What is an Electroscope?

Invented by the British physician William Gilbert around 1600, the electroscope is one of the most important instruments used by scientists for the past many years to study electricity. For centuries, it has been defined as a device made up of conducting material and used for detecting and testing the presence of ionizing radiation or electric charge on a body. Electroscopes detect the charge by the test object’s movement due to Coulomb electrostatic force on it, and the amount of charge on that object is directly proportional to voltage.In the cases where the electric charge of the object is equivalent to its capacitance, electroscopes are regarded as a crude voltmeter. 

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Working of Electroscope

Electroscope often consists of a metal rod with a knob on the top and a pair of metal leaves connected at the bottom. This rod is inserted into a one-hole rubber stopper that is further fitted into a flask. The flask contains the rod's lower part, which includes the metal leaves. In the case of the uncharged electroscope, i.e., when no charge is present, the metal leaves hang straight down. However, when a charged object is brought near the knob of the rod, or we can say the electroscope, the electric charge travels down through the rod and spreads the leaves apart. This spreading of leaves indicates the presence of an electric charge. Note that when a charged object touches the knob of the electroscope, any of the following cases can occur:

If the charge is positive, electrons in the electroscope are attracted to the charge and move out of the leaves in the upward direction. It makes the leaves gain a temporary positive charge, and as like charges repel each other, the leaves separate. 

If the charge is negative, then the electrons in the electroscope repel and move towards the leaves. It makes the leaves gain the temporary negative charge, and as like charges repel, the leaves once again separate. 

In both the above cases, the electrons will return to their original position, and the leaves will relax as soon as the charge is removed. 

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From all these statistics, we can conclude that the working of an electroscope is based on charge induction, the atomic and internal structure of the metal elements, and the notion that unlike charges attract while charges repel each other. Moreover, the electroscope cannot identify whether the charge is positive or negative; rather, it only determines the presence of charge. 


Types of Electroscope

In general, the electroscopes are classified into the following three types:

Pith-ball Electroscope: As the name suggests, it consists of one or two small balls that are made up of a lightweight non-conductive substance and known as pith. To determine whether the object is charged or not by using this electroscope, the object is brought close to the uncharged pith-ball. Force of attraction between the ball and object shows that the object is charged.

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The Pith-ball electroscope is used to detect static electric charges and to determine the polarity of unknown charges. As shown in the figure, the pith-ball A is made up of molecules consisting of positive and negative charges close together. Here, bringing a charged object B close to the pith-ball A makes the charges inside it to separate slightly. Note that if the object is positively charged (as shown in the diagram), then the electrons, i.e., negative charges in the molecules will be attracted to it. They will move to the side of molecules close to the object. On the other hand, the positive charges, i.e., nuclei will be repelled, and move to the side of molecules away from the object. As the electrons are comparatively closer to the external charges than the nuclei, the force of attraction due to them is stronger than the force of repulsion due to nuclei. Though the separation of charges is microscopic, yet the total force (due to a large number of atoms in the pith-ball) is enough to pull the pith-ball towards the external charge. In this way, the pith-ball electroscope is used to detect the presence of a static charge on an object as if the object (when brought near to the ball) gets attracted to it. Moreover, by getting attracted to the object with the opposite charge and repelled by one with the same charge, the pith-ball can help you to determine the polarity of the charge on an object. 


Gold-leaf Electroscope: It consists of a vertical conductive rod with a metal ball on the top and two thin and parallel strips of gold leaf attached at the bottom. Invented by Abraham Bennet in 1787, this electroscope is comparatively more sensitive than a pith-ball one. To prevent the gold leaf from drafts of air, it is kept in a glass bottle. The gold leaves, which are kept in a glass flask to prevent them from the effect of air, spread apart into inverted “V” whenever a charged object is brought close to them.

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A gold-leaf electroscope is used for detecting electric charge present in a body and identifying its polarity. Its operation is based on the principle of electrostatic induction and like charge repulsion. In this electroscope, two thin leaves and an electrically conductive material are hung adjacent and virtually in contact with each other. Since the leaves are very thin, they possess no rigidity and hang down limply. When these leaves gain charge, they get separated. The angle that forms amid them depends on the amount of charge on them. If the instrument is shielded and the capacitance is fixed, then the angle may be with some precision to static voltage. Note that this electroscope actually indicates potential, not charge, and also that the voltage is determined by measuring the separation angle. 


Needle Electroscope: It consists of a plate connected to a support stand and a pivoted free-swinging needle on either side of the stand. If a charged object is brought near to the plate, then the needle will gain the same charge and will swivel away.

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In this electroscope, the plate, support stand, and needle are all made of a conducting material that allows both the free flow of electrons and the distribution of excess charge throughout the electroscope. Here, the presence of charge in not just the electroscope but also the nearby object can be determined by observing the deflection of the needle. The working of this electroscope often emphasizes the induction process of charging. In this process, the presence of the charged object that’s brought near to the plate of the electroscope influences electrons within the electroscope to move accordingly. With the charged object held close to the plate to which the electroscope is touched, the electrons will start flowing between the electroscope and the ground, providing the electroscope with an overall charge. Now, if the charged object is pulled away, the needle deflects, thus indicating the overall charge on the electroscope. 


Uses of Electroscope

An electroscope can be used to: 

  • Detect the presence of ionizing radiation or electric charge on a body

  • Identify and compare the magnitude of charges

  • Calculate the force between two charges 

  • Detect the nature and relative amount of charges.