My last post mentioned flux meters and the kinds of measurements they can be used for. The most basic instrument for making point measurements of flux density (usually referred to as a Gauss meter) is probably more common. It is simple to use, but there are some measurement accuracy hazards to be aware of.
Gauss Meters
Gauss meters usually use a Hall effect probe which is only sensitive in one axis. Our instrument came with a transverse probe (a Hall effect chip on a narrow strip of insulating material about 4mm wide and 1mm thick), sensitive to the component of flux density orthogonal to the plane of the probe, and an axial probe. The axial probe is sensitive to the component of flux density parallel to the probe axis (the Hall chip is mounted at the end of a long cylinder).
Magnetic Measurement
Because the probe is (usually) only sensitive in one axis, in order to measure peak flux density, this axis needs to be aligned with the local field. This is easy enough if using the axial probe to measure inside a long solenoid, or the transverse probe to measure the flux density in the centre of a button magnet. In the latter case, if the probe is held flat against the magnet surface at the exact centre of the magnet then it will automatically be aligned with the field. Anywhere else and it won’t, even if it is still held flat on the magnet’s surface. The flux plot below illustrates that the flux lines are only orthogonal to the magnet surface at the centre.
It’s also worth noting just how rapidly both direction and magnitude change with position over the surface of the magnet, so in order for measurements to be really meaningful, it’s useful to know exactly where that tiny Hall chip is with respect to the physical outline of the probe.
