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How to easily, using standard DIY equipment measure the strength of magnetic field generated by a permanent magnet?

Narrowing down the "loose language" of the above:

strength of magnetic field: either flux density B at given point relative to the magnet or magnetic flux ΦB over area enclosed by a loop made of wire - whichever will be easier to measure, either of those is fine.

standard DIY equipment: commonly found household items, rudimentary tinkering tools. Soldering tools, multimeter, simple electronic parts, or maybe an easy to make spring-based dynamometer - anything of this class of complexity.

The distance of measurement is such that the field is easily noticeable through simplest methods e.g. another magnet held in hand exerts perceptible force - distance of maybe 5cm away at most.

The measurement doesn't need to be very accurate - error of order of 50% is quite acceptable. Simplicity is preferred over accuracy.

Rationale: trying to estimate what coil I need to generate sufficient amount of power to light a LED with a frictionless generator based on that magnet (knowing speed of movement of the magnet and location of the coil relative to the path of the magnet). If you know other simple methods of doing that (without need for measuring the field), they are most welcome them too.

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3 Answers

up vote 1 down vote accepted

For what it's worth: http://www.coolmagnetman.com/magmeter.htm - a home-made device based on a Hall effect device - for about $40.

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That's pretty nice - I'll accept if nobody comes up with something even more simple. –  SF. Dec 8 '12 at 21:08
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The easiest method is to put a magnetic compass on one of the magnet's axes of symmetry, and orient the compass and magnet such that the magnet's field is perpendicular to the earth's. Then the tangent of the deflection angle is equal to the ratio of the fields.

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That's nifty, although probably beter suited for weaker fields. Still - to know the absolute value of the field from the ratio I should know the local strength of earth's magnetic field. How would I go about finding it? –  SF. May 15 '13 at 7:44
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Check out this explanation of http://en.wikipedia.org/wiki/Magnetometer#Fluxgate_magnetometer :

https://www.youtube.com/watch?feature=player_detailpage&v=kmCvrGVtC0M#t=127s

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This needs a few words about the described device or it is just an outgoing like which is discouraged in answered. –  dmckee Dec 8 '12 at 18:44
    
While the idea behind the device is very neat, I'm not quite sure how to go about the driving circuit other than piecing something together on 555. Alternatively, pick a tiny transformer, run 230V AC through primary in line with some load, short the secondary circuit with a non-electrolitic capacitor and measure the voltage on the capacitor. Still, how to calibrate that? (at home!) –  SF. Dec 8 '12 at 21:15
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