Electric Current
Electricity and Magnetism

Electrical animals

Stories from Physics for 11-14 14-16 IOP RESOURCES

A number of animals, including sharks, platypuses, echidnas, sturgeon and catfish are sensitive to electrical stimuli, a sense known as electroreception.

  • Eel vs torpedo −  The electric eel, which lives in fresh water, can generate a current of 1 A with a power output of 500 W. By contrast, the torpedo fish is found in saltwater habitats, and the ions dissolved in sea water mean the magnitude of the torpedo’s current diminishes less than the electric eel’s output. This allows the torpedo to produce currents of up to 50 A with a peak power output of nearly a kilowatt.
  • Poking fun with fish −  One of the first scientists to experiment with the electrical behaviour of animals was John Walsh, who was awarded the Copley Medal by the Royal Society in 1773, for a paper on the torpedo fish. He describes his incredulity on first experiencing a shock from a torpedo fish (also known as the electric ray). The list of his subsequent experiments reveals a dedication to research undiminished by pain.

Walsh reports poking the electric fish with a number of different materials and records his experiences:

Touched the upper and lower sides of the same flank with Spoons; Shock, twice. Repeated it with Spoons; a Shock. With Sealing Wax; nothing. Repeated with spoons; Six times. With sealing wax, twice; nothing.

Despite the similarity of the sensation of shocks from the torpedo fish to other electric shocks, critics doubted that the effect was electrical as the spark and crackling sound characteristic of electrical discharge were absent. Walsh attempted in vain to reproduce these phenomena by poking a patient torpedo fish with a piece of tin foil attached to a stick of sealing wax.

The debate about the missing spark led Henry Cavendish to construct, and demonstrate in public, an artificial torpedo that produced shocks without sound or sparks. The first model torpedo was made of a piece of wood, cut to the shape of the fish, and connected to several Leyden jars. The wooden version, however, did not conduct well and Cavendish built a second version out of pieces of leather with thin pewter plates attached to each side, connected to the Leyden jars by glass-insulated wires. The device was then placed inside a sheep’s skin soaked in brine.

In a paper on the artificial torpedo, Cavendish argued that both the “degree of electrification” (equivalent to electrical potential) and “quantity of electric fluid” (a measure of current) determined the nature of electrical phenomena, pre-empting Volta’s discussion of tension and charge in capacitors.

In 1775, in his London home, Walsh finally succeeded in eliciting an elusive spark from a torpedo fish by holding a piece of tin foil pasted onto glass near a fish held in the air.

  • Grasping the eel by the tail −  Between 1776 and 1777, George Baker, a ship’s captain, exhibited five electric eels in an apartment on Piccadilly, opposite St. James’s Street in London. Visitors could experience a mild shock by placing their fingers in the eels’ tank but were told that the best way to experience the eels’ abilities was to grasp the animal firmly by its head and tail. Some visitors reported that their limbs were numbed for several hours, while others were brought to tears or fell to the floor. When an interested physician attempted to purchase a single eel, Baker set the price at 50 guineas (over £53 at a time when the average annual wage of a female domestic servant was £16).
  • Electric shark attacks −  The heads of sharks, skates and rays are studded with tiny pores, known as ampullae of Lorenzini, which help the animals sense electric fields. Researchers have discovered that the pores contain a jelly which is the most highly proton conductive substance in the natural world.

Early submarine fibre optic cables suffered several insulation failures that were believed to have been caused by shark bites. It is speculated that either acoustic vibrations or the relatively high magnetic field around the cable (the voltage lines to power the optical repeaters required three times the current of previous coaxial telephone cables) attracted the sharks. Subsequent cables were protected with fish bite tape.

References

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