The Hall Effect

One of the most underappreciated achievements in modern physics, which led to an indispensable tool for measuring the strength of electromagnetic fields, happened 140 years ago at Johns Hopkins.

On an autumn day in 1879, Edwin Hall was at his bench in Johns Hopkins University’s physics lab, a converted kitchen off Baltimore’s North Howard Street called “the Annex.” The third-year graduate student, just 23, had set two electromagnets facing each other and slid a tiny strip of gold foil carrying an electric current in the gap. He used a sensitive galvanometer to measure the voltage at different points in the strip. After months of work, he finally saw what he was looking for: a jump in the instrument’s needle indicating that the magnetic field had pushed the electric current toward one side of the strip. He had observed a voltage in a direction that was perpendicular to both the magnetic field direction and the applied current.

Edwin Hall’s discovery that Tuesday in late October of 1879, which came to be called the Hall effect, may be one of the most underappreciated achievements in the history of Hopkins. Over the past 140 years, the Hall effect has become an indispensable tool for measuring the strength of electromagnetic fields.

When Hall’s paper was published, it was hailed as a major advance in the basic science of magnetism and electricity. He had shown, contrary to what was assumed, that the carrier of the electric current carried a negative rather than a positive charge. His discovery was compared to the greatest achievements of Michael Faraday, who in 1845 produced the first experimental evidence that light and magnetism were related. Hall’s work also challenged the views of the eminent British physicist James Clerk Maxwell, whose landmark Treatise on Electricity and Magnetism had been published six years earlier.

Hall also helped demonstrate that, just three years after Hopkins opened its doors, its radical, research-oriented approach to American higher education was already bearing fruit.

But the discovery didn’t come into its own until the rise of semiconductor and transistor technology in the mid-20th century, when sensors based on the Hall effect, that detect and measure magnetic fields, became smaller and more powerful. Today, billions of these sensors are in use worldwide to measure the position, speed, and proximity of one thing relative to another. They’ve been wired into car crankshafts, tachometers, anti-lock brakes, satellites, smart phones, burglar alarms, computer keyboards, disk drives, joysticks, GPS systems, go-kart speed controls, electronic tavern beverage dispensers, and a thick catalog of other devices. New applications are on the horizon. Scientific laboratories around the world are building on Hall’s early work. Since 1985, four Nobel Prizes have been awarded for research related to the Hall effect.

Hall noted in his paper that “a more complete and accurate study of the phenomenon … will probably occupy me for some months to come.” In fact, he would study the Hall effect for much of the rest of his life.

… there are a few billion of us using [Hall sensors] in everyday life and it all started with Hall here at Johns Hopkins University.

—N. Peter Armitage

N. Peter Armitage, professor in the Department of Physics and Astronomy, estimates that today tens of thousands of scientists use Hall sensors in the lab or are directly involved in research related to the Hall effect. “Then there are a few billion of us using them in everyday life,” he says, “and it all started with Hall here at Johns Hopkins University.”

photo of Peter Armitage
N. Peter Armitage

In early 1880, Hall was back in the Annex following up on his first discovery by substituting iron and other ferromagnetic metals for the gold foil in his apparatus. He found a bigger electrical response that sometimes produced a positive charge. This second enigmatic phenomenon, called the “anomalous Hall effect,” would not be fully understood until the 21st century.

Hall’s second great discovery would baffle scientists for decades. It seemed so perverse because it was a product of the fundamental nature of the atom. Scientists only began to fully grasp the behavior of subatomic particles in the late 1920s, with the development of quantum mechanics. According to quantum theory, particles are also waves, energy is organized into discrete packets called “quanta,” and both the location and momentum of a particle can’t be measured at the same time.

“The anomalous Hall effect is an intrinsically quantum mechanical thing,” Armitage says. “How it all worked wasn’t really understood until the last 10 years.”

In 1979, Chia-Ling Chien, the Jacob L. Hain Professor of Physics at Johns Hopkins, organized a symposium on the 100th anniversary of Hall’s historic discovery and later co-edited a book with Charles Westgate, now professor emeritus, titled The Hall Effect and Its Applications. As it turned out, that centenary roughly coincided with the start of a renaissance in research related to Hall’s second great discovery, the anomalous Hall effect.

Today, Chien spends much of his time studying a phenomenon called the spin Hall effect. (Electrons have “spin,” which is very roughly analogous to the spin of a gyroscope.) Eventually, Chien says, he hopes his work will lead to the development of permanent, high-density data storage devices or powerful new quantum computers.

“That’s where the future lies,” he says.

Mentor and Protégé

Edwin Herbert Hall was born six years before the Civil War and grew up in what is now the town of Gorham, Maine, where his father was a farmer, town selectman, and justice of the peace. At the age of 18, Edwin graduated at the head of his class from Bowdoin College, where he served as editor of the college newspaper, and began working as a high school principal. He considered studying law or going into journalism but turned to physics, he told a friend, “because it was progressive and satisfied my standards of intellectual and moral integrity.”

photo of Chia-Ling Chien
Chia-Ling Chien

Friends and colleagues described Hall as a talented but modest man, subject to fits of depression. He sometimes sounded haunted by self-doubt. “I am in some respects distinctly handicapped in all my scientific endeavours, being unskilful of hand and slow of apprehension,” he said. “On the other hand, I am very persistent, and fond of wrestling with a difficult problem in my own slow way; any success I may have attained is to be attributed to these two qualities.”

In his essay in The Hall Effect and Its Applications, the late Owen Hannaway, a Hopkins professor and historian of science, wrote that Hall’s brutally candid self-evaluation and lofty ideals might sound odd to modern Americans, “but science as progress with moral integrity was very much part of the ethos of early Hopkins.” Many faculty members thought American society had been coarsened by the materialism and “unrestrained entrepreneurship” of the post-Civil War’s Gilded Age, Hannaway wrote, and believed it was the university’s mission to “ennoble” society through the practice of what its professors called “pure science.”

Eventually, I hope this work will lead to the development of permanent, high-density data storage devices or powerful new quantum computers. That’s where the future lies.

—Chia-Ling Chien

Hall applied to the physics program at Harvard, which was closer to home, but was told by Harvard physics professor John Trowbridge that he would be better off in Baltimore. Hopkins had opened inauspiciously in 1876 in two former boarding houses along North Howard Street at the western edge of Mount Vernon Place. But at the insistence of the formidable Henry Augustus Rowland, the founder of Hopkins’ physics program, the trustees had shelled out the extravagant sum of $6,429 on instruments and supplies. In 1879, Trowbridge published a survey of university physics labs in America and reported that Rowland’s Annex had almost seven times as much equipment as did Harvard.

When the 21-year-old Hall arrived at Hopkins in the fall of 1877, Rowland, 28, was already one of the most respected physicists in the country.

According to Rowland’s friend and biographer Thomas Mendenhall, he “was often a forbidding figure, intolerant of mediocrity, so devoted to the truth that his frank criticism could be devastating.” Generally, he “gave as little attention as possible to administration and teaching,” focusing on his own experiments and leaving students to find their own way. At the start of one semester, Rowland was asked what he planned to do with his graduate assistants during the academic year.

“Do with them? I intend to neglect them,” he said.

Unsurprisingly, Rowland left a few disgruntled students and annoyed colleagues in his wake. But the ferociously intellectual young professor and the determined Hall seemed to hit it off. Hall would later call Rowland the “greatest of all my teachers,” and add that “my feelings toward him have been for more than twenty years a blending of immense admiration and respect with warm affection.”

Despite this warm relationship, the discovery of the Hall effect may have created tension between mentor and protégé.

In his second year at Hopkins, Hall was troubled while reading James Clerk Maxwell’s two-volume A Treatise on Electricity and Magnetism, published in 1873. Maxwell had predicted that an electric current would not be deflected from its path across a conductor by a magnetic field. To Hall, this didn’t make sense. “This statement seemed to me to be contrary to the most natural supposition in the case considered,” he wrote.

Hall asked Rowland if he could test Maxwell’s claim with an experiment, and Rowland agreed. But the professor may have had qualms about his student’s work. To some extent, Henry Rowland owed his job to Maxwell. The university’s first professor of physics had little or no formal training as a physicist before coming to Hopkins. Instead, he was an enthusiastic and extraordinarily gifted amateur who had basically taught himself physics while earning a civil engineering degree at Rensselaer Polytechnic Institute in Troy, New York.

The winter after his graduation from Rensselaer, Rowland was tinkering in his mother’s kitchen in Newark, New Jersey, when he came up with a mathematical formula similar to Ohm’s law for electric circuits that could help engineers design and build magnetic circuits. Because he lacked academic credentials, he couldn’t find an American publisher for his paper. So the brash young investigator sent a copy to Maxwell at Cambridge University.

Maxwell was so impressed that he arranged to have Rowland’s paper published in Europe. When Hopkins President Daniel Coit Gilman hired Rowland as the school’s first professor of physics in 1875, it was due in large part to his international reputation. And that reputation was due in large part to Maxwell’s support.

In preparation for Hopkins, Rowland made a grand tour of European physics labs, including Maxwell’s, in 1875 and 1876. The two English-speaking scientists seemed to have bonded, personally as well as professionally. Maxwell later wrote a mock-epic poem praising Rowland as “Rowland of Troy, that doughty knight,” “the irrepressible professor” and “Childe Rowland,” who “in busy Baltimore … brews the bantlings [offspring] of his brain.”

Rowland generally subscribed to Maxwell’s theory of electromagnetism set forth in his Treatise, according to historians. But like Hall, he had his doubts about Maxwell’s claim that magnets couldn’t deflect electric currents in metals. He had quietly conducted an experiment similar to the one proposed by Hall, without results.

Armitage says that it should be remembered “how much guts it took” for Hall, “an unknown student at an upstart university,” to challenge the views of Maxwell, one of the leading physicists of his age. Hall’s friend, Percy Williams Bridgman of Harvard, wrote that his “outstanding personal characteristic was his utter honesty and integrity, coupled with an independence and strength of character which enabled him to trust his own judgment and steer his own course once he had made his carefully reasoned decision.”

Credit Where It’s Due?

If Rowland had misgivings about the political wisdom of Hall’s experiment, he did not show it—at least at first. Hall wrote in his paper that Rowland had advised and assisted the work and, crucially, had suggested that Hall use a strip of gold foil instead of another, thicker piece of metal in the apparatus. Without that change, Hall might never have detected the change in voltage. Rowland also arranged for the immediate publication of Hall’s paper.

Then Rowland did something that some of his colleagues would later question. He decided not to put his name on the paper. He never explained why, inspiring persistent speculation over who deserved more credit, the professor or his student.

Rowland would go on to become a founder and the first president of the American Physical Society; he would be showered with international honors and celebrated in the popular press.

By the end of the 19th century, Rowland was considered by many to be the greatest American physicist of his era. “That’s no exaggeration,” Chien says. “Nobody else comes close to his achievement. If the Hall effect was attached to his name, it would be even greater. Unfortunately, he let Hall have all the credit. And the Hall effect turned out to be as important, perhaps even more important than some of Rowland’s later work.”

The Hall effect was difficult to reconcile with Maxwell’s Treatise and appeared to support the theories of Maxwell’s rivals. The timing of Hall’s paper, “On a New Action of the Magnet on Electric Currents,” was also awkward. By coincidence, Maxwell died in Cambridge at the age of 58, just nine days after Hall’s initial experiment and less than a month before the publication of “On a New Action” in a Hopkins journal.

Was Rowland wary of being seen as betraying Maxwell, without whose patronage he might never have been hired at Hopkins? Was Rowland skeptical about Hall’s results and anxious to protect his own reputation? Maybe Rowland felt that he didn’t deserve any credit, despite his role as mentor and advisor, because he hadn’t done the work. Or maybe he was confident that he was destined to do great things and didn’t need to put his stamp on every paper published by his students.

“Give me time and apparatus and if our university is not known, it will not be my fault,” Rowland once promised President Daniel Coit Gilman.

Whatever Rowland was thinking, Chien says, it seems likely that if his name had appeared on the paper, the Hall effect would today be called the Rowland effect.

While Hall remained at Hopkins, his opportunities for academic advancement were limited. Under the German university system, which Hopkins adopted, a single professor-for-life reigned over the department. So he accepted a job as an instructor of physics at Harvard, where he would spend the rest of his career.

The year 1880 marked a dynamic new period in Rowland’s research. Prior to the 1880s, he mainly studied electricity and magnetism. Afterward, he shifted his focus to the study of light. He is best known today for his design, production, and use of diffraction gratings, metal plates inscribed with tens of thousands of closely spaced, parallel lines that act like high-precision prisms to split light into its constituent colors. The spectral lines they produced were used to identify the chemical elements in materials and objects, including the sun and stars.

Rowland didn’t invent diffraction gratings, but he produced and used the most advanced, accurate, and exquisitely crafted gratings of their kind. He built more than 100 of them over the years using a machine he designed called a “ruling engine,” which was equipped with Tiffany diamonds to score the metal and a finely machined screw to keep the lines evenly spaced. Eventually, he could cram up to 110,000 lines onto a concave metal plate about the size of a standard sheet of paper. Rowland used his gratings in his own research and sold them at cost to physicists, chemists, and astronomers around the world.

In 1890, Rowland married Henrietta Harrison of Baltimore. Around the same time, he learned that he had diabetes, which was then untreatable. Over the next decade, a time of tremendous progress in physics, Rowland felt forced to take precious time away from his beloved “pure science” to provide for Henrietta and their three children after his death.

He tried selling some of his patents and developed a new multiplex telegraph technology, but the devastating economic panic of 1893–1897 made it difficult to commercialize his work. In the late winter of 1901, Rowland wrote to the Stockholm Committee nominating himself for the first Nobel Prize in Physics, scheduled to be awarded in the fall. It was one last desperate bid to make his research more marketable. The committee coldly informed him that Nobel candidates couldn’t nominate themselves. (They still can’t.)

Rowland died of diabetes at the age of 52 on April 16, 1901. Although the arrangements for his funeral weren’t made public at the time, Rowland had requested that his body be cremated and his ashes interred in a wall of his basement lab at Hopkins’ Physical Laboratory, located on the northwest corner of West Monument Street and Linden Avenue. They were later moved to a Rowland family plot in Newark, New Jersey.

The physicist Albert Michelson, who launched the University of Chicago’s physics program, became the first American scientist to win a Nobel Prize in 1907. According to the historian of science John David Miller, Harvard’s Trowbridge wrote to Henrietta Rowland that if her husband had lived, he would have been the first American science laureate.

Dauntless Persistence

A year after leaving Hopkins, Hall married Caroline Bottum of Vermont, who had worked as his assistant at Brunswick High School in Maine.

He would never again match his scientific achievements in the Annex, but he would help educate generations of American physics students. He launched an effort to improve and overhaul secondary- and college-level physics courses and produced a set of 40 experimental exercises that were published in 1886. They were endorsed by the National Education Association and came to be known as the National Physics Course.

In 1907, Hall suffered what his friend Bridgman called a “severe nervous breakdown” that forced him to take a year off from work. Hall’s son Frederic died of complications of rheumatic fever while a college senior in 1910, further devastating the elder Hall. “Those who knew him more intimately knew that he had passed through dark times of discouragement or even despair, over which he triumphed by sheer force of character,” Bridgman wrote.

After his retirement in 1921, Hall felt frustrated that researchers were still struggling to quantify the Hall effect in different metals and resumed his earlier research. He published results for gold, palladium, cobalt, and nickel in 1925. He worked six hours a day in his lab until just a few weeks before his death in November 1938, at the age of 83, 59 years after his discovery at Hopkins. “The dauntlessness of his experimental attack on this problem was characteristic of the man,” Bridgman wrote.

Like Rowland, Hall never won a Nobel Prize. But more than a half-dozen scientists following in his footsteps have. A Google search for “the Hall effect” yields 360 million results. Bridgman wrote 80 years ago that “the implications of Hall’s discovery have not even yet been adequately appraised.” That’s as true today as it was then.