the electrode have the same type of charge. There are various forms this can take but it is usual for a metal case to be used. The conical flask models do not allow for the case to be earthed. The central electrode is usually of a good electrically conducting metal such as brass. The leaf is either in an inverted "V" shape at the end or more commonly attached to the side. In the inverted "V" one side acts as the central electrode but both are displaced when charged.

How to charge an electroscope

The three main methods of charging an electroscope are:

  • Charging by contact. Rub an insulating rod to charge it up through friction. Then stroke it across the top plate of the electroscope. This will transfer charge from the insulator to the electroscope. This method loses quite a bit of charge when the rod is removed.
  • Charging by induction. The Charged rod is brought close to the plate of the electroscope but not touching it. Momentarily touching the plate with a finger earths the electroscope. Then when the rod is removed the electroscope is charged with the opposite charge to the rod.


  • Charging with an EHT or Van de Graaff generator. A probe or insulated crocodile clip connected to one of these high voltage sources to charge up the gold leaf electroscope. The case should be connected to either the base of the VDG or the other terminal on the EHT unit. Don’t forget to switch off!.

The electrostatic series:

More ELECTROPOSITIVE
Perspex
Glass
Silk
Wood
Sulphur
Cotton
Ebonite
Indian rubber
Polythene
More ELECTRONEGATIVE

If two materials are rubbed together, the material higher in the list will gain a positive charge, while the lower material will gain a negative charge. So a Cotton duster rubbed on a perspex rod produces a positive charged rod. But the same cooton rubbed on a polythene rod produces a negatively charged rod.

Adding a replacement gold leaf.

This has been the subject of a lot of discussion on the Scitech-l list. The "spittle method" seems to have few drawbacks. Pre-cut gold leaf is placed on the paper surface on a raised platform ( a book is ideal). It is best moved with a dry paintbrush. It should be manoevered to the edge in a draught free room. Take the central electrode from the gold leaf electroscop and remove any vestiges of the original leaf. This should be clean and free from grease. If not clean with ethanol. Moisten your finger with a little spit and transfer it to the electrode at the point where the original leaf hung. Bring the electrode over the leaf and lower onto the leaf. The leaf should stick as the electrode is raised.

one method of attaching gold leaf

Other maintenance tasks

The clear glass at the front, and frosted glass at rear can be cleaned with any proprietry glass cleaning product. Use this too for dusting the inside. Replacement galss panels are available from the normal educational suppliers


Something to try

As an aside, a simple electronic electroscope can be made with only two components (4 if you count the clip and battery).
This is worth doing primarily as the sensitivity is quite impressive, but is a good talking point.
The instructions are based on an idea from:
Eskimo.com link
However the FET (Field Effect Transistor) used was a 2N3819 n-channel JFET. Although the technical data sheet used also had this listed as a MOSFET.(!)

The pin-out and circuit diagram are here

circuit diagram


Connect the cathode of the LED to the negative of the pp6 clip.
Connect the Source of the FET (see diagram) to the positive of the clip.
Join the anode of the LED to the drain of the FET with solder.
Turn the gate up as an aerial.

image of actual electroscope

--D.B.Ferguson 20:32, 2 May 2008 (BST)




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