A hairline crack was introduced in a 27.9 cm long by 7.62 cm wide by 1.3 cm thick (11 by 3 by 0.5 in.) 6061-4 aluminum specimen by repeated bending. The size of the crack was limited by stopping the bending at the initial stage of crack formation. Measurements were then made using an AC magnetic bridge which detected the presence of the crack and enabled characterization of its form. The extent, the orientation, and the depth from the surface along the crack length were obtained from the measurement. This paper outlines the principle of the bridge operation and details related to interpreting the output from the bridge as the bridge is moved over the surface of the aluminum specimen. Verification of the bridge measurements was made by destructive evaluation of the specimen using a scanning electron microscopic examination.