page_title: Electrostatic induction
section_title: Charging an object by induction
hierarchical_section_title: Electrostatic induction / Charging an object by induction
caption_attribution_description: English: Drawing of a gold leaf electroscope, an antique scientific instrument invented in 1787 by British clergyman Abraham Bennett that detects electric charge. It consists of a pair of delicate gold leaves hanging parallel to each other from a brass post, protected by a glass jar. The brass post projects from the mouth of the jar and ends in an electrode to which charged objects can be applied. When a charge is applied to the post, the leaves, carrying the same charge, repel each other, and diverge into a "V" shape. This example shows electrostatic induction of charge in the instrument by holding a charged dielectric rod near it. The positive charge on the rod causes the mobile charges in the brass post to separate, with negative charges being attracted into the top electrode, while positive charges are repelled into the leaves, causing them to separate. Alterations to image: Added colored plusses and minuses showing areas of charge.
context_page_description: Electrostatic induction, also known as "electrostatic influence" or simply "influence" in Europe and Latin America, is a redistribution of electric charge in an object, caused by the influence of nearby charges. In the presence of a charged body, an insulated conductor develops a positive charge on one end and a negative charge on the other end. Induction was discovered by British scientist John Canton in 1753 and Swedish professor Johan Carl Wilcke in 1762. Electrostatic generators, such as the Wimshurst machine, the Van de Graaff generator and the electrophorus, use this principle. Due to induction, the electrostatic potential is constant at any point throughout a conductor. Electrostatic Induction is also responsible for the attraction of light nonconductive objects, such as balloons, paper or styrofoam scraps, to static electric charges. Electrostatic induction laws apply in dynamic situations as far as the quasistatic approximation is valid. Electrostatic induction should not be confused with Electromagnetic induction.
context_section_description: However, the induction effect can also be used to put a net charge on an object.  If, while it is close to the positive charge, the above object is momentarily connected through a conductive path to electrical ground, which is a large reservoir of both positive and negative charges, some of the negative charges in the ground will flow into the object, under the attraction of the nearby positive charge.  When the contact with ground is broken, the object is left with a net negative charge.
  This method can be demonstrated using a gold-leaf electroscope, which is an instrument for detecting electric charge.  The electroscope is first discharged, and a charged object is then brought close to the instrument's top terminal.  Induction causes a separation of the charges inside the electroscope's metal rod, so that the top terminal gains a net charge of opposite polarity to that of the object, while the gold leaves gain a charge of the same polarity.  Since both leaves have the same charge, they repel each other and spread apart.  The electroscope has not acquired a net charge: the charge within it has merely been redistributed, so if the charged object were to be moved away from the electroscope the leaves will come together again.
  But if an electrical contact is now briefly made between the electroscope terminal and ground, for example by touching the terminal with a finger,  this causes charge to flow from ground to the terminal, attracted by the charge on the object close to the terminal.  This charge neutralizes the charge in the gold leaves, so the leaves come together again.  The electroscope now contains a net charge opposite in polarity to that of the charged object.  When the electrical contact to earth is broken, e.g. by lifting the finger, the extra charge that has just flowed into the electroscope cannot escape, and the instrument retains a net charge.  The charge is held in the top of the electroscope terminal by the attraction of the inducing charge.  But when the inducing charge is moved away, the charge is released and spreads throughout the electroscope terminal to the leaves, so the gold leaves move apart again.
  The sign of the charge left on the electroscope after grounding is always opposite in sign to the external inducing charge. The two rules of induction are:
  If the object is not grounded, the nearby charge will induce equal and opposite charges in the object.
  If any part of the object is momentarily grounded while the inducing charge is near, a charge opposite in polarity to the inducing charge will be attracted from ground into the object, and it will be left with a charge opposite to the inducing charge.
caption: Gold-leaf electroscope, showing induction, before the terminal is grounded.
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