Alternating current field measurement (ACFM) has been proven to be a promising technique for detecting and sizing surface cracks using an induced current. Due to the confined induced current region, the induced current cannot bypass long and narrow transverse cracks on a pipe string, which makes the conventional ACFM method insensitive to transverse cracks. In this paper, a broad full 360º circumferential current induced by a novel feed-through ACFM probe is presented for detecting transverse cracks on a pipe string effectively. The distortion of the circumferential current and characteristic signals of a transverse crack are analysed by both simulations and experiments. Our results show that the induced circumferential current is disturbed by a transverse crack on the surface of the pipe string. Characteristic signals obtained from the distorted space magnetic field associated with the disturbed circumferential current can be used to evaluate a small opening transverse crack. The results indicate that the induced circumferential current can be employed to detect small opening transverse cracks on a pipe string effectively.