Michael Faraday

(1791-1867)


Faraday was a British chemist and physicist who contributed significantly to the study of electromagnetism and electrochemistry.

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Michael Faraday was born on 22 September 1791 in south London. His family was not well off and Faraday received only a basic formal education. When he was 14, he was apprenticed to a local bookbinder and during the next seven years, educated himself by reading books on a wide range of scientific subjects. In 1812, Faraday attended four lectures given by the chemist Humphry Davy at the Royal Institution. Faraday subsequently wrote to Davy asking for a job as his assistant. Davy turned him down but in 1813 appointed him to the job of chemical assistant at the Royal Institution.

A year later, Faraday was invited to accompany Davy and his wife on an 18 month European tour, taking in France, Switzerland, Italy and Belgium and meeting many influential scientists. On their return in 1815, Faraday continued to work at the Royal Institution, helping with experiments for Davy and other scientists. In 1821 he published his work on electromagnetic rotation (the principle behind the electric motor). He was able to carry out little further research in the 1820s, busy as he was with other projects. In 1826, he founded the Royal Institution's Friday Evening Discourses and in the same year the Christmas Lectures, both of which continue to this day. He himself gave many lectures, establishing his reputation as the outstanding scientific lecturer of his time.

In 1831, Faraday discovered electromagnetic induction, the principle behind the electric transformer and generator. This discovery was crucial in allowing electricity to be transformed from a curiosity into a powerful new technology. During the remainder of the decade he worked on developing his ideas about electricity. He was partly responsible for coining many familiar words including 'electrode', 'cathode' and 'ion'. Faraday's scientific knowledge was harnessed for practical use through various official appointments, including scientific adviser to Trinity House (1836-1865) and Professor of Chemistry at the Royal Military Academy in Woolwich (1830-1851).

As a chemist, Faraday discovered benzene, investigated the clathrate hydrate of chlorine, invented an early form of the bunsen burner and the system of oxidation numbers, and popularized terminology such as anode, cathode, electrode, and ion.

However, in the early 1840s, Faraday's health began to deteriorate and he did less research. He died on 25 August 1867 at Hampton Court, where he had been given official lodgings in recognition of his contribution to science. He gave his name to the 'farad', originally describing a unit of electrical charge but later a unit of electrical capacitance.

 

Faraday's inventions
Magnetism
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Faraday's greatest work was with electricity. In 1821, soon after the Danish chemist, Hans Christian Orsted, discovered the phenomenon of magnetism, Humphry Davy and William Hyde Wollaston tried but failed to design an electric motor. Faraday, having discussed the problem with the two men, went on to build two devices to produce what he called electromagnetic rotation which is a continuous circular motion from the circular magnetic force around a wire. A wire extending into a pool of mercury with a magnet placed inside would rotate around the magnet if charged with electricity by a chemical battery. This device is known as a homopolar motor. These experiments and inventions form the foundation of modern electromagnetic technology. Unwisely, Faraday published his results without acknowledging his debt to Wollaston and Davy, and the resulting controversy caused Faraday to withdraw from electromagnetic research for several years.
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Faraday's first dynamo
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Ten years later, in 1831, he began his great series of experiments in which he discovered electromagnetic induction. He found that if he moved a magnet through a loop of wire, an electric current flowed in the wire. The current also flowed if the loop was moved over a stationary magnet. This was the first transformer (inductor), although Faraday used it only to demonstrate the principle of electromagnetic induction and did not realise what it would eventualybe used for.

His experiments established that a changing magnetic field produces an electric field. This relation was mathematically modelled by faraday's law, which afterwards became one of the four Maxwell equations. These in turn evolved into the generalization known as field theory.

Faraday then used this principle to construct the first dynamo (in the form of a copper disk rotated between the poles of a permanent magnet), the predecessor of modern dynamos and generators.
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Faraday’s first electric generator

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In 1831-1832 Michael Faraday discovered that a potential difference is generated between the ends of an electrical conductor that moves perpendicular to a magnetic field. He also built the first electromagnetic generator called the 'Faraday disc', a type of homopolar generator, using a copper disc rotating between the poles of a horseshoe magnet. It produced a small DC voltage, and large amounts of current.

This design was inefficient due to self-cancelling counterflows of current in regions not under the influence of the magnetic field. 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, and induces waste heating of the copper disc.

Later homopolar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field effect in one current-flow direction.

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Farady's disk
This exemplar of Faraday's disk is currently in the Marischal museum in Aberdeen. It is approximately 75 centimeters long and 26 centimeters wide and measures 19 centimeter in the diameter. This apparatus can be considered to be the first dynamo ever made, capable of producing electricity by making a copper disk spin between two magnets. It was made and used in several experiments by Michael Faraday around the 1850's.

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On this particular model, there is actually no magnet; the copper disk is so thin that the earths own magnetic field is sufficient to make the apparatus work. There was also some mercury on the contacts on the disk, but it is now absent.

Michael Faraday had constructed Faraday's disk to demonstrate and visualise the theory about currents and magnetic fields he had discovered. Unfortunately, the disk was very inefficient in producing current and was never actually used as a practical power source, but only as a tool of demonstration. Other scientists, with Nikola Tesla amongst them as one of the earliest to take profit of Faraday's invention, later improved and changed Faraday's disk. Tesla patented a version of Faraday's disk which had less frictional losses and Hippolyte Pixii constructed the first efficient dynamo by adding a commutator to it. Sir Mark Oliphant was inspired by it to build what became the world's largest homopolar generator (producing 500 Megajoules).

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Faraday's disk helped a wide range of scientists, such as Sir Joseph Larmour, to understand more phenomena. In Larmour's case, the newly found link between magnetic fields and current lead him to an explanation of several phenomena related to sunspots.

Zénobe Gramme used the principle of Faraday's disk to design the first commercial power plants in Paris, discovering what is since then known as the basic concept of a spinning endless loop of wires.

Later, Joseph Newmann invented the axletree dynamo, which was believed to be a perpetual motion dynamo but was never patented.

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Faraday law of electrolysis

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In 1832, Michael Faraday reported that the quantity of elements separated by passing an electrical current through a molten or dissolved salt was proportional to the quantity of current passed through the circuit. This became the basis of the first law of electrolysis. He also popularized terminology such as anode, cathode, electrode, and ion.

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Faraday effect
In 1845 he discovered what is now called the Faraday effeact. The plane of polarization of linearly polarized light propagated through a material medium can be rotated by the application of an external magnetic field aligned in the propagation direction. He wrote in his notebook, "I have at last succeeded in illuminating a magnetic curve or line of force and in magnetising a ray of light". This established that magnetic force and light were related.

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Diamagnetism

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In 1845 he also discovered the phenomenon that he named Diamagnetism - a very weak form of magnetism that is only exhibited in the presence of an external magnetic field. This phenomenon can can be used for levitation.

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Faraday cage
In his work on static electricity, Faraday demonstrated that the charge only resided on the exterior of a charged conductor, and exterior charge had no influence on anything enclosed within a conductor. This is because the exterior charges redistribute such that the interior fields due to them cancel. This shielding effect is used in what is now known as a Faraday cage.

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The Faraday cage is a practical application of the effect demonstrated in school as the Faraday ice-pail experiment. If an ice pail, or any other hollow conductor is given a charge on its inside, then the charge will spread all over the outside surface of the conductor in such a way as to produce no electric field inside. The Van de Graaff generator uses this effect to produce such a large potential on the outside surface of its dome. In large Van de Graaff generators it is possible to have a lab or office actually inside the dome, and even have people there, while the machine is on! The idea that a hollow metal conductor will protect the interior from electric fields can be used using Faraday screens, cans or cages.

A Faraday can is nothing more complicated that a metal biscuit barrel for example. If an electronic watch is placed inside such a container, and then a spark is drawn to the container either from a Van de Graaff generator or induction coil, then after the demonstration, the watch will be unharmed.

A more impressive demonstration of the Faraday cage effect is that of an aircraft being struck by lightning. This happens frequently, but does not harm the plane or passengers. The metal body of the aircraft protects the interior. For the same reason, and if it were not for the highly flammable nature of petrol, a car would be a very safe place to be in a thunderstorm.

Demonstration of the Faraday cage effect: A human-size Faraday cage has been constructed. Lets allow someone to stand inside while the discharge from a Tesla coil is directed towards it. To increase the dramatic content of the demonstration, the person inside is asked to hold a fluorescent tube, which does not light, while similar tubes balanced against the sides of the cage do light when the discharge is enabled. Our cage measures 1m square and is 2m tall. It is built from 1cm pitch steel mesh. The mesh is secured to a strong 'speedframe' framework, and riveted all round the frame except for the door section which is hinged on, and has two retainer catches to ensure the door stays shut while the demonstration is taking place.

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When the cage is charged, the pith balls move away from the outside of the cage, indicating the presence of an electric field, but the pith balls inside the cage are not repelled, indicating that there is no electric field inside the conductor.

 

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Benzene
Faraday also dabbled in chemistry, discovering chemical substances such as benzene, inventing the system of oxidization numbers, and liquefying gases.

 
   
 
 
 
     

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