Eddy-current sensor measures bolt loading

Thin wire welded to bottom of hole down center of bolt permits measurement of tension in bolt. Bolt lengthens under strain, but wire is not loaded, so gap between wire and eddy-current gap transducer mounted on bolt head indicates bolt loading. Eddy-current transducer could measure gap within 0.05 mm. Method does not require separate "standard" for each bolt type, and is not sensitive to dirt or oil in bolt hole, unlike ultrasonic probes.