What is an Electroscope?

An electroscope is a scientific device that is used to detect the presence of an electric charge on a body. In the year 1600, British physician William Gilbert invented the first electroscope with a pivoted needle called versorium.

Electroscope detects the charge based on the Coulomb electrostatic force which causes the motion of test charge. Electroscope can be regarded as a crude voltmeter as the electric charge of an object is equal to its capacitance. An instrument that is used to measure the charge quantitatively is known as an electrometer.

Working of Electroscope

The working principle of an electroscope is based on the atomic structure of elements, charge induction, the internal structure of metal elements and the idea that like charges repel each other while unlike charges attract each other.

An electroscope is made up of a metal detector knob on top which is connected to a pair of metal leaves hanging from the bottom of the connecting rod. When no charge is present the metals leaves hang loosely downward. But, when an object with a charge is brought near an electroscope, one of the two things can happen.

  • When the charge is positive, electrons in the metal of the electroscope are attracted to the charge and move upward out of the leaves. This results in the leaves to have a temporary positive charge and because like charges repel, the leaves separate. When the charge is removed, the electrons return to their original positions and the leaves relax.
  • When the charge is negative, the electrons in the metal of the electroscope repel and move toward the leaves on the bottom. This causes the leaves to gain a temporary negative charge and because like charges repel, the leaves again separate. Then when the charge is removed, the electrons return to their original position and the leaves relax.

An electroscope responds to the presence of a charge through the movement of electrons either into or away from, the leaves. In both cases, the leaves separate. It is important to note that the electroscope cannot determine if the charged object is positive or negative – it is only responding to the presence of an electrical charge.

Types of electroscope

There are two classical types of electroscopes and they are as follows:

  • Pith-ball electroscope: Pith-ball electroscope was invented by John Canton in the year 1754. It consists of one or two small light balls that are a lightweight non-conductive substance called pith. In order to find if the object is charged or not, it is brought near uncharged pith ball. If the ball gets attracted towards the object it means the object is charged.
  • Pith-Ball Electroscope

    Figure 1: The Pithball electroscope from the 1870s showing attraction to a charged particle

  • Gold-leaf electroscope: Gold-leaf electroscope was developed by Abraham Bennet in the year 1787, which is more sensitive than pith-ball electroscope. It consists of a vertical metal rod which has two parallel strips of thin flexible gold leaf hang to it. To prevent the gold leaf from drafts of air, it is kept in a glass bottle. The gold leaves spread apart into inverted “V” when a charged object is brought near to it.
  • Gold Leaf Electroscope

    Figure 2: Gold Leaf Electroscope showing electrostatic induction

Uses of electroscope

Following are the uses of electroscope:

  • It is used to detect the static charges
  • The nature of electric charges can be determined using an electroscope
  • The magnitudes of two different charges can be compared using an electroscope

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A uniform heavy chain of length 'a' initially has length 'b' hanging off of a table. The remaining part of chain a - b, is coiled on the table. If the chain is released, the velocity of the chain when the last link leaves the table is