 d

The sketches assume the car can be approximated by a compact magnetic dipole whose induced magnetic moment is parallel to the local geomagnetic field. The host contributes negligible local contrast. Real steel also has remanent magnetization and shape-dependent magnetization, so the car's actual profile need not follow this idealization. A proton-precession magnetometer observeswhen $|\mathbf{b}|\ll |{\mathbf{B}}_0|$. This scalar total-field magnetic anomaly must not be confused with a fixed vertical field component. For induced $\mathbf{m}=m{\hat{\mathbf{B}}}_0$, the magnetic dipole field givesReversing both the inducing field and induced moment leaves this total-field magnetic anomaly unchanged, exactly for the magnitude definition as well as in the weak-anomaly approximation.^[ref]