(This is from an old article published by the Insitute of Physics - click on the graphic to go to the original archived article)

Some things like iron nails are known for their magnetic properties, but why should frogs levitate in a magnetic field? The trick is to get the magnetic field right – you can’t just use any old bar magnet to make a frog levitate.

Frogs, like everything around and inside us, are made up of millions and billions of atoms. Each of these atoms contains electrons that whizz around a central nucleus, but when atoms are in a magnetic field, the electrons shift their orbits slightly. These shifts give the atoms their own magnetic field so when a frog is put in a very strong magnetic field, it is essentially made up of lots of tiny magnets. And there’s nothing special about frogs. All materials – including strawberries, water and gold – are ‘diamagnetic’ to some extent, but some are more convenient to levitate than others.

Frogs are convenient not only because they have a high water content, which is a good diamagnetic material, but also because they fit easily inside a tube-shaped Bitter electromagnet. Bitter electromagnets use a very large electric current to create an extremely strong magnetic field which magnetises the frog in such a way that its magnetisation is in the opposite direction to the applied field. This means that the magnetised frog is pushed up from a region of high magnetic field into one of lower field, and levitates.

Is it possible to make a human levitate?

Like frogs, humans are about two thirds water, so if you had a big enough Bitter electromagnet, there’s no reason why a human couldn’t be levitated diamagnetically. None of the frogs that have taken part in the diamagnetic levitation experiments have experienced any adverse effects, which bodes well for any future human guinea pigs.

But many a magician has made their lovely assistant levitate without the need for huge magnetic fields, so how do they do it?

Using magic

Most levitation tricks use optical illusions and misdirection to focus your attention away from what is really happening. One classic trick, called Balducci levitation, is often performed by street entertainers. By standing at a certain angle so that the audience can see only one foot and the back heel of the other, the entertainer can then stand on the tip toes of the hidden foot, lift the visible foot completely off the ground and pretend to be levitating.

Smoke and mirrors

Levitating a reclining assistant is another oft-repeated trick that also relies on optical illusion and misdirection, as well as more than a bit of physics. The lovely assistant comes on stage with a flourish and lies down on a couch. The magician then covers them in a cloth from head to toe before ‘levitating’ them up and across the stage. As the audience are still reeling from the wonder of seeing the lovely assistant floating in mid air, there’s a flash of light and a crack and the magician whisks the cloth away to reveal nothing – the assistant has gone!

This is Asrah levitation and the ‘magic’ is that an assistant shaped shield is placed over the assistant along with the cloth. The couch is hollow and once the cloth and shield are in place, the assistant conceals themselves inside the couch which is then taken off stage. The shield and cloth are levitated using wires, threads and pulleys which are almost impossible to see against the carefully chosen dark stage background. Finally, as the cloth is whipped away, the shield, which is made up of very thin black material, collapses and goes unseen as the magician waves his arms and points to the assistant who has magically reappeared at the back of the auditorium.

Magicians don’t often give away their secrets, but when they do, the ‘magic’ that they reveal is often just physics – magnets, lights, smoke, pulleys – which is used to confound your senses.