Measuring magnetism

In the last diary entry, I mentioned I use two types of rotor: the standard magnetised Type 2 and the more highly magnetised Type 2+. 

Thoughts which have long intrigued me have been: how much more magnetised is the Type 2+ rotor, - and as a separate question, - are the magnets in the 14 'magnet tiles', 4 magnets per 'tile', - are they each of equal strength.

Underlying these thoughts has been the basic question: is it possible to measure the strength of the magnets.

In this blog I want to show how I devised a way to make some measurements using the force generated on a soft iron cylinder held a constant distance away from each magnet. 

The apparatus uses the technology from a kitchen scales of the digital type, the working principle of which is a strain gauge Wheatstone bridge.

The finished test rig looked like this; the force displayed is in grams:

The detail of the 'binocular cantilever strain gauge' looks like this:

The schematic representation of the arrangement looks like this:

The deformation on applying load looks like this:

The wiring diagram looks like this:

The voltage change across the Wheatstone bridge, created by tension and compression of the four strain gauges, is very small; it needs to be amplified and processed for display in the LCD screen as grams weight.

Conclusion:

What did I learn from this little experiment: in truth not much ! The individual magnets of each kind of rotor all seemed to be equally magnetised; the force exerted by each of the Type 2 magnets was, as seen in the picture, around 424 grams whilst for Type 2 + magnets it was around 200 grams more.

One limitation of the system was found to be that the rotors are not precisely circular; this had the effect of reducing the air gap in one place and widening it in another; since the 'pull' exerted on the soft iron cylinder is very dependent on the distance from the surface of the magnet, this limited the reliability and precision of the experiment.

But it was a fun thing to do, - and I had had the 'guts' of the kitchen scales sitting in a drawer for over 10 years awaiting some useful purpose. It was nice finally to discover why I had been keeping it all those years !