The common torpedo, or eyed electric ray, is a species of fish found in the Mediterranean Sea and the eastern Atlantic Ocean. It has an unusual capability of stunning its prey, with what we now understand to be an electric shock.
The torpedo was known to ancient Greeks and Romans who believed it to possess magical and medicinal properties. Fishermen who were unfortunate to catch a torpedo sometimes felt as if they had been struck hard, and at other times felt a numbness in their arms. The torpedo’s “shock” is painful to a human, but not dangerous.
The study of electricity made rapid progress in the mid 1700s. In the latter half of the century, electrical scientists inevitably became interested in the torpedo’s shock. They were familiar with the electrical shock from a Leyden jar (the first capacitor). The torpedo’s shock seemed eerily similar.
However, despite the similarity, several scientists refused to accept that the shock from the fish was due to electricity. There was one nagging problem. The Leyden jar always produced a spark when it was discharged, but there was no spark when the torpedo struck.
Henry Cavendish, perhaps the most enigmatic character in 18th century science, was the one to crack the problem. He correctly reasoned that the key to understanding the torpedo’s shock was to distinguish between the “quantity of electricity” and the “degree of electrification”.
Cavendish painstakingly demonstrated this distinction using multiple Leyden jars of different sizes. He showed that a big jar and a small jar, when charged to the same degree as shown by an electroscope, gave shocks of different strengths. The bigger jar gave a stronger shock for the same “degree of electrification”. Yet they both produced sparks of the same length.
In other words, he argued that the size or even the occurrence of a spark was independent of the strength of the shock.
Some other scientists might have stopped at this theoretical explanation, but not Cavendish. He set about trying to recreate the torpedo’s shock artificially, without the spark.
Cavendish connected several Leyden jars together to form a battery of jars with a very large effective capacity. He then charged his battery of jars to a very low degree of electrification, as indicated by his electroscope. When he touched the jars, he got a strong shock. And as he had expected, there was hardly any spark. It was so tiny that it could be seen only through a magnifying glass.
Not satisfied with just the battery of jars, Cavendish went even further. He made an artificial torpedo fish using wood and leather, immersed it in wet sand and hooked it up to his battery. He wanted to recreate all the conditions of the torpedo’s strike, not just the shock without the spark.
Cavendish invited a group of distinguished electrical scientists including Joseph Priestley to come and try out his artificial torpedo. The guests played with the new equipment and were very impressed. Even those who had been disinclined to believe the torpedo’s shock to be electrical were convinced.
The new concepts that Cavendish introduced perhaps seem vague and not well defined. It is true that his “degree of electrification” is some distance away from the mathematical definition of electric potential and potential difference (or voltage), which had to wait till the early years of the 19th century.
Still, the distinction between “quantity” and “degree” was an important first step towards the equation Q = CV (capacitance formula) that every physics student would know today.