Search coil
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
A search coil is a device for measuring eye movements. It uses coils that are embedded into a tightly-fitting contact lens or a rubber ring that adheres to the eye. An alternating magnetic field with horizontal and vertical components is used to induce electric currents in these coils. These currents depend on the eye position (i.e. on the angle by which the eye is rotated) and thus provide a measure of the eye position.
A more crude search 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 The coil is placed in the magnetic field to be measured and quickly withdrawn to a region of space with a negligible magnetic field.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 <math>\ \mathbf{B}</math>. The search coil can be calibrated by repeating this in a known magnetic field.