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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Hello again,
I have a magnetic sensor that prefers 30 to 70mT.
I'm trying to figure out an approximation for this since I'm using a 0.020" thin flexible magnet sheet. ill use two ultra-thin foils of steel to tighten the field since I need it to be a narrow long linear field. The plates will be north and south, not across as if it were a 'keeper'.
So, the question is: can I adjust the height and width (cant change thickness) to increase or decrease strength ? And is there a device or method I can build to measure strength ?
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
OK, only took 6 tries to get the captcha.
First you need to consider the way magnetic poles are placed on the flexible magnet sheet. On the back side (that sticks to refrigerator door) are parallel stripes alternating North and South poles. Simple way to check this, and determine the pole spacing:
Stick two magnets together face to face, and gently slide one with respect to the other. If the magnet stripes are at right angles to each other, the sliding friction is pretty smooth in all directions of motion. If the magnet stripes are parallel, then sliding in that direction is smooth. Sliding perpendicular to the stripes you will feel a cogging effect, for reasons that should be obvious.
Next step is to determine the strength. Flux density is highest when magnet sheet is stuck on a steel surface that's not too thin. If you can't get a figure in gausses or teslas from the Internet, we can figure it out from basic principles. For example, measure (or look up) the lifting strength per square inch when magnet is kept flat and the force is perpendicular to the interface plane.
I use an analog circuit based on a 100 - 0 - 100 uA moving coil meter for 100 - 0 - 100 mT using battery, resistors and two potentiometers (zero offset and gain), no electronics required. An analogue meter seems better suited to visualising a magnetic field than a digital display. _________________________________________
_________________________
Calibration is not easy as making a known flux density reference field is non-trivial;
Helmholtz coils seemed too much of a project,
nickel saturates with a fairly flat B-H curve to give a maximum flux density of close to 500mT so used in a magnetic circuit it can be used to create a fairly well defined magnetic field strength,
two magnets can be used to create a moderately accurate flux density _________________________________________________
__________ something I've considered but not tried;
A two-pin led with bi-polar leds (e.g. red and green) attached to the probe and driven by the meter circuitry would give a rough visual indication of field strength and polarity.
Carefully moving/scanning the probe around, in the dark, should produce an image of the magnetic field in a camera with long time exposure settings.
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Strength of Magnetism
