Diamagnetic Properties of Bismuth
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The following article was originally published in the journal Młody Technik (eng. Young Technician) (8/2014):
Magnetism, strictly speaking, is not a single, uniform phenomenon; rather, this term covers a whole array of physical effects associated with magnetic fields, which can be produced either by electric currents or by magnetic materials.
Magnetism manifests in several forms — chief among them are ferromagnetism, paramagnetism, and diamagnetism. Today, we’ll focus on the latter.
In everyday life, we most often notice ferromagnetism because its effects are the most obvious. Ferromagnetic substances, such as iron, cobalt, nickel, and others, are strongly attracted by a permanent magnet. Paramagnetic substances are also attracted, but much more weakly. Examples include liquid oxygen, platinum, sodium, and cobalt(II) chloride hexahydrate.
Diamagnetic substances, on the other hand, are repelled by a magnet, regardless of the magnetic field’s polarity! Diamagnetic interaction is relatively easy to observe if you use bismuth.
Torsion pendulum
Bismuth is a metal and the heaviest non-radioactive element. Technically, none of its isotopes are perfectly stable, but the half-lives of most of them are so vast that the probability of radioactive decay is truly negligible. For instance, the half-life of 209Bi is 1.9×1019 years, which is at least a billion times longer than the estimated age of the entire universe — that’s how long it would take for half the atoms in a sample of bismuth to decay. Bismuth also stands out for its color: it has a beautiful metallic shine with a pinkish hue (Photo. 1).
You only need a half-centimeter (~0.2 in) piece of bismuth for this experiment:
Surprisingly, although bismuth belongs to the heavy metals, it’s completely harmless and non-toxic. It’s also relatively easy to acquire because — thanks to its striking crystal forms — it’s quite popular among collectors. This is a great advantage for any amateur experimenter.
Diamagnetism generates such small forces that, to observe it clearly, it’s best to use a torsion balance (also known as a Cavendish balance), slightly modified according to my design (Photo. 2). It’s a very simple apparatus, suitable for anyone who enjoys hands-on experiments.
The balance is built in a straightforward way. It consists of a light bar (in this case, a drinking straw) suspended in the middle by a thread. One end of the beam holds a small piece of bismuth, and for balance, the other arm of the scale carries a movable counterweight — made of a few turns of insulated copper wire. In its resting position, one arm of the balance touches a simple stop: an ordinary needle. The suspension point of the thread should allow slight twisting, so that when no force acts on the arm, it gently rests on the needle. Even the smallest force is then enough to deflect the scale from equilibrium.
Henry Cavendish lived from 1731 to 1810. He was a British chemist and physicist, and a member of the prestigious Royal Society. Born into an aristocratic family, he studied at the University of Cambridge — but never completed a degree. Nevertheless, after inheriting a substantial fortune, he established his own laboratory and conducted research. Known as a recluse and something of an eccentric, he performed pioneering experiments in chemistry and electricity, and he improved Michell’s torsion balance, which later came to be called the Cavendish balance.
You’ll also need a magnet — ideally a small neodymium magnet (Photo. 3).
Initially, there’s no external force, and the bar is in equilibrium (Photo. 4A, indicated by the arrow). When you bring the magnet near the bismuth, the equilibrium is disturbed (Photo. 4B)!
Explanation
Electrons in a material typically occupy orbitals that effectively behave like current loops with almost zero resistance. One might therefore expect diamagnetism to be common, since any applied magnetic field would induce loop currents opposing the flux change — much like in superconductors, which are essentially perfect diamagnets.
However, because electrons are tightly bound by the nuclear charge and further restricted by the Pauli exclusion principle, many materials do display diamagnetism but respond only minimally to an external field.
Superconductors are sometimes considered perfect diamagnets because they expel external magnetic fields entirely — this is known as the Meissner effect.
Finally, note that a diamagnetic substance shows no magnetic properties at all when it isn’t placed in an external magnetic field.
References:
- Jackson R., Tyndall J., the Early History of Diamagnetism, Annals of Science, 4, 2014
- de Marcillac P., Coron N., Dambier G., Leblanc J., Moalic JP., Experimental detection of alpha-particles from the radioactive decay of natural bismuth, Nature, 2003, 693(422), pp. 876-878
- Mulay L.N., Boudreaux E.A., Theory and applications of molecular diamagnetism, Wiley, Nowy Jork, 1976
- Nave C.L., Magnetic Properties of Solids, Hyper Physics, 09.11.2008
All photographs and illustrations were created by the author.
Marek Ples