Metal detecting. Why only down? The search coil shielded or the author too ignorant?

[AlwaysLearning]

"The current moving through the transmitter coil creates an electromagnetic field. The polarity of the magnetic field is perpendicular to the coil of wire."

Why does the magnetic field only push downwards? If I pass an electric current through a coil, I'd be willing to bet my left foot that the magnetic field takes off in every direction.

What's going on here? The author either was too ignorant to explain the coil was shielded or didn't bother to make note that "down" is all that matters when it comes to metal detecting ...unless you're blind & treasure floats in the air:shock:

"Each time the current changes direction, the polarity of the magnetic field changes. This means that if the coil of wire is parallel to the ground, the magnetic field is constantly pushing down into the ground and then pulling back out of it."

When I think of polarity, I think of value: - & +Me

I have no idea what I'm talking about, but this is what I'm going to assume,

I pass an AC current through a conductor. So the current heads in one direction. What happens to the magnetic field? It starts heading in one direction.

AC current changes direction. What happens to the magnetic field? It starts heading the other direction.

Let's call the point where the magnetic field first left off value "-"

Let's call the point where the magnetic field stops & heads back the other direction to where it started value "+"

^Is this what they mean when they speak of polarity?

Logic seems correct, but then you're left with the question, who's to say what gets the negative value & what gets the positive value? Perhaps when you speak of polarity w/ magnetic fields, you don't specify negative & positive values. Rather you just think of them as opposites & be satisfied with that.

?

 

[godfreyl]

the author too ignorant?

No

I have no idea what I'm talking about

Yes

Perhaps when you speak of polarity w/ magnetic fields, you don't specify negative & positive values.

No, you don't. Google magnetism or look it up in Wikipedia.

 

[betwixt]

No figting please!

The concept of the current through the coil pushing or pulling is what's wrong. It is true that the N and S poles reverse when the current is applied in the oposite direction but to equate that to "pull" or "push" is incorrect and in any case not the principle used in a metal detector. You will note that a bar magnet will stick to a ferrous item both ends around, in your wording, it pulls in both polarities so clearly the notion of "push" in one direction is not right.

However, in a metal detector, it isn't the amount of attraction or repulsion that is measured, it's the effect the object has on the inductance/reluctance or in some case the balance of two inductances/reluctances when in close proximity. The current through the coils is monitored and the disturbance caused by current beng induced into nearby conductive objects is what the detector reports to the user.

Brian.