The standard ampere is most accurately realized using a watt balance, but is in practice maintained via Ohm's Law from the units of electromotive force and resistance, the volt and the ohm, since the latter two can be tied to physical phenomena that are relatively easy to reproduce, the Josephson junction and the quantum Hall effect, respectively.

In a similar way, accurate AC / DC non-contact ammeters have been constructed using Hall effect magnetic field sensors.

This phenomenon arising due to the nature of charge carriers in the conductor came to be known as the Hall effect, but it was not properly explained at the time, since the electron was experimentally discovered 18 years later.

The quantum Hall effect was discovered by Klaus von Klitzing in 1980 when he observed the Hall conductivity to be integer multiples of a fundamental constant.

Shortly after, in 1982, Störmer and Tsui observed the fractional quantum Hall effect where the conductivity was now a rational multiple of a constant.

The study of topological properties of the fractional Hall effect remains an active field of research.

Devices used for this include Hall effect sensors, current clamps, current transformers, and Rogowski coils.

* Hall effect current sensor transducers

In some contexts it is meaningful to speak of fractions of a charge ; for example in the charging of a capacitor, or in the fractional quantum Hall effect.

The quasiparticles of the fractional quantum Hall effect are also known as composite fermions, which are electrons with an even number of quantized vortices attached to them.

The Hall effect is the production of a voltage difference ( the Hall voltage ) across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current.

The Hall effect was discovered in 1879 by Edwin Herbert Hall while he was working on his doctoral degree at Johns Hopkins University in Baltimore, Maryland.

The Hall effect is due to the nature of the current in a conductor.

This cannot explain the opposite sign of the Hall effect observed.

As a result, the Hall effect is very useful as a means to measure either the carrier density or the magnetic field.

One very important feature of the Hall effect is that it differentiates between positive charges moving in one direction and negative charges moving in the opposite.

The Hall effect offered the first real proof that electric currents in metals are carried by moving electrons, not by protons.

The Hall effect also showed that in some substances ( especially p-type semiconductors ), it is more appropriate to think of the current as positive " holes " moving rather than negative electrons.

The sample inhomogeneity might result in spurious sign of the Hall effect, even in ideal van der Pauw configuration of electrodes.

For example, positive Hall effect was observed in evidently n-type semiconductors.

The Hall effect in an ionized gas ( plasma ) is significantly different from the Hall effect in solids ( where the Hall parameter is always very inferior to unity ).

Many devices now sold as Hall effect sensors in fact contain both the sensor as described above plus a high gain integrated circuit ( IC ) amplifier in a single package.

Hall effect sensors are readily available from a number of different manufacturers, and may be used in various sensors such as rotating speed sensors ( bicycle wheels, gear-teeth, automotive speedometers, electronic ignition systems ), fluid flow sensors, current sensors, and pressure sensors.

Hall sensors can detect stray magnetic fields easily, including that of Earth, so they work well as electronic compasses: but this also means that such stray fields can hinder accurate measurements of small magnetic fields.

To solve this problem, Hall sensors are often integrated with magnetic shielding of some kind.

Many automotive Hall effect sensors have a built-in internal NPN transistor with an open collector and grounded emitter, meaning that rather than a voltage being produced at the Hall sensor signal output wire, the transistor is turned on providing a circuit to ground through the signal output wire.

Some types of brushless DC electric motors use Hall effect sensors to detect the position of the rotor and feed that information to the motor controller.

Hall effect keyboards use magnets and Hall effect sensors instead of an actual switch.

The most common magnetic sensing devices are solid-state Hall effect sensors.

Common types include opto-isolator slotted disk sensors and Hall effect sensors.

Hall effect sensors typically use a rotating target attached to a wheel, gearbox or motor.

The magnetic piston ( 1 ) in this pneumatic cylinder will cause the Hall effect sensors ( 2 and 3 ) mounted on its outer wall to activate when it is fully retracted or extended.

Hall effect sensors are used for proximity switching, positioning, speed detection, and current sensing applications.

Hall sensors are commonly used to time the speed of wheels and shafts, such as for internal combustion engine ignition timing, tachometers and anti-lock braking systems.

Hall effect sensors may require analog circuitry to be interfaced to microprocessors.

Photos of Conference Rooms and the General Assembly Hall can be made when these rooms are not being used for meetings.

" The Covered Hall ") This abode is somewhat like Hades from Ancient Greek religion: there, something not unlike the Asphodel Meadows can be found, and people who have neither excelled in that which is good nor excelled in that which is bad can expect to go there after they die and be reunited with their loved ones.

According to science and technology historian Bert S. Hall: " It goes without saying, however, that historians bent on special pleading, or simply with axes of their own to grind, can find rich material in these terminological thickets.

One can walk towards Galileo Hall and see the warts ( especially those near the entrances of buildings ) being used as racks for unicycles and skateboards.

In a plasma, the Hall parameter can take any value.

For example, a Hall sensor integrated into a ferrite ring ( as shown ) can reduce the detection of stray fields by a factor of 100 or better ( as the external magnetic fields cancel across the ring, giving no residual magnetic flux ).

Hall effect devices used in motion sensing and motion limit switches can offer enhanced reliability in extreme environments.

Due to building damage received during Hurricane Katrina the Hall of Fame is temporarily located at Gate B in the New Orleans Superdome and can be visited for free at every Saints home game.

Fans can view videotapes on Saints history and the Saints Hall of Famers as well participate in interactive exhibits throughout the Hall.

Cohodas Hall can be seen in the background.

Other examples of storytelling voice-overs can be heard in Annie Hall, Gattaca, Fight Club, Megamind, Ratatouille, Spirit: Stallion of the Cimarron, Tangled, The Magic School Bus, The Emperor's New Groove, Kronk's New Groove, Blade Runner, The Rugrats Movie, The Shawshank Redemption, Big Fish, How to Train Your Dragon, Moulin Rouge !, The Postman Always Rings Twice, Young Winston, Rugrats, Raising Arizona, Goodfellas, Clash of the Titans, and Star Quest: The Odyssey.

The cylindrical Hall thruster can be more readily scaled to smaller sizes due to its nonconventional discharge-chamber geometry and associated magnetic field profile.

While the value of α can be estimated from the values of the constants appearing in any of its definitions, the theory of quantum electrodynamics ( QED ) provides a way to measure α directly using the quantum Hall effect or the anomalous magnetic moment of the electron.

An inscription can be found on the wall of University College, Oxford the High Street at Oxford ( now the location of the Shelley Memorial ), marking the spot where Cross Hall stood until the early 19th century.

Examples can be found today in the buildings surrounding Piazza Spalato ( today Piazza Insurrezione ), the railway station, the new part of City Hall, and part of the Bo Palace hosting the University.

The integer quantum Hall effect is very well understood, and can be simply explained in terms of single-particle orbitals of an electron in a magnetic field ( see Landau quantization ).

Most integer quantum Hall experiments are now performed on gallium arsenide heterostructures, although many other semiconductor materials can be used.

It was later acknowledged to have come from a January, 1970 concert at the Royal Albert Hall, with the guitar parts overdubbed and the original guitar part removed — this can be heard in the original Royal Albert Hall show on 9 January 1970.