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Helmoholtz Coils

Helmoholtz Coils

The term Helmholtz coils refers to a device for producing a region of nearly uniform magnetic field. It is named in honor of the German physicist Hermann von Helmholtz.

Description

A Helmholtz pair consists of two identical circular magnetic coils that are placed symmetrically one on each side of the experimental area along a common axis, and separated by a distance equal to the radius of the coil. Actually a slightly larger separation improves the field uniformity. Each coil carries an equal electrical current flowing in the same direction. A cylindrical region extending between the centers of the two coils and approximately 1/5 of their diameter will have a nearly spatially uniform magnetic field.

Mathematics

The calculation of the exact magnetic field has mathematical complexities and involves the study of Bessel functions. An approximate calculation gives the correct value at the center point. If the radius is R, the number of turns in each coil is n and the current flowing through the coils is I, then the magnetic field, B at the midpoint between the coils will be given by :

B = ^ \frac.

Quadrupole Magnetic Field

This is a Magnetic Field produced by two electric coils; see for example: [http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magquad.html]

External links

- [http://www.netdenizen.com/emagnet/helmholtz/idealhelmholtz.htm On-Axis Field of an Ideal Helmholtz Coil]
- [http://www.netdenizen.com/emagnet/helmholtz/realhelmholtz.htm Axial field of a real Helmholtz coil pair] Category:Magnetic devices

Hermann von Helmholtz

Hermann Ludwig Ferdinand von Helmholtz (August 31, 1821 – September 8, 1894) was a German physician and physicist. In the words of the 1911 Britannica, "his life from first to last was one of devotion to science, and he must be accounted, on intellectual grounds, one of the foremost men of the 19th century".

Early life

Helmholtz was the son of the Potsdam Gymnasium headmaster, Ferdinand Helmholtz, who had studied classical philology and philosophy, and who was a close friend of the publisher and philosopher Immanuel Hermann Fichte. Helmholtz's work is influenced by the philosophy of Fichte and Kant. He tried to trace their theories in empirical matters like physiology. As a young man, Helmholtz was interested in natural science, but his father wanted him to study medicine at the Charité because there was financial support for medical students. Helmholtz wrote about many topics ranging from the age of the Earth to the origin of the solar system.

Conservation of energy

His first important scientific achievement, an 1847 physics treatise on the conservation of energy was written in the context of his medical studies and philosophical background. He discovered the principle of conservation of energy while studying muscle metabolism. He tried to demonstrate that no energy is lost in muscle movement, motivated by the implication that there were no vital forces necessary to move a muscle. This was a rejection of the speculative tradition of Naturphilosophie which was at that time a dominant philosophical paradigm in German physiology. Drawing on the earlier work of Sadi Carnot, Émile Clapeyron and James Prescott Joule, he postulated a relationship between mechanics, heat, light, electricity and magnetism by treating them all as manifestations of a single force (energy in modern terms). He published his theories in his book Über die Erhaltung der Kraft (On the Conservation of Force, 1847). Helmholtz is thought to be the first person to put forward the idea of the heat death of the universe in 1854.

Sensory physiology

The sensory physiology of Helmholtz was the basis of the work of Wilhelm Wundt, a student of Helmholtz, who is considered one of the founders of experimental psychology. He, more explicitly than Helmholtz, described his research as a form of empirical philosophy and as a study of the mind as something separate. Helmholtz had in his early refutal of the speculative early nineteenth century tradition of Naturphilosophie stressed the importance of materialism, and was focusing more on the unity of "mind" and body.

Ophthalmic optics

In 1851, Helmholtz revolutioned the field of ophthalmology with the invention the ophthalmoscope; an instrument used to examine the inside of the human eye. Helmholtz's interests at that time were increasingly focused on the physiology of the senses. His main publication, entitled Handbuch der Physiologischen Optik (Handbook of Physiological Optics), provided empirical theories on spatial vision, color vision, and motion perception, and became the fundamental reference work in his field during the second half of the nineteenth century. His theory of accommodation went unchallenged until the final decade of the 20th century. accommodation Helmholtz continued to work for several decades on several editions of the handbook, frequently updating his work because of his dispute with Ewald Hering who held opposite views on spatial and color vision. This dispute divided the discipline of physiology during the second half of the 1800's.

Acoustics and aesthetics

In 1863 Helmholtz published a book called On the Sensations of Tone as a Physiological Basis for the Theory of Music, once again demonstrating his interest in the physics of perception. This book influenced musicologists into the twentieth century. Helmholtz invented the Helmholtz resonator to show the height of the various tones.

Electromagnetism

In 1871 Helmholtz moved from Bonn to Berlin to become a professor in physics. He became interested in electromagnetism. Oliver Heaviside stated that there were longitudinal waves in Helmholtz theory. Although he did not make major contributions to this field, his student Heinrich Rudolf Hertz became famous as the first to demonstrate electromagnetic radiation. Helmholtz had predicted E-M radiation from Maxwell's equations, and the wave equation now carries his name. A large German association of research institutions, the Helmholtz Association, is named after him.

Students and associates

Other students and research associates of Helmholtz at Berlin included Max Planck, Heinrich Kayser, Eugen Goldstein, Wilhelm Wien, Arthur König, Henry Augustus Rowland, A. A. Michelson, and Michael Pupin. Leo Koenigsberger, who studied at Berlin while Helmholtz was there, wrote the definitive biography of him in 1902.

Notes

Bibliography

- Cahan, D. (1993) Hermann Ludwig Ferdinand von Helmholtz and the Foundations of Nineteenth Century Science, Los Angeles: University of California Press, ISBN 0520083342

External links

-
- [http://www.bartleby.com/30/125.html On the Conservation of Force] Introduction to a Series of Lectures Delivered at Carlsruhe in the Winter of 1862–1863, English translation Helmholtz, Hermann Helmholtz, Hermann Ludwig Ferdinand von Helmholtz, Hermann Helmholtz, Hermann von Helmholtz, Hermann von ja:ヘルマン・フォン・ヘルムホルツ

Coil

A coil is a series of loops.

General applications

loop A coil is made of materials, usually rigid, which can fashioned into a spiral or helical shape. Flexible materials like wire, rope, hose, or cable can also be coiled into empty loops, or wound around a central drum or spindle. Some common applications of coils include:
- A coil spring is the most common type of spring.
- A set of stairs fashioned in a coil shape, which are called spiral staircases.
- A Slinky is a coil-shaped toy.
- A coil stamp is a type of postage stamp sold as strips one stamp wide.
- A boiler coil is an element in a water heater.
- Evaporator coils are used in air conditioning and other refrigeration cycles.
- Coil is a colloquial term applied to contraceptive intrauterine devices. See also: list of coil knots

Electromagnetic

list of coil knots list of coil knots In electrical engineering, a electromagnetic coil is formed when a metallic or conductive wire is looped around a core to create an electronic inductor or electromagnet. A transformer coil has a primary coil and a secondary coil that transfers energy from one electrical circuit to another by magnetic coupling without moving parts. An extra coil (sometimes referred to as a tickler coil) is usually a third coil placed in relation to a primary coil and secondary coil. Some common electromagnetic coils include:
- A bifilar coil is a coil that employs two parallel windings.
- A Barker coil is used in low field NMR imaging.
- A Braunbeck coil is used in geomagnetic research.
- A degaussing coil is used in the process of removing permanent magnetism (magnetic hysteresis) from an object.
- A choke coil (or choking coil) is low-resistance inductor used to block alternating current while passing direct current.
- A Garrett coil is used in metal detectors.
- A Helmholtz coil is a device for producing a region of nearly uniform magnetic field.
- A hybrid coil (or bridge transformer) is a single transformer that effectively has three windings.
- An induction coil (or ignition coil) is an electrical device in common use as the ignition system (ignition coil or spark coil) of internal-combustion engines.
- A loading coil is, in electronics, a coil (inductor) inserted in a circuit to increase its inductance. Archaically called Pupin coils.
- A multiple coil magnet is an electromagnet that has several coils of wire connected in parallel.
- A Maxwell coil is a device for producing almost a constant magnetic field.
- A Oudin coil is a disruptive discharge coil.
- The polyphase coils are connected together in a polyphase system such as a generator or motor.
- A relay coil is the copper winding part of a relay that produces a magnetic field that actuates the mechanism.
- A Rogowski coil is an electrical device for measuring alternating current.
- A single coil is a type of pickup for the electric guitar.
- A solenoid is a mechanical device, based around a coil of wire, that converts energy into linear motion.
- A Tesla coil is category of disruptive discharge coils, usually denoting a resonant transformer that generates very high voltages at radio frequencies.
- A voice coil which is mounted to the moving cone of a loudspeaker. ;Further reading
- Querfurth, William, "Coil winding; a description of coil winding procedures, winding machines and associated equipment for the electronic industry" (2d ed.). Chicago, G. Stevens Mfg. Co., 1958.
- Weymouth, F. Marten, "Drum armatures and commutators (theory and practice) : a complete treatise on the theory and construction of drum winding, and of commutators for closed-coil armatures, together with a full résumé of some of the principal points involved in their design; and an exposition of armature reactions and sparking". London, "The Electrician" Printing and Publishing Co., 1893.
- "Coil winding proceedings". International Coil Winding Association.
- Chandler, R. H., "Coil coating review, 1970-76". Braintree, R. H. Chandler Ltd, 1977. ;External articles
- R. Clarke, "[http://www.ee.surrey.ac.uk/Workshop/advice/coils/ Producing wound components]". Surrey.ac.uk, 2005 October 9th.

Chemistry

loudspeaker In the study of how molecules interact with each other, there are a few specific references to organic coils. During self-assembly, organic elements organize to form this structural pattern. Molecular self-assembly assembles the molecules, without guidance or management from an outside source, into these shapes. Examples of these structural patterns include:
- A coiled coil is a structural motif found in many proteins.
- The DNA coil is a nucleic acid structure that contains the genetic instructions specifying the biological development of all cellular forms of life (and many viruses).
- A random coil is a polymer conformation where the monomers are arranged at random.
- The RNA coil is a nucleic acid structure consisting of a string of covalently-bound nucleotides. As an acronym, COIL denotes the Chemical Oxygen Iodine Laser.

Other uses

Musician names "Coil" is, or is part of, the name for some musicians or their albums. Name
- Coil is a British experimental band.
- Icon of Coil is a Norwegian electronic body music band.
- Lacuna Coil is an Italian goth heavy metal band.
- This Mortal Coil is a British dark cover band. Publication
- Coil is a 1997 album by American band Toad the Wet Sprocket.

External articles

- For the definition of Coil and words related to it, see Wiktionary. ja:巻線

Electrical current

In electricity, current refers to electric current, which is the flow of electric charge. Lightning is an example of an electric current, as is the solar wind, the source of the polar aurora. Probably the most familiar form of electric current is the flow of conduction electrons in a metallic wire. This is how utility companies deliver electricity. In electronics, electric current is most often the flow of electrons through conductors and devices such as resistors, but it is also the flow of ions inside a battery or the flow of holes within a semiconductor.

Relation between current and charge

The symbol typically used for the amount of current (the amount of charge Q flowing per unit of time t) is I, from the German word Intensität, which means 'intensity'. :

I =

Formally this is written as :

i(t) =

or inversely as

q(t) = \int_^ i(x)\, dx

Conventional current

Conventional current was defined early in the history of electrical science as a flow of positive charge. In solid metals, like wires, the positive charges are immobile, and only the negatively charged electrons flow in the direction opposite conventional current, but this is not the case in most non-metallic conductors. In other materials, charged particles flow in both directions at the same time. Electric currents in electrolytes are flows of electrically charged atoms (ions), which exist in both positive and negative varieties. For example, an electrochemical cell may be constructed with salt water (a solution of sodium chloride) on one side of a membrane and pure water on the other. The membrane lets the positive sodium ions pass, but not the negative chlorine ions, so a net current results. Electric currents in plasma are flows of electrons as well as positive and negative ions. In ice and in certain solid electrolytes, flowing protons constitute the electric current. To simplify this situation, the original definition of conventional current still stands. There are also instances where the electrons are the charge that is physically moving, but where it makes more sense to think of the current as the movement of positive "holes" (the spots that should have an electron to make the conductor neutral). This is the case in a p-type semiconductor. The SI unit of electrical current is the ampere. Electric current is therefore sometimes informally referred to as amperage or ampage, by analogy with the term voltage. Though this is a valid term, some engineers frown on it.

The speed of an electric current

The charged particles whose movement causes an electric current do not always move in straight lines. In metals, for example, they follow an erratic path, bouncing from atom to atom, but generally drifting in the direction of the electric field. The speed at which they drift can be calculated from the equation: :

I=nAvQ \!\

where :I is the current :n is number of charged particles per unit volume :A is the cross-sectional area of the conductor :v is the drift velocity, and :Q is the charge on each particle. For example, in a copper wire of cross-section 0.5 mm², carrying a current of 5 A, the drift velocity of the electrons is of the order of a millimetre per second. To take a different example, in the near-vacuum inside a cathode ray tube, the electrons travel in near-straight lines ("ballistically") at about a tenth of the speed of light. However, we know that an electric signal travels much faster than this; usually close to the speed of light. These results show that the speed of the charged particles is not necessarily related to the speed of the electric signal. To understand how signals travel faster than the particles that carry them, it is necessary to understand the properties of electromagnetic waves (see article).

Current density

Current density is the current per unit (cross-sectional) area. Mathematically, current is defined as the net flux through an area. Thus: :

I = j \cdot A

where, in the MKS or SI system of measurement, :I is the current, measured in amperes :j is the "current density" measured in amperes per square metre :A is the area through which the current is flowing, measured in square metres The current density is defined as: :

j=\int_i n_i \cdot x_i \cdot \mathbf

where :n is the particle density (number of particles per unit volume) :x is the mass, charge, or any other characteristic whose flow one would like to measure. :u is the average velocity of the particles in each volume Current density is an important consideration in the design of electrical and electronic systems. Most electrical conductors have a finite, positive resistance, making them dissipate power in the form of heat. The current density must be kept sufficiently low to prevent the conductor from melting or burning up, or the insulating material failing. In superconductors, excessive current density may generate a strong enough magnetic field to cause spontaneous loss of the superconductive property.

Electromagnetism

Every electric current produces a magnetic field. The magnetic field can be visualized as a pattern of circular field lines surrounding the wire. Electric current can be directly measured with a galvanometer, but this method involves breaking the circuit, which is sometimes inconvenient. Current can also be measured without breaking the circuit by detecting the magnetic field it creates. Devices used for this include Hall effect sensors, current clamps and Rogowski coils.

Ohm's law

Ohm's law predicts the current in an (ideal) resistor (or other ohmic device) to be the quotient of applied voltage over electrical resistance: :

I = \frac

where :I is the current, measured in amperes :V is the potential difference measured in volts :R is the resistance measured in ohms

Electrical safety

The danger of an electric shock depends on the current (in milliamperes), duration and the current's path in the body:
- 1 mA causes a tingle
- 5 mA causes a slight shock
- 50 to 150 mA may result in death, e.g. through rhabdomyolysis (muscle breakdown) and resultant acute renal failure
- 1-4 A causes ventricular fibrillation
- 10 A causes cardiac arrest (only at this current will a typical home fuse break the circuit) Currents through the heart and the nervous system are the most dangerous. As most dangerous sources are voltage sources, the current present depends on the resistance of the body between the points of contact and any current limiting built into the source. The comparison between the dangers of alternating current and direct current has been a subject of debate ever since the War of Currents in the 1880s. DC tends to cause continuous muscular contractions that make the victim hold on to a live conductor, thereby increasing the risk of deep tissue burns. On the other hand, mains-frequency AC tends to interfere more with the heart's electrical pacemaker, leading to an increased risk of fibrillation. AC at higher frequencies holds a different mixture of hazards, such as RF burns and the possibility of tissue damage with no immediate sensation of pain.

External links

- [http://www.unitconversion.org/unit_converter/current.html Online Current Converter] - convert between various units of current, such as ampere, biot, abampere, statampere, and so on
- [http://www.unitconversion.org/unit_converter/current-v.html Interactive Current Conversion Table] - convert selected unit to all other units of current
- [http://amasci.com/amateur/elecdir.html Which direction does electricity really flow?] Category:Electromagnetism Category:Magnetism ko:전류 ja:電流 th:กระแสไฟฟ้า

Magnetic field

:For other senses of this term, see magnetic field (disambiguation). In physics, a magnetic field is an entity produced by moving electric charges (electric currents) that exerts a force on other moving charges. (The quantum-mechanical spin of a particle produces magnetic fields and is acted on by them as though it were a current; this accounts for the fields produced by "permanent" ferromagnets.) A magnetic field is a vector field: it associates with every point in space a (pseudo-)vector that may vary in time. The direction of the field is the equilibrium direction of a compass needle placed in the field.

Symbols and terminology

Magnetic field is usually denoted by the symbol

\mathbf \

. Historically,

\mathbf \

was called the magnetic flux density, magnetic induction, or magnetic field strength.

\mathbf

was called the magnetic field (or magnetic field intensity), and this terminology is still often used to distinguish the two in the context of magnetic materials (non-trivial permeability μ). Otherwise, however, this distinction is often ignored, and both symbols are frequently referred to as the magnetic field. (Some authors call H the auxiliary field, instead.) In linear materials, such as air or free space, the two quantities are linearly related: :

\mathbf = \mu \mathbf \

where

\ \mu

is the magnetic permeability (in henries per meter) of the medium. In SI units,

\mathbf \

and

\mathbf \

are measured in teslas (T) and amperes per meter (A/m), respectively; or, in cgs units, in gauss (G) and oersteds (Oe), respectively. Two parallel wires carrying an electric current in the same sense will generate a magnetic field which will cause a force of attraction to each other. This fact is used to generate the value of an ampere of electric current. Note that while like charges repel and unlike ones attract, the opposite holds for currents: if the current in one of the two parallel wires is reversed, the two will repel.

Definition

Like the electric field, the magnetic field can be defined by the force it produces. In SI units, this is: :

\mathbf = q \mathbf \times \mathbf

where :F is the force produced, measured in newtons :

\times \

indicates a vector cross product :

q \

is electric charge that the magnetic field is acting on, measured in coulombs :

\mathbf \

is velocity of the electric charge

q \

, measured in metres per second :B is magnetic flux density, measured in teslas This law is called the Lorentz force law. (More precisely, it is the special case of that law when there is no electric field. It holds in any reference frame, although the force due to the magnetic field may be different in different frames because magnetic fields transform into electric fields under Lorentz transformations. The total force due to the electric and magnetic fields is the same in any frame.)

Current loop

A simpler form of the force equation in a wire current loop is: Force = BLi = (Tesla)x(meter length of wire)x(ampere current of wire). A more complex explanation is that if the moving charge is part of a current in a wire, then an equivalent form of the law is :

\frac   = \mathbf \times \mathbf

In words, this equation says that the force per unit length of wire is the cross product of the current vector and the magnetic field. In the equation above, the current vector,

\mathbf

, is a vector with magnitude equal to the usual scalar current,

i

, and direction pointing along the wire that the current is flowing.

Point charge generating magnetic field

The field can be computed as the sum of the contributions from individual charged particles. The magnetic flux density from a point charge is: :

\mathbf = \frac\mathbf \times \mathbf

which, for constant velocities, can be expanded into the Biot-Savart law: :

\mathbf = \frac\frac\mathbf \times \mathbf

q \

is electric charge generating the magnetic field, measured in coulombs :

\mathbf \

is velocity of the electric charge

q \

that is generating B, measured in metres per second :B is magnetic flux density, measured in teslas

Vector calculus

The most compact and elegant mathematical statements describing how magnetic fields are produced makes use of vector calculus. In free space: :

\nabla \times \mathbf = \mu_0 \mathbf + \mu_0 \epsilon_0 \frac

\nabla \cdot \mathbf = 0

where :

\nabla \times

is the curl operator :

\nabla \cdot

is the divergence operator :

\mu_0 \

is permeability :

\mathbf \

is current density :

\partial \

is the partial derivative :

\epsilon_0 \

is the free-space permittivity :

\mathbf \

is the electric field :

t \

is time The first equation is known as Ampère's law with James Clerk Maxwell's correction. The second term of this equation (Maxwell's correction) disappears in static or quasi-static systems. The second equation is a statement of the observed non-existence of magnetic monopoles. These are two of four Maxwell's equations; the notation is due to Oliver Heaviside.

Energy in the magnetic field

The general relation for nonlinear materials, the differential energy is: :

dU_H = \int_^ H \cdot dB \, dV

Where V is the volume and dV is the differential volume. For linear materials, H is proportional to B, so the above equation can be simplified: :

U_H = \frac\int_^ B \cdot H \, dV

For linear materials and a constant volume: :

U_H = \frac

Energy can produce a force, so :

F = \frac

F = \frac

Where dl is differential distance and A is the surface area. Force per unit area (pressure) is :

P = \frac

In the case of free space (air),

\mu_o = 4 \pi \cdot 10^ \frac

: :

P \approx 398 \, \mbox \, \approx 57.7 \, \frac

at B = 1 tesla :

P \approx 1592 \, \mbox \, \approx 231 \, \frac

at B = 2 teslas This is the force observed when a high permeability, ferromagnetic materials, such as iron and steel alloys, are in the proximity of magnetic fields.

Properties

Maxwell did much to unify static electricity and magnetism, producing a set of four equations relating the two fields. However, under Maxwell's formulation, there were still two distinct fields describing different phenomena. It was Albert Einstein who showed, using special relativity, that electric and magnetic fields are two aspects of the same thing (a rank-2 tensor), and that one observer may perceive a magnetic force where a moving observer perceives only an electrostatic force. Thus, using special relativity, magnetic forces are a manifestation of electrostatic forces of charges in motion and may be predicted from knowledge of the electrostatic forces and the movement (relative to some observer) of the charges. A thought experiment one can do to show this is with two identical infinite and parallel lines of charge having no motion relative to each other but moving together relative to an observer. Another observer is moving alongside the two lines of charge (at the same velocity) and observes only electrostatic repulsive force and acceleration. The first or "stationary" observer seeing the two lines (and second observer) moving past with some known velocity also observes that the "moving" observer's clock is ticking more slowly (due to time dilation) and thus observes the repulsive acceleration of the lines more slowly than that which the "moving" observer sees. The reduction of repulsive acceleration can be thought of as an attractive force, in a classical physics context, that reduces the electrostatic repulsive force and also that is increasing with increasing velocity. This pseudo-force is precisely the same as the electromagnetic force in a classical context. Changing magnetic fields, according to Faraday's law of induction, can induce an electric field and thus an electric current; similar currents can be induced by conductors moving in a fixed magnetic field. These phenomena are the basis for many electric generators and electric motors.

Magnetic field lines

electric motor Formally, the magnetic field is not a vector, it is a pseudovector. That is, it gains an extra sign flip under improper rotations of the coordinate system. (The distinction is important when using symmetry to analyze magnetic-field problems.) This is a consequence of the fact that B is related to two true vectors by a cross product (e.g. in the Lorentz force law). To simplify the study of magnets an arbitrary (but valid) description of magnetic field lines was created. 1 magnetic field line = 1 gauss line. 10,000 gauss lines per square meter is equal to 1 tesla. The total number of lines emanating from a magnet pole is the magnetic flux. Count only north or only south pole lines, i.e. monopole or one sided value. Although the field line orientation is typically indicated in diagrams with an arrow, the arrow should not be interpreted to indicate any actual movement or flow of the field line.

Pole labeling confusions

It is necessary to note that the labeling of north and south on a compass is in opposition to the labeling of the north and south pole of the Earth. If you have two labeled magnets, it is clear that like poles repel, while opposing poles attract. However, this is clearly wrong when using a compass to find the North Pole of the Earth, because the "north" end of the compass points to the "North" Pole. By convention, the pole of a magnet is labelled according to the direction it points, hence when we speak of the "north pole" of a magnet, we really mean the "north-seeking pole". Magnetic field lines point from north to south of a magnet, and hence the natural magnetic field lines run from south to north along the Earth's surface. This choice, along with the choice of sign convention in the Biot-Savart law, is equivalent to choosing a sign convention for electric charge.

Rotating magnetic fields

A rotating magnetic field is a magnetic field which rotates in polarity at non-relativistic speeds. This is a key principle to the operation of alternating-current motor. A permanent magnet in such a field will rotate so as to maintain its alignment with the external field. This effect is utilised in alternating current electric motors. A good rotating magnetic field can be constructed using three phase alternating currents (or even with higher order polyphase systems). Synchronous motors and induction motors use a stator's rotating magnetic fields to turn rotors. In 1882, Nikola Tesla identified the concept of the rotary magnetic field. In 1885, Galileo Ferraris independently researched the concept. In 1888, Tesla gained for his work. Also in 1888, Ferraris published his research in a paper to the Royal Academy of Sciences in Turin.

References

Books
-
-
-

External articles

Information
- Nave, R., "[http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfie.html Magnetic Field]". HyperPhysics.
- "Magnetism", [http://theory.uwinnipeg.ca/physics/mag/node2.html#SECTION00110000000000000000 The Magnetic Field]. theory.uwinnipeg.ca.
- Hoadley, Rick, "[http://my.execpc.com/~rhoadley/magfield.htm What do magnetic fields look like]?" 17 July 2005. Rotating magnetic fields
- "[http://www.tpub.com/neets/book5/18a.htm Rotating magnetic fields]". Integrated Publishing.
- "Introduction to Generators and Motors", [http://www.tpub.com/content/neets/14177/css/14177_87.htm rotating magnetic field]. Integrated Publishing.
- "[http://www.egr.msu.edu/~jurkovi4/Experiment4.pdf Induction Motor-Rotating Fields]". Diagrams
- McCulloch, Malcolm,"A2: Electrical Power and Machines", [http://www.eng.ox.ac.uk/~epgmdm/A2/img89.htm Rotating magnetic field]. eng.ox.ac.uk.
- "AC Motor Theory" [http://www.tpub.com/content/doe/h1011v4/css/h1011v4_23.htm Figure 2 Rotating Magnetic Field]. Integrated Publishing. Journal Articles
- Yaakov Kraftmakher, "[http://www.iop.org/EJ/abstract/0143-0807/22/5/302 Two experiments with rotating magnetic field]". 2001 Eur. J. Phys. 22 477-482.
- Bogdan Mielnik and David J. Fernández C., "[http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JMAPAQ000030000002000537000001&idtype=cvips&gifs=yes An electron trapped in a rotating magnetic field]". Journal of Mathematical Physics, February 1989, Volume 30, Issue 2, pp. 537-549.
- Sonia Melle, Miguel A. Rubio and Gerald G. Fuller "[http://prola.aps.org/abstract/PRE/v61/i4/p4111_1 Structure and dynamics of magnetorheological fluids in rotating magnetic fields]". Phys. Rev. E 61, 4111–4117 (2000). Category:Magnetism Category:Physical quantity Category:Introductory physics ja:磁場 th:สนามแม่เหล็ก

Category:Magnetic devices

This category includes device which use magnetism, but which are not :Category:Electromagnetic components, e.g. not circuit elements. Category:Magnetism

Anza Borrego

Anza-Borrego Desert State Park in southern California is the largest state park in the contiguous United States. The park is located on the eastern side of San Diego County, with portions extending east into Imperial County and north into Riverside County. It is about a two-hour drive from San Diego, Riverside and Palm Springs. The park is named after Spanish explorer Juan Bautista de Anza and the Spanish word borrego, or Bighorn Sheep. 500 miles of dirt roads, 12 wilderness areas and miles of hiking trails provide visitors with an opportunity to experience the wonders of the Colorado Desert. The park features washes, wildflowers, palm groves, cacti, ocotillo and sweeping vistas. Visitors may also have the chance to see greater roadrunner, golden eagles, kit foxes, mule deer and bighorn sheep as well as iguanas, chuckwallas and the red diamond rattlesnake. Listening devices for the hearing impaired are available in the visitor center. Most visitors approach from the east via California Highways S22, S2, or 78. Visitors from San Diego via Highways 79 and 78 have the added pleasure of driving through the mountainous Cuyamaca Rancho State Park--quite a different experience from Anza-Borrego. The highways from the east climb to 2,400 feet or so and then descend about 2,000 feet to the valley. Where the highway breaks out of the high-country vegetation, it reveals the great bowl of the Anza-Borrego desert. The valley spreads below, and there are mountains all around. The highest are to the north--the Santa Rosa Mountains. The mountains are a wilderness, with no paved roads in or out or through. They have the only all-year-flowing watercourse in the park. They are the home of the peninsular bighorn sheep, often called the Desert Bighorn. Few park visitors ever see them; the sheep are justly wary. A patient few observers each year see and count them, to learn how this endangered species is coping with human encroachment.

External link

- [http://www.terragalleria.com/california/california.anza-borrego.html Photos of Anza-Borrego Desert State Park - Terra Galleria]
- [http://www.anzaborrego.statepark.org/ Official park website] Category:California state parks Category:San Diego County, California

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