Joseph Henry

How to use this course

Joseph Henry was the most important American experimental physicist between Benjamin Franklin and Josiah Willard Gibbs. Working in a school basement in Albany, he built the strongest electromagnets of his time, found the rule that matches a magnet’s winding to its battery, sent signals along a mile of wire, and discovered self-induction. At Princeton he showed that the discharge of a Leyden jar oscillates. He then spent his last thirty-two years shaping American science as first Secretary of the Smithsonian Institution. The unit of inductance carries his name.

Each technical lesson opens with a paragraph for the general reader, then gives precise statements in modern units, derivations and exercises with folded solutions. Henry worked before Ohm’s law was known in America and long before Maxwell’s equations. The lessons show how his empirical rules fit the later theory.

Timeline

Albany and Galway, 1797–1819

Joseph Henry was born in Albany, New York, on 17 December 1797, according to the date usually given; some family evidence suggests 1799. His parents, William Henry and Ann Alexander, were Scottish immigrants of modest means. His father died while he was still a child, and the boy was sent to live with his grandmother in Galway, a village in Saratoga County, where he attended the district school and worked in a general store.

Back in Albany, at about thirteen, he was apprenticed to a watchmaker and silversmith. The trade gave him skill with small mechanisms. His real passion at this time was the theatre. He joined an amateur company, wrote and staged plays, and seriously considered a career as an actor.

The turn came at about sixteen, when he picked up a book of Popular Lectures on Experimental Philosophy, Astronomy, and Chemistry left by a lodger. Its questions about why a stone falls and a flame rises seized him. He later said the book opened a new world. He resolved to educate himself, and attended night school to prepare for the Albany Academy.

The Albany Academy, 1819–1830

In 1819 Henry entered the Albany Academy, then one of the best secondary schools in the state, without paying tuition. He supported himself by teaching in a country school and by private tutoring, and assisted in the Academy’s chemistry lectures.

In 1824 he worked as an assistant engineer on the survey of a state road from the Hudson River to Lake Erie through the southern tier of New York. The outdoor work and the responsibility suited him, and he might have become an engineer. In 1826 he was instead appointed professor of mathematics and natural philosophy at the Academy. His teaching load was heavy, and his research had to fit into the summer weeks when the school was closed. One of his pupils in these years was the future novelist Herman Melville.

In 1830 he married Harriet Alexander, his cousin. Her brother Stephen Alexander became an astronomer at Princeton and later worked with Henry on solar heat.

Magnets and sparks, 1827–1832

In 1820 Hans Christian Ørsted had shown that an electric current deflects a compass needle. William Sturgeon in England then bent a varnished iron bar into a horseshoe and wrapped a few turns of bare wire round it, making the first electromagnet. Henry read of these results and improved on them at once. On 10 October 1827 he presented his first paper, on modifications of electromagnetic apparatus, to the Albany Institute.

His key step was to insulate the wire instead of the iron, wrapping copper wire in silk. Insulated wire could be wound in many layers. By about 1830 his Albany magnet supported 750 pounds. In January 1831 he published in Benjamin Silliman’s American Journal of Science the principles of winding, and later that year he built for Silliman at Yale a magnet weighing about 82 pounds that supported 2,063 pounds, the strongest in the world. In 1831 he also built a rocking electromagnetic engine and rang a bell through about a mile of wire strung round a room of the Academy.

Henry was also looking for the reverse effect, an electric current produced by magnetism. His later accounts say he observed induced currents in Albany, but his teaching duties kept him from completing and publishing the work. In 1832 he learned that Michael Faraday had announced the discovery of electromagnetic induction in November 1831. He hurried his own results into print in the American Journal of Science in 1832, including a new effect: a long wire gives a bright spark when its circuit is broken. This was self-induction. In November 1832 he moved to the College of New Jersey at Princeton as professor of natural philosophy.

Electromagnets and the winding rule

For the general reader. An electromagnet is iron wrapped in a coil of wire. More turns and more current make a stronger magnet, but a longer wire also resists the current more. Henry found by experiment that the best winding depends on the battery. A single large cell works best with many short coils in parallel, which he called a quantity magnet. Many cells in series work best with one long coil, an intensity magnet. This rule is a case of what engineers now call impedance matching.

Ampere-turns and the magnetic circuit

For a closed iron path of length \(\ell_{\text{Fe}}\) and permeability \(\mu=\mu_r\mu_0\) with an air gap of length \(g\), Ampère’s law \(\oint\mathbf H\cdot d\mathbf l=NI\) gives, ignoring leakage,

Since \(\mu_r\) is in the thousands until iron saturates near 2 T, a small gap dominates. When the armature touches the poles the gap closes and the field rises toward saturation. The strength of the magnet is controlled by the product \(NI\).

Derivation. The magnetic energy density in the gap is \(B^2/(2\mu_0)\). Opening the gap by \(dx\) at constant flux adds energy \(\frac{B^2}{2\mu_0}A\,dx\), which must equal the work \(F\,dx\) done against the attraction.

Proof. Filling a fixed volume with \(N\) turns requires wire of cross-section proportional to \(1/N\) and length proportional to \(N\), so \(R\propto N^2\). Setting the derivative of \(f(N)=N/(r+cN^2)\) to zero gives \(r+cN^2-2cN^2=0\), so \(cN^2=r\). A battery of \(n\) cells in series has \(r\) multiplied by \(n\), so it calls for a coil with many more turns of finer wire, Henry’s intensity magnet. One large cell with small \(r\) calls for few turns of thick wire, or several coils in parallel, his quantity magnet.

The rocking motor

For the general reader. In 1831 Henry described a machine that turned electrical energy into continuous motion. An electromagnet balanced like a seesaw rocked between two permanent magnets. As it tipped, wires on its ends dipped into cups of mercury and reversed the current, so the magnet was pushed back the other way. It was a toy, but it contained the essential idea of every direct-current motor: a switch that reverses the current at the right moment.

The machine

The device, described in the American Journal of Science in 1831 as a reciprocating motion produced by magnetic attraction and repulsion, rocked about 75 times a minute. Each end of the horizontal electromagnet was attracted by one permanent pole and repelled by the other. The mercury cups acted as a commutator, reversing the polarity at each end of the swing. Revolving electromagnetic engines followed in the next few years, built by William Ritchie in England, Moritz Jacobi in Russia and Thomas Davenport in Vermont.

Derivation. The moving coil cuts magnetic flux, so by Faraday’s law it generates an EMF proportional to its speed that opposes the current (Lenz’s rule). Energy conservation fixes the mechanical power: the source delivers \(\mathcal EI\), of which \(I^2R\) is lost as heat and the remainder \(\mathcal EI-I^2R=\varepsilon I\) does work. The quadratic \(\varepsilon(\mathcal E-\varepsilon)\) peaks at its midpoint. Higher efficiency is possible only by running the motor faster and drawing less power. This back-EMF, not understood at first, explained why early motors drew so much less current when running than when stalled.

The telegraph and the relay

For the general reader. An electric telegraph needs a current strong enough to work a magnet at the far end of a long wire. Early experimenters found that the effect died away after a few hundred feet and concluded that long-distance telegraphy was impossible. Henry showed in 1831 that a battery of many cells in series, driving an intensity magnet, could work through a mile of wire. He later used a relay, a small magnet that closes a second local circuit, so a weak signal could control a strong one. Both ideas were essential to the telegraphs of Samuel Morse and of Charles Wheatstone.

Henry did not know Georg Ohm’s law of 1827, but his rule was equivalent: a long, high-resistance line needs an intensity battery and an intensity magnet with many turns of fine wire. The receiving magnet should have resistance comparable to the line to make best use of the current, by the matching argument of Lesson 04.

The relay

A relay is an electromagnet that pulls an armature to close a contact in a second circuit with its own battery. The line current only has to move a light armature; the local circuit can then drive a heavy sounder or recording instrument, or feed the next section of line. Henry used such an arrangement at Princeton by 1835 to work a large magnet from a distance. In a chain of relays the signal is regenerated at each station, so the range is unlimited in principle.

Overhead lines like Henry’s had small capacitance, and Ohm’s law described them well. Submarine cables, with the sea as an outer conductor, had large capacitance. Their signalling speed falls as the square of the length, a fact that shaped the Atlantic cable projects of the 1850s and 1860s.

Induction and self-induction

For the general reader. A changing magnetic field drives a current in a nearby wire. Faraday published this in 1831, and Henry made the same discovery independently. Henry also noticed something new. A current in a long wire, especially one wound in a coil, resists being switched off. At the moment the circuit breaks, the coil produces a large voltage and a bright spark. The wire induces a current in itself. Today this property is called inductance, measured in henries.

Derivation. In a long solenoid \(B=\mu_0NI/\ell\), so the flux linking all \(N\) turns is \(N\cdot BA=\mu_0N^2AI/\ell\). The energy follows from the work done against the back-EMF while the current rises: \(W=\int_0^I LI'\,dI'=\tfrac12LI^2\). The factor \(N^2\) explains Henry’s finding that coiling a wire strengthens the effect: each turn links the flux of every other turn.

The spark at break

When the circuit is made, the current rises smoothly and nothing dramatic happens. When it is broken, the current must fall from \(I\) to zero in the very short time \(\Delta t\) that the contact takes to separate, so the voltage across the gap is roughly \(LI/\Delta t\). This can reach hundreds of volts from a single cell. The stored energy \(\tfrac12LI^2\) is released in the spark. Henry described this in 1832. In later experiments at Princeton he showed that a person holding the two ends of a long flat ribbon coil felt a shock at break from a battery too weak to give any shock directly. The same principle drives the ignition coil of a petrol engine.

Mutual induction and currents of higher order

For the general reader. At Princeton, Henry studied what happens when the brief current induced in one coil is itself passed through a third coil, and so on down a chain. He called these the induced currents of the second, third and higher orders. He found that their directions alternate and that each coil can transform a large current at low voltage into a small current at high voltage, or the reverse. These are the working principles of the transformer.

Proof of the inequality. Regard \(W\) as a quadratic form in \((I_1,I_2)\). It is the energy of the magnetic field, \(\int B^2/(2\mu_0)\,dV\), so it can never be negative. A quadratic form \(aI_1^2+2bI_1I_2+cI_2^2\) is non-negative for all values exactly when \(a,c\ge0\) and \(b^2\le ac\). Here \(a=L_1/2\), \(b=M/2\), \(c=L_2/2\), which gives \(M^2\le L_1L_2\).

Henry’s cascade

In his “Contributions to Electricity and Magnetism” read to the American Philosophical Society from 1838 to 1842, Henry used flat coils of copper ribbon (few turns, low resistance) and long coils of fine wire (many turns). A short-coil primary fed by a battery, broken suddenly, induced in a long secondary a current of high intensity that gave strong shocks; a ribbon secondary gave instead a large current of low intensity. He then passed the secondary current through another coil to induce a tertiary current, and so on, and found that the directions of successive orders alternate. He also showed that a conducting plate placed between two coils screens the induction, because currents induced in the plate oppose the change.

The oscillatory discharge

For the general reader. When a charged Leyden jar, the first capacitor, discharges through a wire, the spark looks like a single flash. Henry found in 1842 that the current actually surges back and forth many times while dying away, like a plucked string. He also found that one spark could magnetize needles in a cellar thirty feet below, through two floors. These were the first glimpses of electrical oscillations and of their action at a distance, which Heinrich Hertz turned into the discovery of radio waves in 1887–88.

The needles

A steel needle placed inside a small coil is magnetized by a current pulse through the coil. Félix Savary had noticed in 1827 that the polarity of needles magnetized by a Leyden jar discharge varied unpredictably with distance and conditions. Henry repeated and extended these experiments at Princeton. In his paper dated 17 June 1842, on induction from ordinary electricity and the oscillatory discharge, he concluded that the discharge consists of a principal current in one direction followed by reflex actions back and forth, each weaker than the last, until equilibrium is reached. The needle keeps the polarity set by the last half-cycle strong enough to magnetize it, so that polarity depends on the circuit. William Thomson gave the mathematical theory in 1853, and Wilhelm Feddersen photographed the oscillating spark with a rotating mirror in 1858–1862.

Derivation. Kirchhoff’s voltage law round the loop gives the equation, with \(I=\dot q\). Trying \(q=e^{st}\) gives \(Ls^2+Rs+1/C=0\), so \(s=-\gamma\pm\sqrt{\gamma^2-1/(LC)}\). The roots are complex exactly when \(\gamma^2<1/(LC)\), the stated condition. The initial conditions \(q(0)=q_0\), \(\dot q(0)=0\) fix the constants. The quality factor \(Q=\omega_0/(2\gamma)=\frac1R\sqrt{L/C}\), with \(\omega_0=1/\sqrt{LC}\), counts roughly how many radians of oscillation occur while the energy falls by a factor \(e\).

Action at a distance

Henry also reported that a single spark about an inch long, drawn from an electrical machine in an upper room, magnetized needles in a circuit in the cellar about thirty feet below, through two floors each over a foot thick. In 1851 he described the discharge as a series of rebounds propagated outward, like waves in air. Thomson, in the preface to the 1893 English edition of Hertz’s papers, wrote that Henry had come close to demonstrating electromagnetic waves before anyone else.

Sunspots, sound and weather

For the general reader. Henry’s later research was spread across many fields, often in the service of public tasks. With his brother-in-law Stephen Alexander he used a thermopile on the image of the Sun and found that sunspots are cooler than the surrounding surface. For the Lighthouse Board he studied why fog signals are sometimes heard far away and sometimes not at all, and traced much of the effect to wind and temperature bending sound. Through the Smithsonian he organized telegraphed weather reports, the forerunner of the national weather service.

Sunspots

In the 1840s Henry and Alexander projected an image of the Sun onto a screen at Princeton and placed a thermopile on different parts of it. The sunspots gave a smaller thermoelectric current than the bright surface around them. By the Stefan–Boltzmann law, discovered decades later, the radiant flux varies as \(T^4\), so even a modest temperature difference gives a clear contrast.

Sound in a moving, stratified atmosphere

Interpretation. Wind is usually slower near the ground. Downwind, the effective sound speed increases with height and rays curve back down, so sound carries far. Upwind, rays curve upward, leaving a shadow zone at the surface. George Gabriel Stokes had suggested this explanation in 1857. Henry tested it in a long series of trials on fog signals for the Lighthouse Board and argued that wind refraction explained most of the abnormal cases. Tyndall, investigating for Trinity House in 1873, emphasized instead acoustic clouds of irregular temperature and humidity. Both effects are real.

Weather by telegraph

In 1849 Henry began the Smithsonian’s meteorological project. Volunteer observers across the country recorded weather daily, and by the 1850s telegraph companies sent reports to the Smithsonian, where a map showed conditions across the nation each day. Because weather systems in middle latitudes usually travel eastward, reports from the west gave warning of what was coming. The system was suspended during the Civil War and became the model for the national weather service set up in 1870.

Princeton and the Smithsonian, 1832–1878

At the College of New Jersey Henry taught natural philosophy and also chemistry, geology and architecture, and built up the laboratory. He later wrote that he had made several thousand original investigations there. His laboratory assistant for many years was Sam Parker, a free Black man who kept the batteries and apparatus in working order; Henry’s letters also show that he used Parker as a subject in classroom demonstrations of electric shocks. In 1837 Henry travelled to Europe. In London he met Faraday and Charles Wheatstone, and the three men compared experiments. He was impressed by the European scientific establishment and came home determined to raise the standard of American research.

The telegraph brought him into a long and painful dispute with Samuel Morse. Henry had advised Morse freely in the late 1830s and in 1842 wrote in support of a federal grant for Morse’s experimental line. When Morse later claimed sole credit, Henry testified in patent litigation in 1849 that the scientific principles were his own, and Morse attacked him in a pamphlet in 1855. Courts and historians have generally sided with Henry on the science, while crediting Morse and Alfred Vail with the practical system and code.

In 1846 Congress finally established the Smithsonian Institution with the bequest of the English chemist James Smithson, and on 3 December 1846 the Board of Regents elected Henry its first Secretary. Smithson’s will called for “the increase and diffusion of knowledge”. Henry read this to mean original research and its publication, and he resisted pressure to spend the income on a large library and museum. He started the series Smithsonian Contributions to Knowledge in 1848, organized the international exchange of scientific publications, and supported research in meteorology, natural history and ethnology. The family lived in the Smithsonian building. On 24 January 1865 a fire there destroyed many of his papers and Smithson’s own effects.

During the Civil War Henry advised the federal government on technical matters and supported Thaddeus Lowe’s balloon reconnaissance. His record on race was mixed: he allowed abolitionist lectures in the Smithsonian lecture hall in 1861–62 but refused to permit Frederick Douglass to speak there. From 1852 he served on the Lighthouse Board, testing illuminants and fog signals, and from 1871 he was its chairman.

The Academy, death and legacy

Henry was a founding member of the National Academy of Sciences in 1863. He was elected its vice-president in 1866 and, in 1868, its president after the death of Alexander Dallas Bache. He held the office until his death. In 1871 he became the first president of the Philosophical Society of Washington.

On 1 March 1875 the young Alexander Graham Bell, then working on multiple telegraphy, called on Henry at the Smithsonian. Henry told him that his idea for transmitting speech electrically contained the germ of a great invention and urged him to pursue it, even though Bell said he lacked the electrical knowledge. In 1876 Henry was among the judges who saw the telephone at the Centennial Exhibition in Philadelphia.

Henry died in Washington on 13 May 1878 after a short illness, and was buried in Oak Hill Cemetery. Congress held a memorial service in his honour. A bronze statue by William Wetmore Story was unveiled in front of the Smithsonian building in 1883.

Legacy

• Electrical engineering: the multi-layer electromagnet, the matching of coil and battery, the relay, and the demonstration that telegraphy over long lines was possible.
• Physics: independent discovery of electromagnetic induction, the discovery of self-induction, the higher-order induced currents behind the transformer, and the oscillatory discharge that underlies radio.
• Units: in 1893 the International Electrical Congress at Chicago named the practical unit of inductance the henry; it is now an SI unit, \(1\ \mathrm H=1\ \mathrm{V\,s\,A^{-1}}\).
• Institutions: the Smithsonian’s research mission, a national system of weather observation, the Lighthouse Board’s scientific programme, and a strong National Academy of Sciences.

Connections

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

Influenced by

• André-Marie Ampère
• George Gabriel Stokes
• Hans Christian Ørsted
• Félix Savary
• William Sturgeon

Influenced

• Heinrich Hertz
• Alexander Graham Bell 1875

Colleagues and collaborators

• Charles Wheatstone 1837
• John Tyndall
• William Thomson, Lord Kelvin
• Benjamin Silliman 1831

Rivals

• Michael Faraday
• Samuel Morse

Friends

• Alexander Dallas Bache

Family

• Stephen Alexander

Also mentioned in this life

• Ludwig Boltzmann
• Benjamin Franklin
• Josiah Willard Gibbs
• Carl Gustav Jacob Jacobi
• Gustav Kirchhoff
• James Clerk Maxwell
• Georg Simon Ohm

Fields

• electromagnetism 12 lives
• electrical engineering 4 lives
• acoustics 9 lives
• meteorology 6 lives

On CoursesHub

• Classical Electrodynamics

Sources and further reading

Biography

1. Joseph Henry, Wikipedia (with references).
2. Smithsonian Institution Archives, The Joseph Henry Papers Project: Henry’s electromagnetic work.
3. Smithsonian Institution Archives, Henry constructs a large electromagnet for Benjamin Silliman and Henry’s tenure at Princeton.
4. Princeton University, Joseph Henry project, pages on self-inductance and the oscillatory discharge.
5. A. E. Moyer, Joseph Henry: The Rise of an American Scientist, Smithsonian Institution Press, 1997.
6. T. Coulson, Joseph Henry: His Life and Work, Princeton University Press, 1950.
7. N. Reingold et al. (eds.), The Papers of Joseph Henry, 12 vols., Smithsonian Institution, 1972–2008.
8. Joseph Henry, New World Encyclopedia.

Physics

1. J. Henry, On the application of the principle of the galvanic multiplier to electro-magnetic apparatus, Am. J. Sci. 19 (1831).
2. J. Henry, On a reciprocating motion produced by magnetic attraction and repulsion, Am. J. Sci. 20 (1831).
3. J. Henry, On the production of currents and sparks of electricity from magnetism, Am. J. Sci. 22 (1832).
4. J. Henry, Contributions to electricity and magnetism, Trans. Am. Phil. Soc. (1835–1843).
5. J. Henry, On induction from ordinary electricity; and on the oscillatory discharge, Proc. Am. Phil. Soc. 2 (1842).
6. Scientific Writings of Joseph Henry, 2 vols., Smithsonian Institution, 1886.
7. W. Thomson, On transient electric currents, Phil. Mag. 5 (1853).
8. H. Hertz, Electric Waves, transl. D. E. Jones, with a preface by Lord Kelvin, Macmillan, 1893.
9. E. M. Purcell and D. J. Morin, Electricity and Magnetism, 3rd ed., Cambridge University Press, 2013 (chapters on induction and AC circuits).