Squid is an abbreviated form of superconducting quantum interference device. It is a very sensitive instrument for measuring even very small changes in the magnetic flux.

Construction

Squid consists of superconducting ring with two side arms P & Q . X & Y are two very thin insulating junctions (Josephson Junctions). Magnetic flux is applied perpendicular to it.

Working

        It is working on the principle of Josephson effect. Supercurrent flowing through P is branching into two and is passing through the insulating junctions. The supercurrents emerging from the junctions combine together at Q. This supercurrent changes periodically with the change in magnetic flux. The dependence of the supercurrent on the magnetic field through the circular loop looks like an interference pattern as in the fig. This is due to the interference of wave functions of Cooper pairs. Since the magnetic flux is quantized, current changes in a discontinuous manner. Even a very small change in magnetic field of 10^-21 can be detected accurately.

Applications

1)    It s used as a sensitive magnetometer by which e can measure a very small change in magnetic field very accurately.

2)   Used in neurophysics in connection with the change in magnetic field of neurons.

3)   Used to sense weak magnetic fields produced by biological currents in brain.

4)   Used to detect small disturbances in the earth’s magnetic field when a submarine approaches a place.

5)   This confirms tunneling property of electrons.