Magnaflux's Pie Gauge Magnetic Field Indicator
The magnetic particle Pie Gauge is used as an aid for determining the direction of magnetic fields for the detection of discontinuities in ferrous metals. Two major advantages for investing in a pie gauge are that they're easy to use and they can be used indefinitely without deterioration.
The gauge is made of highly permeable material (it's always a good idea to demagnetize it after being used) and is divided into eight even sections by non-ferromagnetic material. The section dividers act as artificial defects that will collect magnetic particles once magnetized. The resulting magnetic particle buildup will illustrate the magnetic field's orientation.