For the 19th century conceptual model of television, see Telectroscope.
Gilbert's versorium
The electroscope is an early scientific instrument used to detect the presence of electric charge on a body. It detects charge by the movement of a test object due to the Coulomb electrostatic force on it. The amount of charge on an object is proportional to its voltage. The accumulation of enough charge to detect with an electroscope requires hundreds or thousands of volts, so electroscopes are used with high voltage sources such as static electricity and electrostatic machines. An electroscope can only give a rough indication of the quantity of charge; an instrument that measures electric charge quantitatively is called an electrometer.
The electroscope was the first electrical measuring instrument. The first electroscope was a pivoted needle (called the versorium), invented by British physician William Gilbert around 1600.[1][2] The pith-ball electroscope and the gold-leaf electroscope are two classical types of electroscope[2] that are still used in physics education to demonstrate the principles of electrostatics. A type of electroscope is also used in the quartz fiber radiation dosimeter. Electroscopes were used by the Austrian scientist Victor Hess in the discovery of cosmic rays.
Pith-ball electroscope
Pith ball electroscope from the 1870s, showing attraction to charged object
How it works
In 1731, Stephen Gray used a simple hanging thread, which would be attracted to any nearby charged object. This was the first improvement on Gilbert's versorium from 1600.[3]
The pith-ball electroscope, invented by British schoolmaster and physicist John Canton in 1754, consists of one or two small balls of a lightweight nonconductive substance, originally a spongy plant material called pith,[4] suspended by silk or linen thread from the hook of an insulated stand.[5]Tiberius Cavallo made an electroscope in 1770 with pith balls at the end of silver wires.[3] Modern electroscopes usually use balls made of plastic. In order to test the presence of a charge on an object, the object is brought near to the uncharged pith ball. If the object is charged, the ball will be attracted to it and move toward it.
The attraction occurs because of induced polarization[6] of the atoms inside the pith ball.[7][8][9][10] All matter consists of electrically charged particles located close together; each atom consists of a positively charged nucleus with a cloud of negatively charged electrons surrounding it. The pith is a nonconductor, so the electrons in the ball are bound to atoms of the pith and are not free to leave the atoms and move about in the ball, but they can move a little within the atoms. See diagram. If, for example, a positively charged object (B) is brought near the pith ball (A), the negative electrons(blue minus signs) in each atom (yellow ovals) will be attracted and move slightly toward the side of the atom nearer the object. The positively charged nuclei(red plus signs) will be repelled and will move slightly away. Since the negative charges in the pith ball are now nearer the object than the positive charges (C), their attraction is greater than the repulsion of the positive charges, resulting in a net attractive force.[7] This separation of charge is microscopic, but since there are so many atoms, the tiny forces add up to a large enough force to move a light pith ball.
The pith ball can be charged by touching it to a charged object, so some of the charges on the surface of the charged object move to the surface of the ball. Then the ball can be used to distinguish the polarity of charge on other objects because it will be repelled by objects charged with the same polarity or sign it has, but attracted to charges of the opposite polarity.
Often the electroscope will have a pair of suspended pith balls. This allows one to tell at a glance whether the pith balls are charged. If one of the pith balls is touched to a charged object, charging it, the second one will be attracted and touch it, communicating some of the charge to the surface of the second ball. Now both balls have the same polarity charge, so they repel each other. They hang in an inverted 'V' shape with the balls spread apart. The distance between the balls will give a rough idea of the magnitude of the charge.
Gold-leaf electroscope
Gold leaf electroscope showing electrostatic inductionUsing an electroscope to show electrostatic induction. When a charged resinous (-) rod touches the top ball terminal, electrons are conducted down through the post to the needle; so the post repels the needle, turning it counter-clockwise. Charged rods held near the terminal chase like-sign charges from the ball, onto the post and needle; or thru the demonstrator to ground. When the charged rod is removed, the charges spread from needle and post onto the ball; so the needle and post have less charge than before, so the needle is deflected less than after contact charging.
The gold-leaf electroscope was developed in 1787 by British clergyman and physicist Abraham Bennet,[4] as a more sensitive instrument than pith ball or straw blade electroscopes then in use.[11] It consists of a vertical metal rod, usually brass, from the end of which hang two parallel strips of thin flexible gold leaf. A disk or ball terminal is attached to the top of the rod, where the charge to be tested is applied.[11] To protect the gold leaves from drafts of air they are enclosed in a glass bottle, usually open at the bottom and mounted over a conductive base. Often there are grounded metal plates or foil strips in the bottle flanking the gold leaves on either side. These are a safety measure; if an excessive charge is applied to the delicate gold leaves, they will touch the grounding plates and discharge before tearing. They also capture charge leaking through the air that accumulates on the glass walls, increasing the sensitivity of the instrument. In the precision instruments the inside of the bottle was occasionally evacuated, to prevent the charge on the terminal from leaking off through the ionization of the air.
When the metal terminal is touched with a charged object, the gold leaves spread apart in an inverted 'V'. This is because some of the charge from the object is conducted through the terminal and metal rod to the leaves.[11] Since the leaves receive the same sign charge they repel each other and thus diverge. If the terminal is grounded by touching it with a finger, the charge is transferred through the human body into the earth and the gold leaves close together.
The electroscope leaves can also be charged without touching a charged object to the terminal, by electrostatic induction. As the charged object is brought near the electroscope terminal, the leaves spread apart, because the electric field from the object induces a charge in the conductive electroscope rod and leaves, and the charged leaves repel each other. The opposite-sign charge is attracted to the nearby object and collects on the terminal disk, while the same-sign charge is repelled from the object and collects on the leaves (but only as much as left the terminal), so the leaves repel each other. If the electroscope is grounded while the charged object is nearby, by touching it momentarily with a finger, the repelled same-sign charges travel through the contact to ground, leaving the electroscope with a net charge having the opposite sign as the object. The leaves initially hang down free because the net charge is concentrated at the terminal end. When the charged object is moved away, the charge at the terminal spreads into the leaves, causing them to spread apart again.
Gold-leaf electroscopes
Condensing electroscope, Rome University physics dept.
Electroscope from about 1910 with grounding electrodes inside jar, as described above
↑ Gilbert, William; Edward Wright (1893). On the Lodestone and Magnetic Bodies. John Wiley & Sons. p.79. a translation by P. Fleury Mottelay of William Gilbert (1600) Die Magnete, London
In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interactions of atoms and molecules. Electromagnetism can be thought of as a combination of electrostatics and magnetism, two distinct but closely intertwined phenomena. Electromagnetic forces occur between any two charged particles, causing an attraction between particles with opposite charges and repulsion between particles with the same charge, while magnetism is an interaction that occurs exclusively between charged particles in relative motion. These two effects combine to create electromagnetic fields in the vicinity of charged particles, which can accelerate other charged particles via the Lorentz force. At high energy, the weak force and electromagnetic force are unified as a single electroweak force.
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwell's equations. Various common phenomena are related to electricity, including lightning, static electricity, electric heating, electric discharges and many others.
Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be positive or negative. Like charges repel each other and unlike charges attract each other. An object with no net charge is referred to as electrically neutral. Early knowledge of how charged substances interact is now called classical electrodynamics, and is still accurate for problems that do not require consideration of quantum effects.
An electrometer is an electrical instrument for measuring electric charge or electrical potential difference. There are many different types, ranging from historical handmade mechanical instruments to high-precision electronic devices. Modern electrometers based on vacuum tube or solid-state technology can be used to make voltage and charge measurements with very low leakage currents, down to 1 femtoampere. A simpler but related instrument, the electroscope, works on similar principles but only indicates the relative magnitudes of voltages or charges.
A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate electric charge on a hollow metal globe on the top of an insulated column, creating very high electric potentials. It produces very high voltage direct current (DC) electricity at low current levels. It was invented by American physicist Robert J. Van de Graaff in 1929. The potential difference achieved by modern Van de Graaff generators can be as much as 5 megavolts. A tabletop version can produce on the order of 100 kV and can store enough energy to produce visible electric sparks. Small Van de Graaff machines are produced for entertainment, and for physics education to teach electrostatics; larger ones are displayed in some science museums.
Timeline of electromagnetism and classical optics lists, within the history of electromagnetism, the associated theories, technology, and events.
Static electricity is an imbalance of electric charges within or on the surface of a material. The charge remains until it is able to move away by an electric current or electrical discharge. The word "static" is used to differentiate it from current electricity, where an electric charge flows through an electrical conductor.
In electromagnetism, an electrophorus or electrophore is a simple, manual, capacitive, electrostatic generator used to produce charge via the process of electrostatic induction. A first version of it was invented in 1762 by Swedish professor Johan Carl Wilcke. Italian scientist Alessandro Volta improved and popularized the device in 1775, and is sometimes erroneously credited with its invention. The word electrophorus was coined by Volta from the Greek ήλεκτρον, elektron, and φορεύς, phoreus, meaning 'electricity bearer'.
A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. It represents a local region where the air has undergone electrical breakdown and become conductive, allowing charge to continuously leak off the conductor into the air. A corona discharge occurs at locations where the strength of the electric field around a conductor exceeds the dielectric strength of the air. It is often seen as a bluish glow in the air adjacent to pointed metal conductors carrying high voltages, and emits light by the same mechanism as a gas discharge lamp. Corona discharges can also happen in weather, such as thunderstorms, where objects like ship masts or airplane wings have a charge significantly different from the air around them.
Electrostatic induction, also known as "electrostatic influence" or simply "influence" in Europe and Latin America, is a redistribution of electric charge in an object that is 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. See also Stephen Gray in this context. Due to induction, the electrostatic potential (voltage) 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.
An electrostatic generator, or electrostatic machine, is an electrical generator that produces static electricity, or electricity at high voltage and low continuous current. The knowledge of static electricity dates back to the earliest civilizations, but for millennia it remained merely an interesting and mystifying phenomenon, without a theory to explain its behavior and often confused with magnetism. By the end of the 17th century, researchers had developed practical means of generating electricity by friction, but the development of electrostatic machines did not begin in earnest until the 18th century, when they became fundamental instruments in the studies about the new science of electricity.
Atmospheric electricity describes the electrical charges in the Earth's atmosphere. The movement of charge between the Earth's surface, the atmosphere, and the ionosphere is known as the global atmospheric electrical circuit. Atmospheric electricity is an interdisciplinary topic with a long history, involving concepts from electrostatics, atmospheric physics, meteorology and Earth science.
Ion wind, ionic wind, corona wind or electric wind is the airflow induced by electrostatic forces linked to corona discharge arising at the tips of some sharp conductors subjected to high voltage relative to ground. Ion wind is an electrohydrodynamic phenomenon. Ion wind generators can also be considered electrohydrodynamic thrusters.
The Kelvin water dropper, invented by Scottish scientist William Thomson in 1867, is a type of electrostatic generator. Kelvin referred to the device as his water-dropping condenser. The apparatus is variously called the Kelvin hydroelectric generator, the Kelvin electrostatic generator, or Lord Kelvin's thunderstorm. The device uses falling water to generate voltage differences by electrostatic induction occurring between interconnected, oppositely charged systems. This eventually leads to an electric arc discharging in the form of a spark. It is used in physics education to demonstrate the principles of electrostatics.
The versorium was the first electroscope, the first instrument that could detect the presence of static electric charge. It was invented in 1600 by William Gilbert, physician to Queen Elizabeth I.
An electrostatic separator is a device for separating particles by mass in a low energy charged beam.
Franklin bells are an early demonstration of electric charge designed to work with a Leyden jar or a Lightning rod. Franklin bells are only a qualitative indicator of electric charge and were used for simple demonstrations rather than research. The bells are an adaptation to the first device that converted electrical energy into mechanical energy in the form of continuous mechanical motion, in this case, the moving of a bell clapper back and forth between two oppositely charged bells.
Coulomb's inverse-square law, or simply Coulomb's law, is an experimental law of physics that calculates the amount of force between two electrically charged particles at rest. This electric force is conventionally called electrostatic force or Coulomb force. Although the law was known earlier, it was first published in 1785 by French physicist Charles-Augustin de Coulomb. Coulomb's law was essential to the development of the theory of electromagnetism and maybe even its starting point, as it allowed meaningful discussions of the amount of electric charge in a particle.
An electrostatic particle accelerator is a particle accelerator in which charged particles are accelerated to a high energy by a static high voltage potential. This contrasts with the other major category of particle accelerator, oscillating field particle accelerators, in which the particles are accelerated by oscillating electric fields.
Faraday's ice pail experiment is a simple electrostatics experiment performed in 1843 by British scientist Michael Faraday that demonstrates the effect of electrostatic induction on a conducting container. For a container, Faraday used a metal pail made to hold ice, which gave the experiment its name. The experiment shows that an electric charge enclosed inside a conducting shell induces an equal charge on the shell, and that in an electrically conducting body, the charge resides entirely on the surface. It also demonstrates the principles behind electromagnetic shielding such as employed in the Faraday cage. The ice pail experiment was the first precise quantitative experiment on electrostatic charge. It is still used today in lecture demonstrations and physics laboratory courses to teach the principles of electrostatics.
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