Wednesday, October 03, 2007

A non-scientific experiment...

(Follow this LINK for a detailed discussion of magnetic loop detection systems and bicycle.)

Last week, Gary Parker and I discussed an idea for tripping magnetic loop signals. These are common around the Tulsa area. There are several loops buried under the pavement, and Gary came across an idea for placing several magnets on the underside of the bottom bracket. Supposedly the additional magnetic field helps to trigger the detector.

So today, I salvaged three small rare earth magnets from some defunct headsets, attached them to some double-sided adhesive tape, and applied them to the bottom of my left cycling shoe. Yes, I'm a left footer.

There's a double loop detector at our main gate. A double loop looks like a big 0 with a vertical line up the middle. The most sensitive portion is that mid-line. Normally, I position the bike directly above it and the light triggers. But today, I stopped a bit to the right and placed my left foot with the magnets directly onto the center line.

It never triggered, leading me to conclude that small magnets have little effect even in close proximity to the loop. A magnet attached to the bottom bracket would have to be much more powerful due to the rapid weakening of the magnetic field. It decreases by the square of the distance according to the field equations.

But there's another idea worth trying, and that's a shorted coil. I may do that tomorrow if time permits.

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Blogger Thomas said...

Wow what a cool idea! ... What the Hill is a magnetic loop signal? And what have they ever done to you?

7:47 AM  
Blogger Fritz said...

I've thought about trying similar experiments with the magnets advertised in motorcycle magazines.

Magnetic field diminishes with the inverse cube with distance, BTW, not square.

12:47 PM  

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