Search coil
Encyclopedia
A search coil is a device for measuring eye movement using coil
s that are embedded into a tightly-fitting contact lens or a rubber ring that adheres to the eye
. In animal studies, the search coil may be surgically implanted into the sclera
of the eye. Alternating magnetic field
s are generated by magnets positioned around the eye. Through electromagnetic induction
, electric currents are generated in the search coils. The polarity and amplitude of the current generated varies with the direction and angular displacement
of the eye. By measuring these values, the position of the eye can be determined. Search coils can be applied to one or both eyes.
In order to detect eye orientation in more than one dimension (e.g. up/down vs. left/right), multiple magnets oriented orthogonally to each other may be used. Each magnet generates a field using a different frequency, allowing the readings from the search coil to be analyzed by computer to determine displacement in multiple dimensions. Additionally, a second search coil can be added to measure torsional
rotation.
A more crude search coil also called exploring coil is also used in laboratory experiments in schools to measure the magnetic field
in a certain region of space. In this case the search coil consists of a simple wire coil or solenoid connected to a sensitive ammeter
or galvanometer
. The coil is placed in the magnetic field to be measured and quickly withdrawn to a region of space with a negligible magnetic field. As the search coil moves the magnetic flux
linked with the coil changes. This induces a current in the coil which can be registered on the galvanometer
. Since induced current is directly proportional to rate of change of flux linkage and assuming the coil is removed from the magnetic field very quickly, the maximum current measured by the ammeter is proportional to the magnetic field
. The search coil can be calibrated by repeating this in a known magnetic field.
A reference table shows open coil voltage versus magnetic field A/m (or teslas) or short current versus magnetic field.
Coil
A coil is a series of loops. A coiled coil is a structure in which the coil itself is in turn also looping.-Electromagnetic coils:An electromagnetic coil is formed when a conductor is wound around a core or form to create an inductor or electromagnet...
s that are embedded into a tightly-fitting contact lens or a rubber ring that adheres to the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
. In animal studies, the search coil may be surgically implanted into the sclera
Sclera
The sclera , also known as the white or white of the eye, is the opaque , fibrous, protective, outer layer of the eye containing collagen and elastic fiber. In the development of the embryo, the sclera is derived from the neural crest...
of the eye. Alternating magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
s are generated by magnets positioned around the eye. Through electromagnetic induction
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....
, electric currents are generated in the search coils. The polarity and amplitude of the current generated varies with the direction and angular displacement
Angular displacement
Angular displacement of a body is the angle in radians through which a point or line has been rotated in a specified sense about a specified axis....
of the eye. By measuring these values, the position of the eye can be determined. Search coils can be applied to one or both eyes.
In order to detect eye orientation in more than one dimension (e.g. up/down vs. left/right), multiple magnets oriented orthogonally to each other may be used. Each magnet generates a field using a different frequency, allowing the readings from the search coil to be analyzed by computer to determine displacement in multiple dimensions. Additionally, a second search coil can be added to measure torsional
Torsion tensor
In differential geometry, the notion of torsion is a manner of characterizing a twist or screw of a moving frame around a curve. The torsion of a curve, as it appears in the Frenet-Serret formulas, for instance, quantifies the twist of a curve about its tangent vector as the curve evolves In the...
rotation.
A more crude search coil also called exploring coil is also used in laboratory experiments in schools to measure the magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
in a certain region of space. In this case the search coil consists of a simple wire coil or solenoid connected to a sensitive ammeter
Ammeter
An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes , hence the name. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters...
or galvanometer
Galvanometer
A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil in a magnetic field. .Galvanometers were...
. The coil is placed in the magnetic field to be measured and quickly withdrawn to a region of space with a negligible magnetic field. As the search coil moves the magnetic flux
Magnetic flux
Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...
linked with the coil changes. This induces a current in the coil which can be registered on the galvanometer
Galvanometer
A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil in a magnetic field. .Galvanometers were...
. Since induced current is directly proportional to rate of change of flux linkage and assuming the coil is removed from the magnetic field very quickly, the maximum current measured by the ammeter is proportional to the magnetic field
. The search coil can be calibrated by repeating this in a known magnetic field.A reference table shows open coil voltage versus magnetic field A/m (or teslas) or short current versus magnetic field.