SQUID Magnetometer:

To minimize the noise, the gradiometric design of the sensing coil is proposed.

As shown in Figure 30.07, let us consider a first order gradiometer. In this case, a spatially uniform time variation of magnetic field would produce no net current in the coils as the contribution to the current given by the lower coil is canceled by the contribution from the upper coil, which is actually wound in the opposite direction.

Most of the magnetic measurements have been made up to now with first and second order gradiometers in shielded and non-shielded environments, respectively.

Figure 30.08: Schematic of second order gradiometer and sample movement.

Figure 30.08 shows a single piece of superconducting wire configured as a second-order gradiometer. In this geometry, the pick-up coil system is placed in the uniform magnetic field region of the solenoidal superconducting magnet.