If the same amperage is used on two bars of different diameters, what is true about the magnetic field strength at the surface?

When two bars of different diameters are subjected to the same amperage for magnetic particle testing, the relationship between the diameter of the bars and the magnetic field strength is critical to understand. The magnetic field strength (often denoted as H) is proportional to the current and inversely proportional to the cross-sectional area of the material being magnetized.

In the case of bars with different diameters, the smaller diameter bar (25 mm) will have a smaller cross-sectional area compared to the larger bar (51 mm). This means that when the same current is applied, the magnetic field strength generated at the surface of the smaller bar will be relatively higher. Specifically, since field strength is impacted by how much current is concentrated in a given area, the 25 mm bar can be expected to exhibit a field strength that is significantly greater than that of the 51 mm bar.

Therefore, the statement that the field of the 25 mm bar will be approximately twice that of the 51 mm bar reflects the mathematical relationship between the current density and the magnetic field generated, given the size differences in the two bars. This highlights the principle that a smaller diameter leads to a higher field strength under the same amperage conditions.