### A Magnetic Hose

July 7, 2014

electricity and magnetism have a lot in common. The first clue is that they share one of the four fundamental forces, called electromagnetism, the others being gravitation, the strong nuclear force, and the weak nuclear force. You can have electric dipoles producing electric fields in space, and magnetic dipoles producing magnetic fields in space.

One small failure of this analogy is that you can have electric charge and electric monopoles, but we haven't discovered the equivalent magnetic charge and magnetic monopole. This isn't for lack of trying, since there a quite a few reasons why physicists believe that they should exist.[1]

Just as we have wires conducting electricity in electrical circuits, we also have magnetic materials conducting magnetism in magnetic circuits. There are many useful analogies between these two types of circuits, such as those between electromotive force and magnetomotive force; electric current and magnetic flux; and electrical resistance and magnetic reluctance.

One common magnetic circuit is the one made by a horseshoe magnet and its "keeper."

The keeper ensures that the magnetic field is mostly contained in the magnetic. This reduces the tendency for such magnets to demagnetize.

(Photograph by Eurico Zimbres FGEL/UERJ, via Wikimedia Commons.)

The analogy is not perfect, since electric charges flow in electrical circuits, but there's nothing flowing in a magnetic circuit. Also, magnetic conductors, which are magnetic materials, are less perfect in conducting magnetic fields than wires are at conducting electricity. While there's a large difference between electrical conductance in a vacuum and in a wire, magnetic conductance in a magnetic material is only a few hundred to a few thousand times greater than that in a vacuum (a.k.a., "free space"). In a word, magnetic circuits leak.

Analogies are such a useful pedagogical construct that electricity, itself, has an analogy based on hydraulics. This analogy was invented by Oliver Lodge, who was famous for his many contributions to early radio research. The analogy persists to the present. My father often referred to electricity as "juice," which, to my younger mind, meant that the transformers mounted on utility poles were juice storage containers.

It's easy to think of a wire as a hose that carries electricity. This leads to the idea of a "magnetic hose." Just as an "electrical hose" will translate an electrical field from one place to another, a magnetic hose will do the same for a magnetic field. An international team of scientists from the Universitat Autònoma de Barcelona (Barcelona, Spain), the Austrian Academy of Sciences (Innsbruck, Austria), the University of Innsbruck (Innsbruck, Austria), and the Max-Planck-Institut für Quantenoptik (Garching, Germany), has demonstrated such a hose by combining magnetic and superconducting materials.[2-5]

A superconductor, in this case the high temperature superconductor, yttrium barium copper oxide (YBCO), is used as a means of preventing the magnetic fields inside a ferromagnetic iron alloy from leaking out into free space. The principle behind this is the Meissner effect, discovered in 1933 by the German physicists, Walther Meissner and Robert Ochsenfeld, quite a few years after the discovery of superconductivity by Heike Kamerlingh Onnes. The delay, of course, was because of the difficulty of doing experiments at cryogenic temperatures.

In the Meissner effect, magnetic fields, represented here by the magnetic field lines, are excluded from a superconductor.

The effect is so strong that small magnets can be levitated above a superconductor.

(Illustration by Piotr Jaworski, via Wikimedia Commons.)

Lead author of the study, Alvar Sánchez, and his colleagues have been doing research on such devices for several years.[6-7] In 2012, they demonstrated a device built from two coaxial cylinders formed from rolled sheets of superconductor and a ferromagnetic foil.[7] The proposed magnetic hose, as illustrated in the figure, would be built from alternating layers of superconductor and magnetic material.[2] For a proof-of-concept device, just a single layer device was built; namely, a ferromagnetic cylinder coated with a superconductor.[2-5]

Magnetic hose construction.

The hose is constructed from alternating layers of superconducting and magnetic material around a magnetic core.

(Illustration by the author using Inkscape.)

The demonstration device had an iron alloy core surrounded by the YBCO superconductor, and it was 14 centimeters long.[2-3] This device demonstrated a four-fold improvement of magnetic field transfer than what could be obtained without the superconductor.[2-5] mathematical modeling shows that many, thin layers will offer a greater transfer efficiency.[5]

A short magnetic hose, shown in yellow, translates the field of a magnetic dipole. This illustration uses the traditional field line representation of a magnetic field. (Illustration by the author using Inkscape.)

Potential applications for such a magnetic hose exist in such present magnetism-based devices as superconducting quantum interference device (SQUID) microscopes and magnetic resonance imaging devices.[3] Tapering the hose might also allow concentration of magnetic field strength. Similar flux concentrators made from the magnetic alloy, permalloy, are used in magnetic sensors.[3] Although it's suggested that nothing will limit application of such hoses, even at nanometer scale,[5] it should be noted that strong magnetic fields will penetrate a superconductor to a significant depth.

Distributing a magnetic field with a magnetic hose.

(Universitat Autònoma de Barcelona image.)

A good summary of this research was made by lead author, Alvar Sánchez,
"...Until now there was nothing similar [to optical fibers] with which to guide and transport static magnetic fields."[5]

### References:

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