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By Megan McTigue on

Free floating frogs

If you’ve been to see our new exhibition Electricity: The spark of life, you may well have seen a video featuring a frog suspended in space. And you may well be wondering what artist John Gerrard was thinking when he created his levitating frog. Let us explain.

Please note: Electricity: The spark of life ended in April 2019. To find out what exhibitions and activities are open today, visit our What’s On section.

Frogs have a long history of being used in science experiments (normally dead, but not always) but some of their uses are a lot more interesting than a stereotypical dissection carried out by GCSE biology students. One such experiment was carried out in 18th century Italy, by Luigi Galvani, now considered a pioneer of bioelectricity.

Green Frog by Grant Williamson (CC BY-SA 2.0)
A frog. Not levitating.

Who was Luigi Galvani and what was he doing with a frog?

Galvani was an Italian physicist at the Bologna Academy of Science, and in the late 1770s he started to experiment with electricity. During one of his experiments, he skinned a frog in order to use the skin in a static electricity experiment. Not a great day for the frog; however, it turned out to be a good day for Galvani as he was about to discover, what he called ‘animal electricity’.

The discovery was made when a scalpel, which had become charged with static electricity, touched an exposed nerve and caused the frog’s leg to twitch. This led Galvani to believe that there was an organ in the animal that was capable of creating electricity, which would be supplied to the nerves to allow the creature to move.

Insulating stands consisting of two glass blocks partly sunk into a wooden blocks, said to have been used by Galvani in his experiments on frogs

Galvani concluded that animal electricity was similar but not identical to static electricity, and was a unique property of living things.

Not everybody was toad-ally convinced about Galvani’s theory…

Physicist Alessandro Volta disagreed with Galvani about the source of the electricity. He believed it wasn’t specific to the animal, but instead was a physical phenomenon. and so set out to prove that.

Volta said that the “animal” part of Galvani’s animal electricity was not needed. Animals merely responded to normal electricity. There was no difference between animal electricity and electricity.

Once again, the humble frog played a key role. Volta performed various experiments on frogs’ legs and found the key to getting them to move was contact with two different metals. Contact with pieces of the same metal did nothing.

This led Volta to create an early battery to prove that the biological electricity worked on the same chemical principles as electrochemical cells, and therefore it was not a process that was unique to animals.

Early voltaic pile


What happened to Galvani?

Despite Galvani’s theory being disproved, his name gave rise to Galvanism, which in biology referred to the contraction of a muscle when stimulated by an electric current. So it wasn’t all bad.

Mary Shelley read Galvani’s reports on his experiments in the summer before she wrote Frankenstein, and was apparently inspired by his ideas about electricity and reanimation.

Why did John Gerrard use a levitating frog in space?

Frogs have gone far beyond their local habitats in the name of science, with many having been sent into space since the 1950s. As frogs have similar inner ears to humans, they are deemed suitable for studying motion sickness in space.

It was video footage of Astronaut Mark Lee working on the Frog Embryology Experiment in the General Purpose Work Station during the STS-47 mission in 1992 that partly inspired Gerrard’s piece, X.laevis (Spacelab) 2017.

X.Laevis by John Gerrard

The footage shows a frog escaping Mark’s hands and trying to swim through its zero gravity environment and, along with Galvani’s reanimated frog’s legs, Gerrard has created a mesmerising artwork for the exhibition.

Do frogs only levitate in space?

Well funny you should ask that. In 2000, the Ig Nobel prize was awarded to the (now) Nobel prize winner Sir Andre Geim. Along with fellow physicist Sir Michael Berry, Geim used magnets to levitate a frog. Although the Ig Nobel prize is a parody of the far more serious Nobel prize, it’s awarded for science that ‘makes you laugh, then makes you think’.

How do you levitate a frog?

We saved the best bit for last, naturally.

Some materials are diamagnetic, mostly due to their high water content. When placed in a magnetic field, a diamagnetic material will produce its own field, of an equal and opposite force to oppose the original field. Therefore, if the field is strong enough, it can cause the object to levitate.

A frog is a diamagnetic material. We are too, but frogs are more convenient for levitating 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 that 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.

The field needed to cause a frog to float is massive, just under 16 tesla (tesla being the unit used to measure magnetism). A typical fridge magnet will create a field of 5 mT (militesla) so you’d need a LOT of magnets to achieve the same results at home.

Where frogs might find themselves in the future in the name of science is anyone’s guess, but we musn’t frog-et the contributions they’ve made so far.

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