A Faraday cage or Faraday shield is an enclosure used to block electromagnetic fields. A Faraday shield may be formed by a continuous covering of conductive material, or in the case of a Faraday cage, by a mesh of such materials. Faraday cages are named after the English scientist Michael Faraday, who invented them in 1836. A Faraday cage operates because an external electrical field causes the electric charges within the cage's conducting material to be distributed so that they cancel the field's effect in the cage's interior; this phenomenon is used to protect sensitive electronic equipment from external radio frequency interference. Faraday cages are used to enclose devices that produce RFI, such as radio transmitters, to prevent their radio waves from interfering with other nearby equipment, they are used to protect people and equipment against actual electric currents such as lightning strikes and electrostatic discharges, since the enclosing cage conducts current around the outside of the enclosed space and none passes through the interior.
Faraday cages cannot block stable or varying magnetic fields, such as the Earth's magnetic field. To a large degree, they shield the interior from external electromagnetic radiation if the conductor is thick enough and any holes are smaller than the wavelength of the radiation. For example, certain computer forensic test procedures of electronic systems that require an environment free of electromagnetic interference can be carried out within a screened room; these rooms are spaces that are enclosed by one or more layers of a fine metal mesh or perforated sheet metal. The metal layers are grounded to dissipate any electric currents generated from external or internal electromagnetic fields, thus they block a large amount of the electromagnetic interference. See electromagnetic shielding, they provide less attenuation of outgoing transmissions than incoming: they can block EMP waves from natural phenomena effectively, but a tracking device in upper frequencies, may be able to penetrate from within the cage.
A common misconception is that a Faraday cage provides full attenuation. The reception or transmission of radio waves, a form of electromagnetic radiation, to or from an antenna within a Faraday cage is attenuated or blocked by the cage. Near-field high-powered frequency transmissions like HF RFID are more to penetrate. Solid cages attenuate fields over a broader range of frequencies than mesh cages. In 1836, Michael Faraday observed that the excess charge on a charged conductor resided only on its exterior and had no influence on anything enclosed within it. To demonstrate this fact, he built a room coated with metal foil and allowed high-voltage discharges from an electrostatic generator to strike the outside of the room, he used an electroscope to show that there was no electric charge present on the inside of the room's walls. Although this cage effect has been attributed to Michael Faraday's famous ice pail experiments performed in 1843, it was Benjamin Franklin in 1755 who observed the effect by lowering an uncharged cork ball suspended on a silk thread through an opening in an electrically charged metal can.
In his words, "the cork was not attracted to the inside of the can as it would have been to the outside, though it touched the bottom, yet when drawn out it was not found to be electrified by that touch, as it would have been by touching the outside. The fact is singular." Franklin had discovered the behavior of what we now refer to shield. Additionally, the Abbe Nollet published an early account of an effect attributable to the cage effect in his Leçons de physique expérimentale. A continuous Faraday shield is a hollow conductor. Externally or internally applied electromagnetic fields produce forces on the charge carriers within the conductor; the redistributed charges reduce the voltage within the surface, to an extent depending on the capacitance, full cancellation does not occur. If a charge is placed inside an ungrounded Faraday cage, the internal face of the cage becomes charged to prevent the existence of a field inside the body of the cage, this charging of the inner face re-distributes the charges in the body of the cage.
This charges the outer face of the cage with a charge equal in sign and magnitude to the one placed inside the cage. Since the internal charge and the inner face cancel each other out, the spread of charges on the outer face is not affected by the position of the internal charge inside the cage. So for all intents and purposes, the cage generates the same DC electric field that it would generate if it were affected by the charge placed inside; the same is not true for electromagnetic waves. If the cage is grounded, the excess charges will be neutralized as the ground connection creates an equipotential bonding between the outside of the cage and the environment, so there is no voltage between them and therefor no field; the inner face and the inner charge will remain the same. Effectiveness of shielding of a static electric field is independent of the geometry of the conductive material, static magnetic fields can penetrate the shie
IET Faraday Medal
The Faraday Medal is the top medal awarded by the Institution of Engineering and Technology. It is part of the IET Achievement Medals collection of awards; the medal is named after the father of electromagnetism. Faraday is recognized as a top scientist, engineer and inventor, his electromagnetic induction principles have been used in electric motors and generators today. The medal is awarded annually to distinguished individuals who either for notable scientific or industrial achievement in engineering or for conspicuous service rendered to the advancement of science and technology, without restriction as regards to nationality, country of residence or membership of the Institution; the award was first established in 1922 to commemorate the 50th Anniversary of the first Ordinary Meeting of the Society of Telegraph Engineers and is named after Michael Faraday. Each year, the recipient received his/her award at a ceremony held in London, hosted by the IET
A flux tube is a tube-like region of space containing a magnetic field, B, such that the field is perpendicular to the normal vector, n ^. Both the cross-sectional area of the tube and the field contained may vary along the length of the tube, but the magnetic flux is always constant; as used in astrophysics, a flux tube has a larger magnetic field and other properties that differ from the surrounding space. They are found around stars, including the Sun, which has many flux tubes of around 300 km diameter. Sunspots are associated with larger flux tubes of 2500 km diameter; some planets have flux tubes. A well-known example is the flux tube between its moon Io. Flux rope: Twisted magnetic flux tube. Isolate flux tube: Magnetic flux tube that does not have a magnetic field outside the tube. In 1861, James Clerk Maxwell gave rise to the concept of a flux tube inspired by Michael Faraday's work in electrical and magnetic behavior in his paper titled "On Physical Lines of Force". Maxwell described flux tubes as:"If upon any surface which cuts the lines of fluid motion we draw a closed curve, if from every point of this curve we draw lines of motion, these lines of motion will generate a tubular surface which we may call a tube of fluid motion."
The flux tube's strength, F, is defined to be the magnetic flux through a surface, S, with F constant throughout the flux tube as a result of Maxwell's Equation: ∇ ⋅ B = 0. Under the condition that the cross-sectional area, A, of the flux tube is small, F can be approximated as F ≈ B A. If the area is decreased the magnetic field must increase in order to satisfy the condition of constant F. F = ∫ S B → ⋅ d S From the condition of perfect conductivity in ideal Ohm's Law, in ideal magnetohydrodynamics, the change in magnetic flux, Φ, is zero in a flux tube, known as Alfvén's Theorem of flux conservation. With flux conservation, the topology of the flux tube does not change; this effect arises when there is a high Magnetic Reynolds number, Rm >> 1, where induction dominates and diffusion is neglected, allowing for the magnetic field to follow the flow of the plasma resulting in "frozen-in" flux. R m = U L η where U is the velocity scale of the flow L is the length scale of the flow η is the viscosity ∂ B → ∂ t = ∇ → × The rate of change of the magnetic flux is given by: d d t Φ = d d t ∫ S B → ⋅ d S = ∫ S ⋅ d S = 0 In ideal magnetohydrodynamics, if a cylindrical flux tube of length L0 is compressed while the length of tube stays the same, the magnetic field and the density of the tube increase with the same proportionality.
If a flux tube with a configuration of a magnetic field of B0 and a plasma density of ρ0 confined to the tube is compressed by a scalar value defined as λ, the new magnetic field and density are given by: B = B 0 λ 2 ρ = ρ 0 λ 2 If λ < 1, known as traverse compression, B and ρ increase and are scaled the same while transverse expansion decreases B and ρ by the same value and proportion where B/ρ = constant value. Extending the length of the flux tube by λ* gives a new length of L = λ*L0 while the density of the tube remains the same, ρ0, which results in the magnetic field strength increasing by B = λ*B0. Reducing the length of the tubes results in a decrease of the magnetic field's strength. In magnetohydrostatic equilibrium, the following condition is met for the equation of motion of the plasma confined to the flux tube: 0 = − ∇ p + j × B − ρ g where p is the plasma pressure j is the current density of the plasma ρg is the gravitational force With the magnetohydrostatic equilibrium condition met, a cylindrical flux tube's plasma pressure
Electrochemistry is the branch of physical chemistry that studies the relationship between electricity, as a measurable and quantitative phenomenon, identifiable chemical change, with either electricity considered an outcome of a particular chemical change or vice versa. These reactions involve electric charges moving between an electrolyte, thus electrochemistry deals with the interaction between electrical energy and chemical change. When a chemical reaction is caused by an externally supplied current, as in electrolysis, or if an electric current is produced by a spontaneous chemical reaction as in a battery, it is called an electrochemical reaction. Chemical reactions where electrons are transferred directly between molecules and/or atoms are called oxidation-reduction or reactions. In general, electrochemistry describes the overall reactions when individual redox reactions are separate but connected by an external electric circuit and an intervening electrolyte. Understanding of electrical matters began in the sixteenth century.
During this century, the English scientist William Gilbert spent 17 years experimenting with magnetism and, to a lesser extent, electricity. For his work on magnets, Gilbert became known as the "Father of Magnetism." He discovered various methods for strengthening magnets. In 1663, the German physicist Otto von Guericke created the first electric generator, which produced static electricity by applying friction in the machine; the generator was made of a large sulfur ball cast inside a glass globe, mounted on a shaft. The ball was rotated by means of a crank and an electric spark was produced when a pad was rubbed against the ball as it rotated; the globe could be used as source for experiments with electricity. By the mid—18th century the French chemist Charles François de Cisternay du Fay had discovered two types of static electricity, that like charges repel each other whilst unlike charges attract. Du Fay announced that electricity consisted of two fluids: positive, electricity; this was the two-fluid theory of electricity, to be opposed by Benjamin Franklin's one-fluid theory in the century.
In 1785, Charles-Augustin de Coulomb developed the law of electrostatic attraction as an outgrowth of his attempt to investigate the law of electrical repulsions as stated by Joseph Priestley in England. In the late 18th century the Italian physician and anatomist Luigi Galvani marked the birth of electrochemistry by establishing a bridge between chemical reactions and electricity on his essay "De Viribus Electricitatis in Motu Musculari Commentarius" in 1791 where he proposed a "nerveo-electrical substance" on biological life forms. In his essay Galvani concluded that animal tissue contained a here-to-fore neglected innate, vital force, which he termed "animal electricity," which activated nerves and muscles spanned by metal probes, he believed that this new force was a form of electricity in addition to the "natural" form produced by lightning or by the electric eel and torpedo ray as well as the "artificial" form produced by friction. Galvani's scientific colleagues accepted his views, but Alessandro Volta rejected the idea of an "animal electric fluid," replying that the frog's legs responded to differences in metal temper and bulk.
Galvani refuted this by obtaining muscular action with two pieces of the same material. In 1800, William Nicholson and Johann Wilhelm Ritter succeeded in decomposing water into hydrogen and oxygen by electrolysis. Soon thereafter Ritter discovered the process of electroplating, he observed that the amount of metal deposited and the amount of oxygen produced during an electrolytic process depended on the distance between the electrodes. By 1801, Ritter observed thermoelectric currents and anticipated the discovery of thermoelectricity by Thomas Johann Seebeck. By the 1810s, William Hyde Wollaston made improvements to the galvanic cell. Sir Humphry Davy's work with electrolysis led to the conclusion that the production of electricity in simple electrolytic cells resulted from chemical action and that chemical combination occurred between substances of opposite charge; this work led directly to the isolation of sodium and potassium from their compounds and of the alkaline earth metals from theirs in 1808.
Hans Christian Ørsted's discovery of the magnetic effect of electric currents in 1820 was recognized as an epoch-making advance, although he left further work on electromagnetism to others. André-Marie Ampère repeated Ørsted's experiment, formulated them mathematically. In 1821, Estonian-German physicist Thomas Johann Seebeck demonstrated the electrical potential in the juncture points of two dissimilar metals when there is a heat difference between the joints. In 1827, the German scientist Georg Ohm expressed his law in this famous book "Die galvanische Kette, mathematisch bearbeitet" in which he gave his complete theory of electricity. In 1832, Michael Faraday's experiments led him to state his two laws of electrochemistry. In 1836, John Daniell invented a primary cell which solved the problem of polarization by eliminating hydrogen gas generation at the positive electrode. Results revealed that alloying the amalgamated zinc with mercury would produce a higher voltage. William Grove produced the first fuel cell in 1839.
In 1846, Wilhelm Weber developed the electrodynamometer. In 1868, Georges Leclanché patented a new cell which became the forerunner to the world's first used battery, the zinc carbon cell. Svante Arrhenius published
James Clerk Maxwell
James Clerk Maxwell was a Scottish scientist in the field of mathematical physics. His most notable achievement was to formulate the classical theory of electromagnetic radiation, bringing together for the first time electricity and light as different manifestations of the same phenomenon. Maxwell's equations for electromagnetism have been called the "second great unification in physics" after the first one realised by Isaac Newton. With the publication of "A Dynamical Theory of the Electromagnetic Field" in 1865, Maxwell demonstrated that electric and magnetic fields travel through space as waves moving at the speed of light. Maxwell proposed that light is an undulation in the same medium, the cause of electric and magnetic phenomena; the unification of light and electrical phenomena led to the prediction of the existence of radio waves. Maxwell helped develop the Maxwell–Boltzmann distribution, a statistical means of describing aspects of the kinetic theory of gases, he is known for presenting the first durable colour photograph in 1861 and for his foundational work on analysing the rigidity of rod-and-joint frameworks like those in many bridges.
His discoveries helped usher in the era of modern physics, laying the foundation for such fields as special relativity and quantum mechanics. Many physicists regard Maxwell as the 19th-century scientist having the greatest influence on 20th-century physics, his contributions to the science are considered by many to be of the same magnitude as those of Isaac Newton and Albert Einstein. In the millennium poll—a survey of the 100 most prominent physicists—Maxwell was voted the third greatest physicist of all time, behind only Newton and Einstein. On the centenary of Maxwell's birthday, Einstein described Maxwell's work as the "most profound and the most fruitful that physics has experienced since the time of Newton". James Clerk Maxwell was born on 13 June 1831 at 14 India Street, Edinburgh, to John Clerk Maxwell of Middlebie, an advocate, Frances Cay daughter of Robert Hodshon Cay and sister of John Cay, his father was a man of comfortable means of the Clerk family of Penicuik, holders of the baronetcy of Clerk of Penicuik.
His father's brother was the 6th Baronet. He had been born "John Clerk", adding Maxwell to his own after he inherited the Middlebie estate, a Maxwell property in Dumfriesshire. James was a first cousin of both the artist Jemima Blackburn and the civil engineer William Dyce Cay. Cay and Maxwell were close friends and Cay acted as his best man when Maxwell married. Maxwell's parents married when they were well into their thirties, they had had one earlier child, a daughter named Elizabeth. When Maxwell was young his family moved to Glenlair, in Kirkcudbrightshire which his parents had built on the estate which comprised 1,500 acres. All indications suggest. By the age of three, everything that moved, shone, or made a noise drew the question: "what's the go o' that?" In a passage added to a letter from his father to his sister-in-law Jane Cay in 1834, his mother described this innate sense of inquisitiveness: He is a happy man, has improved much since the weather got moderate. He investigates the hidden course of streams and bell-wires, the way the water gets from the pond through the wall....
Recognising the potential of the young boy, Maxwell's mother Frances took responsibility for James's early education, which in the Victorian era was the job of the woman of the house. At eight he could recite the whole of the 119th psalm. Indeed, his knowledge of scripture was detailed, his mother was taken ill with abdominal cancer and, after an unsuccessful operation, died in December 1839 when he was eight years old. His education was overseen by his father and his father's sister-in-law Jane, both of whom played pivotal roles in his life, his formal schooling began unsuccessfully under the guidance of a 16 year old hired tutor. Little is known about the young man hired to instruct Maxwell, except that he treated the younger boy harshly, chiding him for being slow and wayward; the tutor was dismissed in November 1841 and, after considerable thought, Maxwell was sent to the prestigious Edinburgh Academy. He lodged during term times at the house of his aunt Isabella. During this time his passion for drawing was encouraged by his older cousin Jemima.
The 10 year old Maxwell, having been raised in isolation on his father's countryside estate, did not fit in well at school. The first year had been full, obliging him to join the second year with classmates a year his senior, his mannerisms and Galloway accent struck the other boys as rustic. Having arrived on his first day of school wearing a pair of homemade shoes and a tunic, he earned the unkind nickname of "Daftie", he never seemed bearing it without complaint for many years. Social isolation at the Academy ended when he met Lewis Campbell and Peter Guthrie Tait, two boys of a similar age who were to become notable scholars in life, they remained lifelong friends. Maxwell was fascinated by geometry at an early age, rediscovering the regular polyhedra before he received any formal instruction. Despite winning the school's scripture biography prize in his second year, his academic work remained unnoticed until, at the
The Faraday Building was the GPO's first telephone exchange in London. It started life as the Central telephone exchange at the Savings Bank building in Queen Victoria Street, opening for business on 1 March 1902 with just 200 subscribers; the Faraday Building is erected on the former site of Doctors' Commons, the location of the Admiralty and principal Ecclesiastical Court in England. The Post Office’s first London telephone exchange served nearly two-and-a-half square miles of the capital – notable subscribers included the Treasury, the War Office and Fleet Street. Take-up of the telephone by the public was quick so that by 1905 the exchange capacity was extended to 10,000 subscribers, full capacity was exhausted just three years later. To meet the growing demand from businesses in the City, a new common battery exchange was installed in 1906 with a capacity of 15,000 lines; this became'City' exchange and opened in November 1907. In common with other exchanges in London, Central was able to connect subscribers to the Electrophone exchange at Gerard Street.
Electrophone allowed people to listen to performances at certain London theatres and music halls while sitting at home. In 1933, the international telephone exchange was opened at Faraday. In 1935, an automatic exchange was opened with more than 6,000 working lines; the complex task of switching subscribers over to the new exchange involved 60 engineers working for more than 15 months. The construction of the Faraday Buildings obscured the riverside view of St Paul's Cathedral and directly led to the legislation protecting the views of St Paul's, used to thwart large buildings being erected around the various vantage points to see the cathedral; the City of London School and another telephone exchange, Baynard House, were built between the riverside and Faraday Building but are restricted in height to just three levels above ground. During the Second World War, the Faraday Building was transformed into a redoubt where the Cabinet could retreat if the need arose and the Prime Minister could run the war in greater security than Downing Street could provide
A homopolar generator is a DC electrical generator comprising an electrically conductive disc or cylinder rotating in a plane perpendicular to a uniform static magnetic field. A potential difference is created between the center of the disc and the rim with an electrical polarity that depends on the direction of rotation and the orientation of the field, it is known as a unipolar generator, acyclic generator, disk dynamo, or Faraday disc. The voltage is low, on the order of a few volts in the case of small demonstration models, but large research generators can produce hundreds of volts, some systems have multiple generators in series to produce an larger voltage, they are unusual in that they can source tremendous electric current, some more than a million amperes, because the homopolar generator can be made to have low internal resistance. The first homopolar generator was developed by Michael Faraday during his experiments in 1831, it is called the Faraday disc or Faraday wheel in his honor.
It was the beginning of modern dynamos — that is, electrical generators which operate using a magnetic field. It was inefficient and was not used as a practical power source, but it showed the possibility of generating electric power using magnetism, led the way for commutated direct current dynamos and alternating current alternators; the Faraday disc was inefficient due to counterflows of current. While current flow was induced directly underneath the magnet, the current would circulate backwards in regions outside the influence of the magnetic field; this counterflow limits the power output to the pickup wires, induces waste heating of the copper disc. Homopolar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field around the circumference, eliminate areas where counterflow could occur. Long after the original Faraday disc had been abandoned as a practical generator, a modified version combining the magnet and disc in a single rotating part was developed.
Sometimes the name homopolar generator is reserved for this configuration. One of the earliest patents on the general type of homopolar generators was attained by A. F. Delafield, U. S. Patent 278,516. Other early patents for homopolar generators were awarded to S. Z. De Ferranti and C. Batchelor separately. Nikola Tesla was interested in the Faraday disc and conducted work with homopolar generators, patented an improved version of the device in U. S. Patent 406,968. Tesla's "Dynamo Electric Machine" patent describes an arrangement of two parallel discs with separate, parallel shafts, joined like pulleys by a metallic belt; each disc had a field, the opposite of the other, so that the flow of current was from the one shaft to the disc edge, across the belt to the other disc edge and to the second shaft. This would have reduced the frictional losses caused by sliding contacts by allowing both electrical pickups to interface with the shafts of the two disks rather than at the shaft and a high-speed rim.
Patents were awarded to C. P. Steinmetz and E. Thomson for their work with homopolar generators; the Forbes dynamo, developed by the Scottish electrical engineer George Forbes, was in widespread use during the beginning of the 20th century. Much of the development done in homopolar generators was patented by J. E. Noeggerath and R. Eickemeyer. Homopolar generators underwent a renaissance in the 1950s as a source of pulsed power storage; these devices used heavy disks as a form of flywheel to store mechanical energy that could be dumped into an experimental apparatus. An early example of this sort of device was built by Sir Mark Oliphant at the Research School of Physical Sciences and Engineering, Australian National University, it stored up to 500 megajoules of energy and was used as an high-current source for synchrotron experimentation from 1962 until it was disassembled in 1986. Oliphant's construction was capable of supplying currents of up to 2 megaamperes. Similar devices of larger size are designed and built by Parker Kinetic Designs of Austin.
They have produced devices for a variety of roles, from powering railguns to linear motors to a variety of weapons designs. Industrial designs of 10 MJ were introduced for a variety including electrical welding; this device consists of a conducting flywheel rotating in a magnetic field with one electrical contact near the axis and the other near the periphery. It has been used for generating high currents at low voltages in applications such as welding and railgun research. In pulsed energy applications, the angular momentum of the rotor is used to accumulate energy over a long period and release it in a short time. In contrast to other types of generators, the output voltage never changes polarity; the charge separation results from the Lorentz force on the free charges in the disk. The motion is azimuthal and the field is axial, so the electromotive force is radial; the electrical contacts are made through a "brush" or slip ring, which results in large losses at the low voltages generated. Some of these losses can be reduced by using mercury or other liquified metal or alloy as the "brush", to provide uninterrupted electrical contact.
A recent suggested modification is to use a plasma contact supplied by a negative resistance neon streamer touching the edge of the disk or drum, using specialized low work function carbon in vertical strips. This would have the advantage of low resistance within a current range up to thousands of Amps without the liquid metal contact. If the magnetic field is provi