Gold leaf electroscope

What is a gold leaf electroscope?

Gold leaf electroscope is an instrument for detecting and measuring static electricity or voltage.

The following diagram shows a modern gold leaf electroscope:

Why is gold used to make such an electroscope?

Gold is popularly used to make such electroscopes since gold is even sensitive to very minute charges (This is achieved by the malleability of gold - hammering into sheet form and making the mass thin). When we introduce a charged object to the disk of the electroscope ( explained below) we practically do not know the magnitude of the charge. Thus a simple conductor may not be the best option to show a reasonable deflection or response even to minute charges. Thus gold is assumed to be sensitive to charges of any magnitude which makes it more suitable as the indicator. 

Besides gold is also a non corrosive metal. We practically do not use anhydrous materials inside the glass case. The penetration of vapor into the glass cage by insulations may result in the corrosion or rusting of the leaf. Rusted leaves may give inaccurate results including inaccurate angular displacements.

Due to these two reasons, gold is the most suitable option to make the gold leaf electroscope.

Parts of the above electroscope can be labeled as follows:

(Photo credit: www.schoolphysics.co.uk )


When we bring a charged object close to the disc or the metal cap, charge will spread to the plate and the gold leaf. Thus both the plate and the gold leaf will have the same charge. 

Since similar charges repel each other, the gold leaf will move away from the metal plate once a charge is introduced to the disc. 

Furthermore, the magnitude of the charge can be found using the angular scale presented behind the plate-leaf composite. When the magnitude of the charge increases, the angular displacement of the gold leaf will also be larger.

Charging a gold leaf electroscope:

Any gold leaf electroscope can be charged by two methods:

1. Charging by induction
2. Charging by contact


Charging by induction:

(Photo credit: www.uq.edu.au )



Steps :

1. First the electroscope is earthed.
2. When a negatively charged object is brought near the disk, the disk is positively charged while the gold leaf and the metal plate is negatively charged.
3. In the step 2, if the electroscope is earthed without moving the charged objects then the negatively charges flow to earth.
4. In step 3, since the gold leaf and the metal plate is now neutral, they converge and return to the original position
5. Finally when the earth system and the negatively charged rod are removed, the whole system becomes positively charged.

Note: Similarly, if you bring a positive charge and redo the above steps, you can induce the electroscope (plate, disk and electroscope) with a negative charge.


Charging by contact:

This method is not an effective method for charging the electroscope. This is used to identify that the object is charged.

(Photo credit : tutorvista.com)


In this method, as opposed to the previous method, the object is not only brought close to the disk but also touches the disk. For example when the disk is touched by a particular object and if the gold leaf diverges from the initial position, we can say that the object is charged.