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A search coil is an apparatus for measuring eye movement using coils 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 fields 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 The coil is placed in the magnetic field to be measured and quickly withdrawn to a region of space with a negligible magnetic field.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.
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