How can a Hall effect ammeter detect current in a magnetically shielded wire? - Page 1
Forum section: electronics / general

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 26, 2013, 04:46:54 pm
content: Take a piece of iron pipe (in my case, 18" long and 3/4" in diameter) and thread a wire through it. Pass a current through the wire (3A in my case) and then clamp a Hall effect ammeter around the pipe. Surprisingly, the ammeter detects the current flow. Turning the current on and off confirms that it is indeed the current and not stray magnetic fields causing the reading.But how? A Hall effect ammeter works by detecting the magnetic field coming off a wire. With the wire inside the (rather thick) iron pipe, the iron should be able to magnetically shield it so that very little reaches the outside.

author: G7PSK
author_rank: Super Contributor
timestamp: March 26, 2013, 05:06:31 pm
content: Place a piece of iron in a magnetic field and it also becomes magnetized. Run a wire through the pipe and the wires magnetic field will permeate the pipe.

author: Wytnucls
author_rank: Super Contributor
timestamp: March 26, 2013, 05:09:02 pm
content: Magnetic shielding (Wikipedia)Equipment sometimes requires isolation from external magnetic fields. For static or slowly varying magnetic fields (below about 100 kHz) the Faraday shielding described above is ineffective. In these cases shields made of high magnetic permeability metal alloys can be used, such as sheets of Permalloy and Mu-Metal, or with nanocrystalline grain structure ferromagnetic metal coatings. These materials don't block the magnetic field, as with electric shielding, but rather draw the field into themselves, providing a path for the magnetic field lines around the shielded volume. The best shape for magnetic shields is thus a closed container surrounding the shielded volume. The effectiveness of this type of shielding depends on the material's permeability, which generally drops off at both very low magnetic field strengths and at high field strengths where the material becomes saturated. So to achieve low residual fields, magnetic shields often consist of several enclosures one inside the other, each of which successively reduces the field inside it.Because of the above limitations of passive shielding, an alternative used with static or low-frequency fields is active shielding; using a field created by electromagnets to cancel out the ambient field within a volume. Solenoids and Helmholtz coils are types of coils that can be used for this purpose.Additionally, superconducting materials can expel magnetic fields via the Meissner effect.

author: ejeffrey
author_rank: Super Contributor
timestamp: March 26, 2013, 06:04:40 pm
content: Basically shielding doesn't work like that. Ampere's law doesn't give out free passes. If you have a flowing current, the integral of the H field in a loop is set, and there is nothing you can do about it. Shielding can push the flux around, but doesn't get rid of it.

author: krish2487
author_rank: Frequent Contributor
timestamp: March 26, 2013, 07:48:26 pm
content: Besides, you are not really shieding the wire unless you connect the pipe to ground of the circuit in question.Take a BNC cable, the shield in really a woven mesh around the conductor. Of course then the shield is also grounded.Unless you ground the iron pipe. it only gets magnetized as G7PSK said.It is simply not a "true" shield.

author: ejeffrey
author_rank: Super Contributor
timestamp: March 26, 2013, 09:30:12 pm
content: Quote from: krish2487 on March 26, 2013, 07:48:26 pmGrounding or not is not relevant for magnetic shielding.

author: krish2487
author_rank: Frequent Contributor
timestamp: March 27, 2013, 01:24:00 am
content: You are right..My badDunno what I was thinking.Saw the word "shielding" and the mind automatically went to "electrical"..

author: Unknown
author_rank: Unknown
timestamp: March 28, 2013, 01:07:55 am
content: Low frequency or high can be given a magnetic shield, as has been stated; it needs to be within a closed area e.g.a Faraday Cage.However, it's much easer to shield at Radio Frequencies, due the outside magnetic skin effect, but at very low frequencies with bi-polar magnetic fields, the need for more thickness within an enclosed shield area is required.The type's of Mu-Metals for low frequency are normally intended for AC audio applications, or low field strengths, thus at a fixed current field, a substantial and enclosed metal area with thickness is required... due the deep field penetration at low frequencies or bi-polar magnetic field.The pipe under test is being allowed to form a magnet, because it is an open ended round section; if you were tobend the pipe into a circle and join the ends together, then provide two small holes just big enough for the wireto pass in and out of the pipe, you would find the magnetic field would no longer be detectable, by reversing the current... all that should be present is the residual magnetic field set-up before and after your first experiments, where the internal atomic structure domains within the metal, have been aligned to form the magnetic field.Although in reference to the Atomic Structure, their is much still not understood in this area.