At 01:19 AM 5/16/2010, you wrote:
>Hi Spehro,
>
>When I mentioned low mass, I was referring strictly to a compass like
>device, that would indicate a slightly different direction as the
>large steel vehicle interacted with the Earth's magnetic field. Since
>the Earth's magnetic field is relatively weak, I was trying to say
>that the rotating mass should be kept as low as possible in order to
>allow the needle in the compass to move enough in the short time
>periods that it would take the vehicle to move past the detector.
>
>The sensor can weigh 10 pounds, but the internal rotating mass of the
>magnet in the compass should be kept low.
>
>Regards,
>
>Art

It's possible to use local distortions in the earth's magnetic field caused
by the presence of a nearby ferromagnetic object to passively detect
vehicles with sufficient magnetic moment, but the sensitivity falls off
with the distance^3, IIRC.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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