Ι What Is Surface Gauss?

When we consider the strength of a magnet, the terminology can often be confusing as several comparative measurement methods attribute to a magnet’s “strength” in different ways. Broadly speaking, both magnetic flux density and pull force are often summarized as strength. Magnetic flux density—most commonly known as “surface gauss”, but also referred to as “surface field”, “B-field” or “magnetic induction”—is measured in Tesla (the SI unit) or Gauss, where 1 Tesla equals 10,000 Gauss.

Although it is regularly misleadingly referred to as “magnetic field strength”, magnetic flux density is technically defined as the total number of magnetic field lines in a given area. To be precise, one Gauss is defined as being one Maxwell of magnetic flux in a square centimeter of surface area.

It is crucial to distinguish between the material’s intrinsic data and the measurable surface gauss. Remanence is the magnetic flux density held by the magnet whilst it is in a closed magnetic circuit, such as when being tested in a Hysteresisgraph. However, when the magnet is removed from the Hysteresisgraph, it is no longer in a closed loop and is considered to be in an open magnetic circuit. The magnetic flux density in this open circuit condition is what we measure using a Gauss meter and Hall probe. It is precisely in this open circuit state where the distribution of magnetic flux becomes non-uniform, leading to the variations we observe in practical measurement.

The Phenomenon: Strong Edges, "Weak" Center

If you measure a standard magnet in an open circuit, you might notice a puzzling phenomenon. When you move the probe from the geometric center to the edge, the surface gauss reading often jumps significantly.

For a standard block or disc magnet, the surface gauss distribution is never uniform across the pole face:

The Edges & Corners: exhibit the highest surface gauss.
The Geometric Center: exhibits the lowest surface gauss on the pole face.

Consequently, the surface gauss of the geometric center is invariably used as the standard reference point in comparative measurement methods. This is not because it is easy to measure, but because it offers the highest probability of repeatability. Compared to the edges, where values fluctuate wildly with even minimal movement, the center’s relatively flatter gradient makes it the most consistent reference point for routine quality control checks.

The Physics: Understanding the Demagnetizing Field

To understand why surface gauss is distributed this way, we must look at the behavior of magnetic flux lines and the internal forces at play.

The “Crowded Exit” (Flux Concentration)

Imagine the magnetic flux lines trying to travel from the North pole to the South pole. Flux lines always seek the path of least resistance (lowest reluctance).

At the Edges: The path through the air to the opposite pole is shorter and more direct. Consequently, a large volume of flux lines “crowds” towards the edges to exit, resulting in a higher density (surface gauss).
At the Center: The flux lines must travel a longer distance through the air to loop back to the opposite pole.

The Internal Cause (Demagnetizing Field)

This distribution is physically governed by the demagnetizing field.

Definition: The demagnetizing field, also called the stray field (outside the magnet), is the magnetic field (H-field) generated by the magnetization in a magnet.

As shown in the $\vec{H}$ -field diagram above, inside the magnet, this field runs in the opposite direction to the magnetization ( $\vec{M}$ ), effectively trying to “cancel out” the magnet’s strength from the inside.

At the Geometric Center: This internal opposing force is at its strongest, effectively suppressing the net flux output.
At the Edges: This opposing force is weaker, allowing the flux lines to concentrate and exit with higher intensity.