The search coil magnetometer or induction magnetometer, based on an
inductive sensor
An inductive sensor is a device that uses the principle of electromagnetic induction to detect or measure objects. An inductor develops a magnetic field when a current flows through it; alternatively, a current will flow through a circuit contain ...
(also known as
inductive loop
An induction or inductive loop is an electromagnetic communication or detection system which uses a moving magnet or an alternating current to induce an electric current in a nearby wire. Induction loops are used for transmission and reception of ...
and inductive coil), is a
magnetometer
A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, o ...
which measures the varying
magnetic flux
In physics, specifically electromagnetism, the magnetic flux through a surface is the surface integral of the normal component of the magnetic field B over that surface. It is usually denoted or . The SI unit of magnetic flux is the weber ( ...
. An inductive sensor connected to a conditioning
electronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow. It is a type of electrical ...
constitutes a search coil magnetometer. It is a
vector magnetometer
A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, on ...
which can measure one or more components of the magnetic field. A classical configuration uses three orthogonal inductive sensors. The search-coil magnetometer can measure magnetic field from mHz up to hundreds of MHz.
Principle
The inductive sensor is based on
Faraday's law of induction
Faraday's law of induction (briefly, Faraday's law) is a basic law of electromagnetism predicting how a magnetic field will interact with an electric circuit to produce an electromotive force (emf)—a phenomenon known as electromagnetic inducti ...
. The temporal variation of the
Magnetic Flux
In physics, specifically electromagnetism, the magnetic flux through a surface is the surface integral of the normal component of the magnetic field B over that surface. It is usually denoted or . The SI unit of magnetic flux is the weber ( ...
through a ''N'' turns circuit will induce a voltage ''
'' which follows
:
which can be expressed in a simpler way
:
by assuming that the induced magnetic field B is homogeneous over a section ''S'' (the
Magnetic flux
In physics, specifically electromagnetism, the magnetic flux through a surface is the surface integral of the normal component of the magnetic field B over that surface. It is usually denoted or . The SI unit of magnetic flux is the weber ( ...
will be expressed
).
The induced voltage (''
'') may be increased several ways:
* increase the surface (''S''),
* increase the turn number (''N''),
* use a ferromagnetic core.
Search coil using a ferromagnetic core
When a coil is wound around a
ferromagnetic
Ferromagnetism is a property of certain materials (such as iron) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...
core, that increases the sensitivity of the sensor thanks to the apparent permeability of the ferromagnetic core.
Apparent permeability
The magnetic amplification, known as apparent permeability ''
'', is the result of the
magnetization
In classical electromagnetism, magnetization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material. Movement within this field is described by direction and is either Axial or Di ...
of the ferromagnetic core response to an external magnetic field. The magnetization is reduced by the demagnetizing field.
:
where ''
'' is the relative permeability, ''
'' is the demagnetizing coefficient in the ''z'' direction.
The induced voltage will be written
:
The demagnetizing coefficient can easily be computed in the case of simple shapes (spheres and ellipsoids).
Applications
*
Eye tracker: a search coil is used to measure
eye movement using coils that are embedded into a contact lens.
* Education
*
Non Destructive Testing
*
Magnetotellurics
*
Space research
* Natural electromagnetic waves observations on Earth.
References
{{reflist
* Pavel Ripka, ''Magnetic Sensors and Magnetometers'', Artech House Publishers
S. Tumanski, ''Induction Coil Sensors - a Review''
See also
*
Waves (Juno) (Uses a magnetic search coil)
Vision
Magnetometers