The use of magnetic fields as a tool for control and measurement has important medical applications. Present devices include magnets to remove objects from the eye and to facilitate the withdrawal of swallowed safety pins and similar objects, ensuring that the sharp end points backwards. In more sophisticated applications magnetic materials are purposely introduced into the body to accomplish various tasks. Magnetically guided catheters have been used to negotiate the tortuous passageways of blood vessels. Ferrite powders may replace barium sulfate for gastrointestinal x‐ray diagnosis in some applications. Magnetic fields have also been used for various measurements, employing not only fields due to small amounts of material artificially introduced into the body but also the extremely weak fields produced by the electrical or electromagnetic activity of the body itself.