Magnetic Couple Measuring
Below is a blurry picture of the iron core I made for the magnetic encoder
Next, the edges were wrapped with Scotch tape to avoid shorting the windings on the metal core.
Next, 75 turns of fine magnet wire were tightly wrapped around the iron core.
It looks like I could have done 150 or even 200 turns and that would increase both the range and the sensitivity.
The extra long leads are needed to perform strain relief during operation.
They exit the brush head and wind in a circular fashion around it much like a watch spring. As a result, no commutator is needed making the construction fast, easy and inexpensive.
Here you see the brush with the center cut out to allow installation and gluing of the armature.
Below, the armature is inserted in the fresh silicone rubber and held until dry in the vise.
Testing resulted in a range of 1-25 milivolts depending on how thick the wall was with the 1 milivolt reading occurring at 1/2".