Copyright Goodheart-Willcox Co., Inc. 104 Electricity Copyright Goodheart-Willcox Co., Inc. However, with the use of magnetic shielding, magnetism can be led around or directed away from instruments or devices needing protection. By placing a magnetic material such as a piece of iron in the fi eld of a magnet, the fl ux lines of magnetism follow the direction of the iron, because the iron has a greater permeability than the surrounding air. The magnetic fi eld actually becomes a little stronger by this increased permeability. The fi eld now follows the iron and so is kept away from a less desirable area of the circuit. Meters, transformers, and other components are frequently placed in metal cans that effectively shield them against the infl uence of magnetism. In radio equipment, metal fences or partitions are sometimes placed between sections of a circuit to prevent interaction between the circuits. In all communications equipment, shielding is important to ensure the best operating results. S N Current flow DC power Hall voltage Goodheart-Willcox Publisher Figure 12-15. This non-contact Hall effect sensor can be mounted near a conveyor, rotation shaft, or the like. Each time a magnetic fi eld passes near it, a voltage pulse is generated and that pulse can be used for counting purposes. N S S Magnet Nail N Goodheart-Willcox Publisher Figure 12-16. The nail becomes a magnet by induction. Goodheart-Willcox Publisher Figure 12-17. Magnetic induction.