Instruments as Frogs: the Leyden Jar and Voltaic Pile

By the late 18th century, natural philosophers knew that electricity could stimulate muscular motion. What kind of electricity was it, though, and how was it produced? To answer this question, Galvani and Volta each pointed to an instrument used for storing or producing electric current whose operation was more convincingly understood than the frog's kick.

Voltaic Pile
Image 1 Voltaic pile, first half of the 19th century. Image © the Whipple Museum.
Leyden Jar
Image 2 Leyden Jar by Harvey and Peak, second half of the 19th century.Image © the Whipple Museum.

When Galvani hung a frog's leg over his iron banister and saw it kick, a familiar electrical instrument came to mind:

"These results surprised us greatly and led us to suspect that the electricity was inherent in the animal itself. An observation that a kind of circuit of a delicate nerve fluid is made from the nerves to the muscles when the phenomenon of contractions is produced, similar to the electric circuit which is completed in a Leyden jar, strengthened this suspicion and our surprise." (1)

The twitching leg reminded Galvani of the Leyden jar. The Leyden jar was one of the first and most important electrical tools to spread throughout Europe. Pieter van Musschenbroek invented it around 1745 in the Dutch town of Leiden, from which the jar derives its name. Another 'electrician' named Ewald Georg von Kleist, however, invented a jar with the same capabilities independently, and roughly simultaneously.

By filling the jar with water, then directing an electrical current into that water via a metal cap, one can store or 'condense' current within the jar. The water's charge in turn produces the opposite electrical charge in the outer metal plate. 'Electricians' would perform stunning displays of electrical power by establishing a circuit between the water and the outer plate. We now call this a 'capacitor' or 'condenser.'

In his experiments on electricity and frogs, Luigi Galvani imagined that the frog's muscle and nerve acted like the two sides of a Leyden jar. Establishing a circuit between them allows the discharge of 'animal electricity' analogous to the artificial electricity of the jar. Animal motion resulted from distributions of charges in nerves that would, when sufficiently strong, discharge relative to muscles and cause them to contract. To Galvani, the frog comprised an ensemble of capacitors. Electricity created and used by animal bodies for this purpose constituted 'animal electricity.'

Physicians later applied this idea to medicine by creating machines for administering therapeutic shocks. See the Electro-galvanic Machine.

Volta took Galvani's brass hook experiment to demonstrate an opposing claim: the frog's body does not produce its own electrical current, which is merely conducted by the brass and iron through an imbalance of charges. Instead, the brass and iron in contact with the moist, conducting frog body produce 'artificial' electricity that makes the frog kick. This reinterpretation amounted to the entirely new theory of 'contact electricity,' which Volta mobilised in the invention of his battery pile.

Volta first created the pile in 1799 to demonstrate the capacity of moistened metals to produce electric current through contact alone. Little is known about the many voltaic piles owned by the Whipple Museum, in part because it was such a common and generic scientific tool. Many were produced in early 19th century England according to Volta's general design. This example consists of alternating copper and zinc plates separated by 'cells', or cloth pads moistened with saltwater or acid.

References

  1. A. Mauro, 'The Role of the Voltaic Pile in the Galvani-Volta Controversy Concerning Animal vs. Metallic Electricity', Journal of the History of Medicine and Allied Sciences (Oxford: Oxford University Press, 1969). (Find in text ^)

Henry Schmidt

Henry Schmidt, 'Instruments as Frogs: the Leyden Jar and Voltaic Pile', Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, [http://www.hps.cam.ac.uk/whipple/explore/frogs/animalelectricity/instrumentsasfrogs/, accessed 25 November 2017]

 
Back to top ^^
 
Privacy / Web Standards / Copyright Information
© Whipple Museum of the History of Science, University of Cambridge 2006-16
The Whipple Museum [/whipple/]
Explore Whipple Collections
Gallery Challenge [/whipple/gallerychallenge/]
http://www.hps.cam.ac.uk/whipple/explore/frogs/animalelectricity/instrumentsasfrogs/