vroom1776 wrote on 12/31/69 at 16:00:06:Alright don't want to get into a mess...
the equation that governs the change in current due to a change in magnetic filed is the the Faraday equation (the "curl" of electric filed = charge density + change in Magentic induction with time (more or less). The curl basically means what it sounds like... Basically this equation says that a changing magnetic field (in time) causes a voltage which in turn causes a current to flow if a wire is nearby. The change in current within the buried wire is what is measured and triggers the green light
A hall probe measures magnetic field and gives out a voltage due to the hall effect (see above). The greg described it above does not involve a hall probe... ugh. don't want to start a flame war...
Magnetism, electricity, and time (and space, and the weak force) are inextricably interwoven
Try this to see what the heck we are talking about: get a permanent magnet (NOT a refridgerator magnet), an insulated wire, and an amp meter (sometimes call ammeters (sp?) or galvanometer... a DMM should work too). Actually, a small light bulb should work too... Make the wire into a bunch of closely packed coils like a spring. Move the magnet back in forth in there and watch the light flash! Note: if it doesn't work, or barely works, rotate the magnet to a different side and try again... this is essentially how one generates electricity... spin a magnet inside a coil.
Believe it or not,
I got an "A" in my daughter's science project several years ago with this very same experiment.....
....to clarify what you mean. I though the "Hall Effect" was different than a "Hall Probe" as in the effect that a conducting material has on an existing magnetic field. We know that current flow in a wire, creates a magnetic field, but this is about how other objects effect that field when they pass through it....
In our case these roadway sensors are designed (and calibrated?) to sense cars with their big steel chassis and more powerful electrical systems, but bikes have a smaller "foot print" and are often made of materials that are non-magnetic...aluminum and plastic. So their effect is much less...below some transistor base, I guess.
Pages: 
