Alessandro Volta

How to use this course

Before 1800, electricity came in sparks. A Leyden jar could store charge and release it in an instant, but nobody could make charge flow steadily through a wire. Alessandro Volta’s pile changed that. Within weeks it was splitting water, within a decade it had isolated new elements, and within twenty years it had revealed electromagnetism. The unit of electric potential carries his name.

The pile was the end of a long road. Volta spent twenty-five years building instruments to make weak electricity visible and to measure it, and in doing so he gave physics the ideas of tension (potential difference) and capacity. This course follows that road: the biography in Parts I and III, the physics in Part II.

Each technical lesson opens with a paragraph for the general reader, then gives precise statements in modern SI form, derivations and exercises with folded solutions. Volta worked before SI units, before the electron and before Ohm’s law, so each lesson also says what he actually measured and how he described it.

Timeline

Como, 1745–1774

Alessandro Volta was born in Como, on the southern tip of Lake Como, on 18 February 1745. Como was then part of the Duchy of Milan, under Habsburg rule. His father, Filippo Volta, came from a family of minor nobility; his mother, Maddalena, came from the Inzaghi family. Several of his relatives were clergymen, and the family expected Alessandro to follow them into the Church. He remained a practising Catholic all his life but did not take orders.

From 1758 he studied rhetoric and philosophy at the Jesuit school in Como, and in 1761 he entered the Seminario Benzi. There he became friends with Canon Giulio Cesare Gattoni, who owned a small collection of physical apparatus and let the young man use it. Volta taught himself electricity from books: Petrus van Musschenbroek, Jean-Antoine Nollet and the Turin physicist Giambattista Beccaria. He wrote to Nollet and to Beccaria, and Beccaria in particular encouraged him to experiment rather than speculate.

First papers

In 1769 he published De vi attractiva ignis electrici, on the attractive force of the electric fire. Like many papers of the period, it tried to explain electrical effects by attractions between matter and an electric fluid in the manner of Newtonian gravity. A second memoir in 1771, addressed to the naturalist Lazzaro Spallanzani, described a new and simpler electrical machine. These works were derivative, but they showed the habits that marked his later research: building apparatus himself, and testing claims against what the apparatus could actually show.

In 1774 the Austrian administration of Lombardy appointed him to direct the public schools of Como. The next year he began teaching experimental physics there.

Electrophorus and marsh gas, 1775–1778

In a letter to Joseph Priestley dated 10 June 1775, Volta described what he called an elettroforo perpetuo, a perpetual electricity-bearer. It was a resin cake on a metal base, charged once by rubbing, and a metal plate with an insulating handle. Lowered onto the cake, touched, and lifted, the plate came away charged and could give a spark. The process could be repeated many times without recharging the cake. The Swedish physicist Johan Carl Wilcke had described a device on the same principle in 1762, but Volta’s version was simple, robust and well publicized, and it made his name across Europe. Lesson 04 explains how it works.

Inflammable air

In November 1776, boating on Lake Maggiore near Angera, Volta stirred the muddy bottom of a marsh and collected the bubbles that rose. The gas burned with a blue flame. He had found what is now called methane. He distinguished it from the inflammable air (hydrogen) that Henry Cavendish had obtained from metals and acids, and in 1777 he published his findings as letters on the inflammable air native to marshes.

Volta then combined his two interests. He fired mixtures of inflammable air and ordinary air with an electric spark inside a closed vessel, which led to an “electric pistol” and to a eudiometer for measuring how much respirable air a sample contained (Lesson 10). The eudiometer became a standard tool of the new pneumatic chemistry.

In 1777 he travelled through Switzerland and met the naturalist and alpinist Horace-Bénédict de Saussure, who became a lifelong friend.

Pavia and the grand tour, 1778–1791

In 1778 Volta was appointed to the chair of experimental physics at the University of Pavia, the university of Austrian Lombardy, which the government was then reforming. He held the post for about forty years. Pavia gave him students, a budget for instruments and colleagues such as Spallanzani. A lecture hall built under Emperor Joseph II, the Aula Volta, still bears his name.

Around this time he worked out the ideas that would guide him for the rest of his career. He distinguished the quantity of electricity on a conductor from its tension, the electrical “effort” it exerts, and he saw that the ratio between them depends on the conductor, a quantity he called its capacity. In 1780 he invented the condenser, a device that uses this distinction to make tiny charges detectable (Lesson 05).

Travels

In 1781 and 1782 he travelled through Switzerland, Alsace, Germany, the Low Countries, France and England. In Paris he met Georges-Louis Leclerc de Buffon, Pierre-Simon Laplace and Antoine Lavoisier, and worked with Laplace and Lavoisier on whether evaporation and chemical effervescence produce electricity. In London his condenser paper was read to the Royal Society and printed in the Philosophical Transactions for 1782.

In 1785 the students of Pavia elected him rector for the academic year 1785–86. In letters to the Göttingen physicist Georg Christoph Lichtenberg, published between 1787 and 1789, he described a balance for weighing electrical attraction and the calibration of his straw electrometers, the first serious attempt to put electrical tension on a numerical scale (Lesson 06). By 1791, when the Royal Society elected him a Fellow, he was the leading experimental electrician in continental Europe.

Induction and the electrophorus

For the general reader. Rub a plastic sheet and it stays charged for hours. Volta found how to use one charged sheet to fill a metal plate with charge again and again, as many times as you like. The sheet’s charge is never used up. The energy in each spark comes from the hand that lifts the plate.

The cycle

The cake is an insulator with a fixed surface charge density \(-\sigma\) (resin rubbed with fur charges negatively). Its metal base, the “sole”, is grounded. The cover plate has area \(A\) and an insulating handle.

1. Lower the plate onto the cake. Because the cake surface is rough, the plate touches it at only a few points, and almost none of the cake’s charge passes to it. Induction separates charge in the plate: \(+\sigma A\) on its lower face and \(-\sigma A\) on its upper face.
2. Touch the plate with a grounded finger. The negative charge leaves; the plate now carries net charge \(+\sigma A\), held by the attraction of the cake.
3. Lift the plate by the handle. Its charge \(q=\sigma A\) is now free, and its capacity is small, so its potential is high. A spark can be drawn.

Why the cake does not run down

Charge is conserved, and the cake’s charge never leaves it: the plate receives its charge from ground, not from the cake. The cake only acts through its field. It does slowly lose charge through leakage along its surface and into humid air, which is why electrophori worked best in dry weather. Volta’s name “perpetual” was an exaggeration, but a good resin cake could last for weeks.

The maximum charge density is limited by dielectric breakdown of air, at a field of about \(E_b\approx3\ \mathrm{MV/m}\). Taking the field in the thin air gap under the plate to be of order \(\sigma/\varepsilon_0\), the charge density cannot much exceed \(\varepsilon_0E_b\approx 27\ \mu\mathrm{C/m^2}\).

Tension, capacity and the condenser

For the general reader. A litre of water in a narrow tube stands high; the same litre in a wide basin barely covers the bottom. Volta saw that electricity behaves the same way. A given quantity of charge produces a high electrical “pressure”, which he called tension, on a small conductor and a low one on a large conductor. He used this to build an amplifier for charges far too small to detect directly.

Charge and tension

Eighteenth-century electricians often mixed up how much electricity a body held with how strongly it acted. Volta separated the two. Tension, measured by the divergence of an electrometer, is what we now call electric potential (relative to ground). Capacity is the ratio of charge to tension. In modern notation:

The proportionality follows from the linearity of electrostatics. If a charge distribution with total charge \(Q\) solves Laplace’s equation with constant potential \(V\) on the conductor, then multiplying all charges by \(\lambda\) gives another solution with potential \(\lambda V\). So \(Q/V\) is fixed by geometry. Standard results:

The condenser (1780–82)

Volta’s condenser was a metal plate covered with a thin layer of varnish or marble, on which a second metal plate could be laid. He connected the upper plate to a weak source, such as a slightly electrified body or the air, while the lower plate was grounded. With the plates in contact through the thin insulating layer, the capacitance \(C_1\) was large, so the plate soaked up a relatively large charge \(Q=C_1V\) at the source’s low tension \(V\). He then disconnected the source and lifted the upper plate. Its capacitance fell to a much smaller \(C_2\), and since charge is conserved,

An electrometer attached to the lifted plate now showed a clear divergence where none had been visible before. The gain \(C_1/C_2\) could reach hundreds. Volta combined the condenser with a straw electrometer to make the condensing electroscope, the instrument with which he later detected the tension between two touching metals (Lesson 07).

Measuring tension: electrometers

For the general reader. Before there were meters, electricians judged electricity by the length of a spark or the shock in their fingers. Volta wanted numbers. He built electrometers from two fine straws hanging side by side, which spread apart when charged, and he checked their readings with a balance that weighed the pull between charged plates.

The straw electrometer

Two light straws hang from a conducting support inside a glass case. When the support is raised to a potential \(V\), the straws share its charge and repel. Volta graduated the case with a scale and treated the divergence, in “degrees”, as a measure of tension. For small charges the repulsion grows with \(V^2\) (charge on each straw is proportional to \(V\), and the force to the product of charges), so the deflection is not linear in tension over a wide range. Calibration was therefore essential.

The attracted-plate balance

In his letters to Lichtenberg, Volta described a balance with a flat plate hung from one arm above a second plate. Weights in the other pan measured the pull between them. The modern analysis is the one Kelvin later used in his absolute electrometer.

Derivation. At fixed charge, \(U=Q^2d/(2\varepsilon_0A)\) and \(F=-\partial U/\partial d\big|_Q=-Q^2/(2\varepsilon_0A)\), an attraction of magnitude \(Q^2/(2\varepsilon_0A)\). Substituting \(Q=\varepsilon_0AV/d\) gives the formula. (At fixed voltage one must include the work done by the source; the force is the same, as it must be since it depends only on the instantaneous charge distribution.) Because the force depends only on \(V\), the gap and the area, a weighed force gives \(V\) in absolute terms, independent of any electrometer.

Galvani’s frogs and contact tension

For the general reader. In 1791 the Bologna anatomist Luigi Galvani reported that a dissected frog’s leg twitched when a nerve and a muscle were joined by an arc of two different metals. He concluded that animals generate their own electricity. Volta first agreed, then decided the frog was only a sensitive detector. The electricity, he argued, came from the contact of the two metals. Both men were partly right.

The dispute

Galvani’s De viribus electricitatis in motu musculari commentarius (1791) described the frog experiments and proposed that muscle and nerve held opposite charges, like a Leyden jar, discharged through the metal arc. In 1792 Volta noticed that the twitches were strongest when the arc used two different metals, and that a bimetallic arc on his own tongue produced a sour or alkaline taste. He concluded that a weak electric tension arises wherever two different conductors touch. Galvani and his nephew Giovanni Aldini replied with experiments that produced contractions with a single metal, and even with no metal at all, by bringing a nerve into contact with a muscle. Each side could explain away the other’s results. Galvani died in 1798 with the question unsettled. Volta’s pile, which needed no animal at all, persuaded most physicists that he had won. Modern physiology shows that Galvani had also seen something real: nerves and muscles do carry electrical potentials.

Contact tension

Using his condensing electroscope, Volta found that two metals placed in contact and then separated carried small opposite charges. He arranged metals in a series (zinc, lead, tin, iron, copper, silver, gold, roughly in that order) such that each acquires positive tension with respect to those after it.

Volta therefore needed a different kind of conductor to break the cancellation. He distinguished conductors of the first class (metals) from those of the second class (moist bodies and solutions), which did not obey the series law. A circuit of the form metal, metal, moist conductor, metal yields a net tension. This is the pile’s principle as Volta understood it.

Modern view

When two metals with work functions \(\phi_A\) and \(\phi_B\) touch, electrons flow from the one with the smaller work function until the Fermi levels align. The surfaces then differ in electrostatic potential by the Volta potential (contact potential difference)

with \(A\) (smaller work function) at the higher potential. Since \(\Delta V\) depends only on the difference of a quantity attached to each metal, the series law follows at once. For zinc (\(\phi\approx4.3\ \mathrm{eV}\)) and copper (\(\phi\approx4.7\ \mathrm{eV}\)) this is a few tenths of a volt, the size Volta saw. The Kelvin probe measures exactly this quantity today. But a contact potential is an equilibrium property and cannot drive a steady current. The energy of the pile comes from chemistry (Lesson 09).

The pile as a circuit element

For the general reader. Volta stacked discs of zinc and silver or copper, separated by cloth soaked in brine, in a column. Touch the top and bottom with wet hands and you feel a shock; touch them again and you feel another. Unlike a Leyden jar, the pile recharged itself. Its tensions added: twenty cells gave twenty times the tension of one.

Volta’s description

His letter to Banks, dated 20 March 1800 and written in French, described two forms. The column was the stack just described. The crown of cups was a ring of cups of brine, each joined to the next by a bent strip that was zinc at one end and copper or silver at the other. He stressed that the device acted like a weak Leyden battery that recharged itself without end, and compared it to the electric organ of the torpedo fish. The letter appeared in the Philosophical Transactions later in 1800.

The circuit model

Ohm’s law was not formulated until 1827, the year of Volta’s death, so this model postdates him. It is the clearest way to see what the pile did.

The open-circuit voltage \(n\mathcal E\) is what Volta’s electrometer measured: it grows linearly with the number of cells. The current depends on the load. For a high-resistance load (a frog, a human body, an electrometer) series stacking is essential. For a low-resistance load, such as a thick wire to be heated, cells of large area matter more, because \(r\) falls as the plate area grows. Humphry Davy’s great battery at the Royal Institution exploited both.

Polarization

Volta’s piles weakened within hours. Hydrogen bubbles accumulated on the copper, raising the internal resistance and creating a back-emf, and the brine dried out. The problem was solved in 1836 by John Frederic Daniell’s two-fluid cell.

Inside the cell: electrochemistry

For the general reader. Volta believed his pile drew its power from the contact of the metals. Within a few years chemists showed that the zinc slowly dissolved while the pile ran, and that the amount dissolved matched the electricity produced. The pile was a chemical engine. Its invention created electrochemistry.

The reactions

In a zinc–brine–copper cell, zinc is oxidized and hydrogen is evolved at the copper, which is not consumed:

With standard electrode potentials \(E^\circ(\mathrm{Zn^{2+}/Zn})=-0.76\ \mathrm V\) and \(E^\circ(\mathrm{H^+/H_2})=0\), the standard cell emf is about 0.76 V. In neutral brine, with polarization, real cells gave less.

Contact versus chemistry

Volta’s contact theory faced a fatal objection: a steady current dissipates energy, and an equilibrium contact potential has no energy to give. Davy and later Michael Faraday insisted on the chemical origin, and by the mid-nineteenth century the conservation of energy settled the matter. Yet the contact theory was not empty. The emf of a cell can be written as a sum of potential jumps at every interface, metal–metal and metal–electrolyte. The metal–metal jump is Volta’s contact potential; it is real, and it is included in the total. Only the sum around the whole circuit is measurable, and that sum is fixed by \(\Delta G\).

Airs: methane, the eudiometer and the expansion of air

For the general reader. Volta was also a chemist of gases. He discovered methane, invented a way to measure how much oxygen an air sample contains by exploding it with hydrogen, and showed in 1793 that air expands by the same amount for each degree of heating. This last result is now usually credited to later French scientists.

The eudiometer

Volta’s eudiometer was a graduated glass tube, closed at the top and standing over water, with two wires sealed into it so that a spark could be passed. A measured volume of air was mixed with an excess of inflammable air (hydrogen) and fired. Water formed and condensed, and the gas volume shrank.

This follows from Avogadro’s law, which Volta did not have; he calibrated empirically. Equal volumes of gas contain equal numbers of molecules, three volumes of gas disappear for each volume of oxygen, and the water occupies negligible volume.

Uniform expansion (1793)

Earlier measurements of the thermal expansion of air disagreed, partly because samples contained water vapour. Volta took care to dry his air and found that it expands by equal amounts for equal steps of temperature.

Paris, Napoleon and honours, 1794–1814

In 1794 the Royal Society awarded Volta its Copley Medal for his papers on Galvani’s experiments. On 22 November of the same year he married Teresa Peregrini, from a Como family. They had three sons: Zanino (1795–1869), Flaminio (1796–1814) and Luigi (1798–1876).

The 1790s were turbulent for Lombardy. French armies under Napoleon Bonaparte took Milan in 1796; Austrian and Russian forces returned in 1799 and the University of Pavia was closed for a time; the French came back in 1800. Volta built his first pile late in 1799 and sent his description to Banks in March 1800. The response was immediate. William Nicholson and Anthony Carlisle used a pile to decompose water into hydrogen and oxygen within weeks. Humphry Davy took up electrochemistry and by 1807–08 had isolated potassium, sodium, calcium, barium, strontium and magnesium.

Paris, 1801

In autumn 1801 Volta travelled to Paris and presented his work to the Institut de France. Napoleon, then First Consul, attended the session in November, had him awarded a gold medal, and became an enthusiastic supporter. Volta became a foreign associate of the Institut. Honours from the Napoleonic Kingdom of Italy followed: membership in its national institute, the Legion of Honour (1805), a seat in the Senate (1809) and the hereditary title of count (1810).

Volta did not seek political influence and spent little time in the Senate. His scientific output slowed after 1801, though he continued to teach and to defend his contact theory against the growing chemical school.

Last years and legacy, 1814–1827

His son Flaminio died in 1814 at the age of eighteen. After Napoleon’s fall, Lombardy returned to Austria. The new government kept Volta in office and placed him at the head of the philosophical faculty at Pavia. In 1819 he retired from public life and divided his time between Como and his country house at Camnago. He died in Como on 5 March 1827, just after his 82nd birthday, and was buried at Camnago.

Legacy

• The battery: the first source of continuous current. Every electrochemical cell descends from the pile.
• New science: the pile led directly to electrolysis (1800), the isolation of the alkali and alkaline-earth metals by Davy (1807–08), Hans Christian Ørsted’s discovery that currents deflect a compass (1820), André-Marie Ampère’s electrodynamics, Georg Simon Ohm’s law (1827) and Faraday’s laws of electrolysis (1833–34).
• Concepts: tension and capacity, the precursors of potential and capacitance, and the habit of quantitative electrical measurement.
• The volt: in 1881 the International Electrical Congress in Paris named the practical unit of electromotive force after him.
• Memorials: the Tempio Voltiano, a museum on the Como lakefront built for the centenary of his death, holds his instruments and relics. The international physics congress held at Como in September 1927 for the centenary of his death was the occasion on which Niels Bohr first presented his idea of complementarity.

Connections

People linked to Alessandro Volta across this collection. A name without a link has no page here yet. See Alessandro Volta in the network of all 337 lives →

Influenced by

• Benjamin Franklin
• Henry Cavendish
• Giambattista Beccaria
• Jean-Antoine Nollet

Influenced

• André-Marie Ampère
• Georg Simon Ohm
• Hans Christian Ørsted
• Humphry Davy
• Jöns Jacob Berzelius
• Michael Faraday

Colleagues and collaborators

• Alexander von Humboldt
• Antoine Lavoisier 1782
• Joseph Priestley 1775
• Pierre-Simon Laplace 1782
• Georg Christoph Lichtenberg 1787-1789
• Lazzaro Spallanzani

Rivals

• Luigi Galvani 1791-1798

Friends

• Horace-Bénédict de Saussure 1777-1799

Also mentioned in this life

• Amedeo Avogadro
• Niels Bohr
• Charles-Augustin de Coulomb
• Enrico Fermi
• Galileo Galilei
• Carl Friedrich Gauss
• Joseph Louis Gay-Lussac
• Josiah Willard Gibbs
• William Thomson, Lord Kelvin

Fields

• electricity 5 lives
• electrochemistry 7 lives
• experimental physics 16 lives

Sources and further reading

Biography

1. Alessandro Volta, Wikipedia; and the more detailed Italian article, Alessandro Volta, Wikipedia (Italian).
2. Museo Galileo, Florence, Alessandro Volta: biography.
3. Alessandro Volta, Encyclopaedia Britannica.
4. G. Pancaldi, Volta: Science and Culture in the Age of Enlightenment, Princeton University Press, 2003.
5. M. Pera, The Ambiguous Frog: The Galvani–Volta Controversy on Animal Electricity, Princeton University Press, 1992.
6. J. L. Heilbron, Electricity in the 17th and 18th Centuries, University of California Press, 1979.

Physics

1. A. Volta, On the electricity excited by the mere contact of conducting substances of different kinds, Phil. Trans. R. Soc. 90 (1800) 403–431. Journal page.
2. A. Volta, Del modo di render sensibilissima la più debole elettricità sia naturale, sia artificiale, Phil. Trans. R. Soc. 72 (1782).
3. L. Galvani, De viribus electricitatis in motu musculari commentarius, Bologna, 1791.
4. Voltaic pile, Wikipedia.
5. Volta potential and Electrophorus, Wikipedia.
6. A. J. Bard and L. R. Faulkner, Electrochemical Methods, 2nd ed., Wiley, 2001 (interfacial potentials, Nernst equation).
7. E. M. Purcell and D. J. Morin, Electricity and Magnetism, 3rd ed., Cambridge University Press, 2013 (capacitance, energy and forces on conductors).