This is a very handy device used to indicate the strength of a magnet, especially when debugging problems with a magneto or charging a magnito's magnet.
This is a modern version of the old fashioned mechanical gauss meters that were used many years ago when the old gas engines and tractors from the early 1900's into the 60's were built. Some of those old magneto's sparks have gotten weak or non existent. This Gauss meter can help you determine whether this may be the fault of the magnet or the coil.
For Measuring the strength of a magnet
Shown here measuring the magnetic flux of a horseshoe magnet off an International magnito from a 1929 Farmall Regular Tractor.