A little frog (alive !) and a water ball levitate inside a Ø32mm vertical bore of a Bitter solenoid in a magnetic field of about 16 Tesla at the Nijmegen High Field Magnet Laboratory.
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(Click on the images to get larger ones)
The image of a high-temperature superconductor levitating above a
magnet in fog of liquid nitrogen can hardly surprise anyone these
days – it has become common knowledge that superconductors
are ideal diamagnetics and magnetic field must expel them. On the
other hand, the enclosed photographs of water and a frog hovering
inside a magnet (not on board a spacecraft) are somewhat
counterintuitive and will probably take many people (even
physicists) by surprise. This is the first observation of magnetic
levitation of living organisms as well as the first images of
diamagnetics levitated in a normal, room-temperature environment
(if we disregard the tale about Flying Coffin of Mohammed as such
evidence, of course). In fact, it is possible to levitate
magnetically every material and every living creature on the
earth due to the always present molecular magnetism. The
molecular magnetism is very weak (millions times weaker than
ferromagnetism) and usually remains unnoticed in everyday life,
thereby producing the wrong impression that materials around us
are mainly nonmagnetic. But they are all magnetic. It is just
that magnetic fields required to levitate all these
“nonmagnetic” materials have to be approximately 100
times larger than for the case of, say, superconductors.
Whether an object will or will not levitate in a magnetic field B
is defined by the balance between the magnetic force F = M
B and
gravity mg = 
V g where is the
material density, V is the volume and g = 9.8m/s2.
The magnetic moment M = (
/ µ0)VB
so that F = (/µ0)BVB = (/2µ0)VB2.
Therefore, the vertical field gradient B2
required for levitation has to be larger than 2µ0
g/.
Molecular susceptibilities are typically 10-5
for diamagnetics and 10-3 for
paramagnetic materials and, since is most often a few g/cm3,
their magnetic levitation requires field gradients ~1000
and 10 T2/m, respectively. Taking l = 10cm as a
typical size of high-field magnets and B2 ~
B2/l as an estimate, we find that
fields of the order of 1 and 10T are sufficient to cause
levitation of para- and diamagnetics. This result should not come
as a surprise because, as we know, magnetic fields of less than
0.1T can levitate a superconductor (= -1) and,
from the formulas above, the magnetic force increases as B2.
If the above is too complicated for you, read the more simple explanation.
The water and the frog are but two examples of magnetic
levitation. We have observed plenty of other materials floating
in magnetic field - from simple metals (Bi and Sb),
liquids (propanol, acetone and liquid nitrogen) and various
polymers to everyday things such as various plants and living
creatures (frogs and fish). We hope that our photographs will
help many – particularly, non-physicists – to
appreciate the importance of magnetism in the world around us.
For instance, it is not always necessary to organize a space
mission to study the effects of microgravity– some
experiments, e.g. plants or crystal growth, can be performed
inside a magnet instead. Importantly, the ability to levitate
does not depend on the amount of material involved, V, and
high-field magnets can be made to accommodate large objects,
animals or even man. In the case of living organisms, no adverse
effects of strong static magnetic fields are known – after
all, our frog levitated in fields comparable to those used in
commercial in-vivo imaging systems (currently up to 10T). The
small frog looked comfortable inside the magnet and, afterwards,
happily joined its fellow frogs in a biology department.
A.K. Geim, J.C. Maan.
in collaboration with
H.A.Carmona (University of Sao Carlos)
P.C.Main (Universty of Nottingham)
More photographs
of levitating objects.
The best of the press coverage so far:
"If frogs can fly, there is no reason why John Major
cannot be Prime Minister"
And the best of public response so far (have
a good laugh!)
Go to the HFML
Home Page.
Last updated on 27-jan-98, ln
