Eddy Current Braking

Increasing the thickness of an aluminum plate run between magnets in an eddy brake system will increase resistance, but will increasing the volume of the material outside the eddy current (i.e. increasing the total diameter or thickness outside the area passing through the magnets) increase resistance?
- Chris (age 35)
Carlsbad, CA 92008

First, we should clarify for other readers, that increasing the amount of aluminum reduces the electrical resistance. That allows larger eddy currents to flow, increasing the braking power. 

In adding more aluminum, say by increasing the diameter, remember that the magnet fields spread out so that the extra aluminum may well be still in the weaker part of the fields even if it isn't right between the magnets. I think it will increase the braking, but I'm not sure. Adding aluminum outside the reach of the magnetic fields is trickier. It will allow larger eddy currents to flow, since it offers another electrical pathway. However, fields from the currents out  there won't exert forces on the magnets. (I think there's a reciprocity theorem that the places where the magnets exert fields are the same as the places where current loops would make fields that exert forces on the magnets.) So I'm not sure what effect that will have on the braking. Maybe some reader or colleague knows that, or you could try the experiment in a little model.

Mike W. 

(published on 02/18/2014)