MOTION MOUNTAIN

40 1 electricity and fields
F I G U R E 17 The magnetotactic bacterium
Magnetobacterium bavaricum with its magnetosomes
(© Marianne Hanzlik).
Ref. 20
Ref. 21
Challenge 20 r
just below the hairs, as shown in Figure 16. The second magnetic sense of migrating
birds is an inclination compass that tell them the angle between the magnetic field lines
and the vertical. This system is based on magnetically sensitive protein molecules, socalled
cryptochromes. The mechanism is located in the eye and is based on blue light.
This second magnetic sense, which is still not properly understood, is used by birds to
decidethegeneraldirectioninwhichtofly.
Can humans feel static magnetic fields? So far, thereis no definite answer. Magnetic
microcrystals are present in the human brain, but whether humans can feel magnetic
fieldsisstillanopenissue.Maybe youcandevise a way to test the this possibility?
In contrast, oscillating or pulsed magnetic fields can be felt by humans. There is anecdotalevidence
that0.2 T oscillating at170 kHz leads to numbness in fingers for a few
days. Beneficial effects of pulsed fields on well-being arealsoclaimed, but are questionable;
ontheotherhand,oscillating magneticfieldshave positive effect on bone fracture
healing.
Magnetism and electricity
Are magnetism and electricity related? In the early 19th century, the François Arago*
discovered that they were. He explored a ship that had survived a bad thunderstorm. At
that time, ships where made of wood.The ship had been struck by lightning; as a result,
the ship needed a new compass.Thus lightning has the ability to demagnetize compasses.
Arago knew that lightning is an electrical phenomenon. He concluded that magnetism
and electricity must be related.
In short, magnetism must be related to themotion of electric charges. If magnetism is
related to motion, it must be possible to use magnetism and electricity to move matter.
* François Arago (b. 1786 Estagel, d.1853Paris) was physicist and politician; he was a friend of Alexander
vonHumboldt.
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net