Electromagnetic induction

The Lorentz force causes a conducting rod moving through a magnetic field to develop a nonuniform charge distribution. If the rod is immersed in a conductive medium that is stationary relative to the field, an electrical circuit is formed. As far back as 1832, Michael Faraday noted that ocean currents should generate electric fields as they move through Earth's magnetic field. Indeed, some modern profiling systems that detect and map ocean currents are based on that principle.

Electroreception is relatively common and found in animals ranging from aquarium fish to duck-billed platypuses. Due to the weakness of Earth's magnetic field, however, the electromotive force induced in an animal moving at a realistic speed can be detected only by a highly sensitive electroreceptive system. In 1974, Adrianus Kalmijn suggested that sharks and their close cousins, rays, possess such a system. Those fish, collectively known as elasmobranchs, possess several hundred long canals that begin at tiny pores in the skin and end blindly inside the body (figure 1a). The canals, which feature exceptionally resistive walls and an interior filled with a highly conductive "jelly," essentially function as electrical cables. At the ends of the canals are the ampullae of Lorenzini—collections of cells that are extremely sensitive to small changes in voltage. Because the canals are highly conductive, almost all the induced voltage drop occurs at the ampullae (figure 1b). The ampullae's exact detection threshold has been debated, but a conservative estimate is 2 µV/m, the field that would be produced by a 1.5-V battery with one electrode in New York Harbor and the other off Cape Hatteras, North Carolina, 750 km south! Given that extraordinary sensitivity, magnetoreception using induction is theoretically possible. Depending on its compass direction, a shark or ray moving horizontally through the ocean at 1 m/s (about 2 miles per hour) could generate a voltage gradient at the receptor as high as 25 µV/m, well above the detection threshold.

In the several decades since the hypothesis was first proposed, however, several findings have emerged that complicate matters. First, although they are exquisitely sensitive to changes in voltage, the electroreceptors of elasmobranchs were found to be incapable of detecting DC voltages. In addition, ocean currents are also conductors moving through Earth's magnetic field and thus create electric fields of their own. Michael Paulin addressed both problems in 1995 by suggesting that sharks and rays might pay attention only to the oscillating electric fields that arise as their heads sway rhythmically back and forth during swimming. In addition to creating AC voltages that the animals can detect, the head motion might function as a high-pass filter, removing irrelevant stimuli associated with ocean currents.

As one might guess, sharks (and even rays) are not ideal experimental animals, and the evidence for their magnetic sense is not as complete as for that in many other species. The few experiments that have been done mostly involved training captive animals to respond to the presence of local magnetic field gradients generated by an electromagnet. Given their extremely sensitive electroreception, however, it is unclear whether the animals responded to the magnetic field or to the electric fields induced as the magnet was turned on and off. In addition, it has never been demonstrated that electromagnetic induction is responsible for any of the observed magnetic behavior. In a 2001 experiment by Michael Walker, rays lost their ability to detect magnetic field gradients when small magnets (but not nonmagnetic brass bars) were inserted into their nasal cavities. Since a magnet that moves with the detector should not affect an induction-based system, Walker and his colleagues interpreted the results to mean that induction was not involved. But because the bodies of rays are flexible, the possibility remains that the magnets moved slightly relative to the electroreceptors and thus affected an induction-based system.

It is also possible that freshwater and terrestrial animals have induction-based mechanisms based on internal conducting rods or loops such as neural circuits. However, electromagnetic induction appears unlikely to be a widespread mechanism for magnetoreception because only elasmobranchs are known to have the extreme electrical sensitivity required. Most animals with electroreceptors have electric thresholds two to five orders of magnitude higher—too high for magnetoreception. For example, the electric fish Eigenmannia (glass knifefish), a relatively electrosensitive animal, would need to swim at 400 mph (nearly 180 m/s) to detect Earth's field using induction.

Ferrimagnetism

The only conclusively demonstrated magnetoreceptors are found in various phytoplankton and bacteria, which contain chains of crystals of ferrimagnetic minerals, either magnetite (Fe3O4) or greigite (Fe3S4), as shown in figure 2 and on the cover. The torque on the chain is so large that it rotates the entire organism to align with Earth's field. The field generally has a vertical component, and some of those organisms use magnetoreception to sense what direction is "down" and to move toward the deeper, less oxygenated mud they prefer. The 1963 discovery by Salvatore Bellini of magnetotaxis in certain bacteria, followed by Richard Blakemore's 1975 description of the crystals, led to the detection of magnetite in a diverse array of magnetoreceptive species, including honeybees, birds, salmon, and sea turtles.

Ferromagnetic and ferrimagnetic minerals are natural choices for a compass mechanism, due to their powerful interaction with magnetic fields caused by spontaneous ordering of electron spins. Certain compounds of ferromagnetic elements, including magnetite, maghemite (Fe2O3), and greigite, are ferrimagnetic, meaning that although neighboring spins are antiparallel, the material still has a net moment because the moments in one direction are larger than those in the other. In both ferro- and ferrimagnetic minerals, the minimization of energy that comes from spin alignment is superseded at larger distances by other contributions to the total energy primarily magnetostatic energy. Thus larger volumes of those minerals are broken up into clearly defined domains on the order of 0.1–1 µm in diameter, each of which has a powerful magnetic moment in the absence of an external field. A single cuboidal domain 60 nm on a side has an interaction with Earth's field roughly equal to kT.

In the presence of moderately strong external fields, energetically favorable domains expand at the expense of neighboring domains, and the material as a whole becomes a magnet. Lacking a source for such fields, however, animals' internal compass needles are limited to their minerals' original domain size. Particles larger than the typical domain will develop multiple domains with moments in different directions (figure 3a). Particles smaller than a certain size (about 30 nm for magnetite, depending on the aspect ratio) have their moments randomized by thermal energy, even though the local spins are still aligned. In the single-domain range, the magnetic interaction µB, where µ is the magnetic moment of the particle and B is Earth's field strength, must be about six times greater than kT; otherwise even a tethered compass will be tumbled too much by thermal interactions to be reliable (figure 3b). Bacteria thus may have found the best strategy: long chains of single-domain particles. However, the most sensitive measurements of magnetic field strength are found when the ratio of µB to kT is about 2.

Exactly how the rotation of a single-domain particle creates an action potential in a neuron is not known, but the existence of diverse mechanical sensors in cells offers many possibilities. One is that the particles strain or twist hair cells, stretch receptors, or other mechanical receptors as they attempt to align with the geomagnetic field. Another is that the rotation of intracellular magnetite crystals might open ion channels directly if cytoskeletal filaments connect the crystals to the channels.

The small size and ferric nature of those putative compasses make them almost impossible to unambiguously locate in a body. They are below the resolution limit of light microscopy and are dissolved by many common tissue preservatives. In addition, iron is one of the most common metals found in organs and accumulates in a number of degenerative processes, including hemochromatosis, Parkinson's disease, and blood coagulation. Iron is also widespread in both outdoor and lab environments. Thus searching for a magnetite-based compass is even worse than finding a needle in a haystack—it is like finding a needle in a stack of needles.

Date: 2016-01-14; view: 419