How Is the Surface Field Measured?
The surface field is measured with a gaussmeter: a meter with a thin Hall-effect probe that’s placed flat against the center of a pole face. The reading is the magnetic flux density at that spot, in gauss (G) or millitesla (mT); 10 G = 1 mT. A higher reading means a more intense field at the surface, but gauss is not the same as pull force.
How to Take a Good Reading
- Zero the meter away from any magnets.
- Hold the probe flat against the center of the pole face. The field varies across the face, so position matters (see below).
- Keep the probe square to the surface; tilting lowers the reading.
- Record the reading and where you took it, and take several readings at the same spot to compare magnets fairly.
Why the Center Isn’t Always the Highest Reading
On thin discs and blocks, the field is stronger toward the edges than at the center. If you sweep the probe across the face, you’ll often find a higher number just inside the rim of a disc or near the corners of a block. Past the edge, the field bends sideways, so an axial probe’s reading drops off quickly and can even reverse.
Some suppliers quote that edge peak, which makes their magnets look stronger on paper than an identical magnet measured at the center. The difference can be large: on the 1/2 × 1/8 Inch disc above, the peak reads about 15% higher than the center, and near a corner of a thin 1 × 1 × 1/8 Inch block the reading is about twice the center’s. When you compare gauss figures, check where they were measured.
Thicker magnets behave differently. When a disc’s thickness is about half its diameter, the reading is nearly even across most of the face, and on a cylinder at least as long as it is wide, the center reads highest.
Rings are the exception. There’s no material at the center, so measure on the face partway between the inner and outer edge.
Gauss Isn’t Pull Force
Surface field depends mostly on a magnet’s shape (thickness compared with width), while pull force depends on size. Calculated center-of-face readings for our N52 discs show this clearly:
| N52 disc | Surface field | Pull force |
|---|---|---|
| 1/4 × 1/8 Inch | about 5,130 G | 2.9 lb |
| 1/2 × 1/4 Inch | about 5,130 G | 13.55 lb |
| 1/2 × 1/8 Inch | about 3,240 G | 8.15 lb |
| 1 × 1/4 Inch | about 3,240 G | 33.7 lb |
| 1 × 1/2 Inch | about 5,130 G | 54.5 lb |
The 1/4 × 1/8 Inch and 1 × 1/2 Inch discs have about the same surface field, yet the larger one pulls nearly 19 times harder. To compare holding power, use pull force. More on gauss vs. pull force.
Typical Field Strengths for Scale
| Source | Field |
|---|---|
| Earth’s magnetic field | 0.25 to 0.65 G |
| Flexible fridge magnet | about 50 G |
| Neodymium disc, at the pole face | about 2,000 to 6,500 G |
| N52 material (remanence, Br) | 14,300 to 14,800 G |
| Hospital MRI scanner (1.5 to 3 T) | 15,000 to 30,000 G |
About the Calculated Values
Surface field values above are calculated at the center of the pole face for an N52 disc (Br about 14.5 kG). Measured readings vary with probe size and position.