As was mentioned in Part 2, when there is a defect in a pipe, the applied magnetic field leaks around that defect; and the increased magnetic field is picked up in a sensor – a Hall Effect sensor (refer to the Figure above). Here is the basic definition of a Hall Effect sensor.
Hall Effect Sensors consist basically of a thin piece of rectangular p-type semiconductor material such as gallium arsenide (GaAs), indium antimonide (InSb) or indium arsenide (InAs) with current passing through it (see Figure below). When the device is placed within a magnetic field, the magnetic flux lines exert a force on the semiconductor material which deflects charge to either side of the semiconductor slab. This movement of charge is a result of force exerted by the magnetic field.
When the magnetic field lines move around the defected pipe surface, they flow through the Hall Sensor and cause the charged particles in the semiconductor to migrate to one side of the rectangle. This migration creates a voltage, the Hall Voltage; and thus, a signal is received that an anomaly exists in the pipe structure.
The reason the charged particles are deflected is based in the physics between current and magnetic field. The magnetic field, B; the velocity of the current (or charge, q), v; and the magnetic force, F; are somewhat orthogonal to each other. Please refer to the Figure below.
Although orthogonal is perhaps not the right word, let us just say that where ever v and B form a surface or plane, then F will be perpendicular to that plane. Thus, as more B flows though the Hall Sensor, then more voltage is generated within the sensor, and that indicates a pipe defect.