Edwin Hall

Edwin Herbert Hall (November 7, 1855 – 1938) was an American physicist renowned for discovering the Hall effect in 1879, a key electromagnetic phenomenon that reveals the sign and density of charge carriers in conductors through the generation of a transverse voltage in the presence of a magnetic field and electric current.^[1]^[2] Born November 7, 1855, in Great Falls, Maine, Hall graduated at the top of his class from Bowdoin College in 1875 with an A.B. degree and later earned an A.M. from the same institution in 1878 before pursuing graduate studies at Johns Hopkins University, where he received his Ph.D. in 1880.^[3]^[4]^[5] His doctoral research at Johns Hopkins, under the guidance of Professor Henry A. Rowland, led to the observation of the Hall effect on October 28, 1879, using a thin strip of gold foil in a magnetic field, which he published that year and which provided early evidence that charge carriers in metals are negatively charged electrons.^[1]^[2] After a year of study in Europe, Hall joined the faculty at Harvard University in 1881 as an instructor, rising to full professor in 1895 and to the Rumford Professor of Physics in 1914, serving until his retirement in 1921, during which time he taught for four decades and trained numerous physics educators while conducting research on galvanomagnetic and thermoelectric effects.^[3]^[4]^[5] In 1886, he developed an influential high school physics curriculum adopted by the National Education Association, and later in life, after retiring, he resumed investigations into the Hall effect, publishing findings in 1925.^[1] Hall died on November 20, 1938, in Cambridge, Massachusetts, at the age of 83, leaving a legacy that underpins modern technologies such as magnetic field sensors and quantum Hall effect studies, which earned later Nobel Prizes.^[4]^[3]

Early Life and Education

Early Life

Edwin Herbert Hall was born on November 7, 1855, in Great Falls (later North Gorham), Maine, to Joshua Emery Hall and Lucy Ann Hilborn Hall.^[5] His father, born in 1823, worked as a farmer and lumberer while serving in public roles as a town selectman, Maine legislator, and justice of the peace, instilling in the family a sense of civic duty and discipline.^[5] His mother, born in 1825, had taught in a local district school and worked in a textile factory; she was well-read, with talents in literature and drawing that encouraged intellectual pursuits within the household.^[5] Hall grew up as one of five children in a modest rural family, though tragedy marked his early years: his two sisters and one brother died in infancy or childhood, leaving only Hall and his younger brother, Frederic Winslow Hall (born 1860), to reach maturity.^[5] His father died of abdominal illness—possibly peritonitis—in 1864 at age 41, when Edwin was just nine, further shaping the family's resilience amid economic and emotional hardships.^[5] The household emphasized practical values, with Hall contributing as a farmhand until age 12 and continuing such labor through his boyhood, fostering perseverance in the face of rural Maine's demanding environment.^[5] Early education was limited to the local district school, supplemented by family resources that nurtured self-directed learning and inquiry, though formal preparation for higher studies began later at Gorham Seminary.^[5] This foundational period in rural isolation honed Hall's independent character, paving the way for his academic pursuits.^[5]

Formal Education

Hall received his early formal education at Bowdoin College in Brunswick, Maine, where he enrolled in the fall of 1871 at the age of fifteen. He graduated in 1875 with a Bachelor of Arts degree, having ranked first in his class as valedictorian. During his undergraduate years, Hall balanced his studies with teaching positions in local district schools, gaining practical experience while pursuing a curriculum that included foundational courses in physics and mathematics.^[5] Following a brief period of teaching after graduation, Hall advanced his studies in physics at Johns Hopkins University, entering the graduate program in the fall of 1877. Under the supervision of Henry A. Rowland, the university's newly appointed professor of physics, Hall conducted research that culminated in his doctoral thesis on an experiment performed in 1879, leading to the award of his Ph.D. in 1880. This work was supported by Hall's role as a teaching assistant in the physics department from 1880 to 1881. Additionally, Bowdoin College conferred an A.M. degree on Hall in 1878 in recognition of his early achievements.^[5] Rowland's laboratory at Johns Hopkins served as a conduit for contemporary European advancements in physics, as the professor had toured leading facilities, including James Clerk Maxwell's laboratory in Cambridge, during 1875 and 1876 prior to assuming his position. This exposure influenced the rigorous, experimental approach in Rowland's program, where Hall engaged with cutting-edge techniques and theoretical discussions drawn from British and continental traditions, preparing him for independent research without requiring personal travel abroad at that stage.^[1]^[5]

Professional Career

Early Teaching Positions

After graduating from Bowdoin College in 1875 with a Bachelor of Arts degree, Edwin Hall assumed his first professional role as principal of Gould Academy, a college preparatory school in Bethel, Maine, serving from 1875 to 1876.^[5] In this position, he managed the academy's operations, overseeing staff and curriculum for high school-level students, which provided him with initial administrative and educational experience in a rural setting.^[5] Hall then moved to Brunswick, Maine, where he served as principal of Brunswick High School from 1876 to 1877.^[5] Here, he continued in a leadership capacity, handling both administrative responsibilities and instructional duties, with assistance from Caroline Eliza Bottum, who later became his wife and worked as his teaching aide.^[5] These secondary school roles, while offering practical insights into education, lacked advanced facilities for scientific inquiry, prompting Hall to reflect on his career path. In personal recollections, Hall noted that after two years of school teaching, he pursued science not out of deep passion but because it aligned with his values of intellectual and moral integrity, marking a pivotal shift toward advanced studies.^[5] This period honed his teaching skills amid the demands of administration but highlighted the limitations of such environments for his emerging research interests.^[5]

Career at Harvard University

In 1881, Edwin Herbert Hall joined the faculty of Harvard University as an instructor in physics, marking the beginning of a distinguished academic career that spanned four decades.^[5] He served in this initial role until 1888, during which time he focused on teaching elementary physics with an emphasis on clear, meticulous explanations to foster student comprehension.^[5] In 1888, Hall was promoted to assistant professor, a position he held until 1895, followed by his advancement to full professor from 1895 to 1914.^[5] In 1911, he was elected to the National Academy of Sciences.^[5] These promotions reflected his growing influence within the department and his commitment to advancing physics education. Hall played a key role in developing Harvard's physics curriculum and laboratory instruction during his tenure. In 1886, he authored the Harvard Descriptive List of Elementary Physical Experiments, a pamphlet outlining 40 practical exercises that became a national standard for secondary school physics preparation and admission requirements at Harvard.^[5] This work, aligned with President Charles W. Eliot's initiatives, helped establish rigorous laboratory-based training as essential for undergraduate entry, influencing physics education across the United States for decades.^[5] As a professor, Hall continued to refine these instructional methods, integrating hands-on experimentation into the core of Harvard's physics program. In 1914, Hall assumed the prestigious Rumford Professorship of Physics, a position endowed to support research in heat and light, which he held until his retirement in 1921.^[5] Beyond teaching and curriculum development, he mentored approximately 12 graduate students in experimental physics, cultivating enduring professional relationships and even presenting a silver loving cup to his colleagues in 1906 to commemorate 25 years of service at Harvard.^[5] Notably, during the 1919 Boston police strike, Hall volunteered as one of the first special police officers, serving as a strikebreaker to maintain order in the city.^[5] Upon retirement, he became professor emeritus, continuing his association with Harvard until his death in 1938.^[5]

Scientific Contributions

Discovery of the Hall Effect

In 1879, while pursuing his doctoral studies at Johns Hopkins University, Edwin Hall conducted an experiment to test whether a magnetic field could directly influence the electric current within a fixed conductor, beyond merely inducing electromotive forces at its ends. He prepared a thin strip of gold leaf, about 10 cm long, 1 cm wide, and 0.001 cm thick, mounted flat on a glass plate to ensure rigidity. The strip was connected longitudinally to a battery via mercury pools or brass blocks secured with screws for low-resistance contacts, allowing a steady current to flow along its length. This assembly was positioned between the poles of a large electromagnet, which produced a uniform magnetic field perpendicular to the plane of the strip. To detect any potential difference across the width of the strip, Hall attached fine wires to its edges and connected them to a sensitive Thomson reflecting galvanometer.^[6]^[7] On October 28, 1879, with a current of approximately 1 ampere flowing through the gold strip and a magnetic field of about 0.15 tesla applied, Hall observed a persistent deflection of the galvanometer needle, indicating a transverse voltage of around 3 microvolts across the strip's width. This deflection reversed direction when either the current or the magnetic field was reversed, confirming the effect's dependence on their relative orientations. Hall repeated the measurements with strips of silver, platinum, and iron, finding similar transverse voltages proportional to both the current strength and the magnetic flux density, though the magnitude varied with the material. These observations marked the first experimental evidence of a magnetic field's direct action on charge carriers inside a conductor, rather than on the conductor as a whole.^[6]^[7] Hall's theoretical insight stemmed from considering the current as composed of moving charged particles subject to a magnetic force, analogous to the deflection of charged particles in free space. He reasoned that if the magnetic field attracted or repelled the current's constituent "particles of electricity," it would accumulate positive charges on one side of the strip and negative on the other, creating a transverse electric field that eventually balanced the magnetic deflection in steady state. This provided the first empirical confirmation of magnetic deflection occurring within solid conductors, resolving prior theoretical debates about whether such forces acted only externally. Although the full Lorentz force law was not yet formulated, Hall's prediction aligned with the principle that the steady-state condition equates the magnetic force on carriers to the restoring electric force due to charge separation.^[6]^[1] The resulting Hall voltage V_H arises from this force balance and can be derived as follows. For a conductor with charge carriers of density n and charge e, carrying current I in the x-direction (width w, thickness d) under magnetic field B in the z-direction, the drift speed is v_d = \frac{I}{n e w d}. The magnetic Lorentz force per carrier deflects them in the y-direction with magnitude e v_d B. In equilibrium, this equals the electric force e E_H from the Hall field, yielding E_H = v_d B = \frac{I B}{n e w d}. Thus, the Hall voltage across the width is

V_H = E_H w = \frac{I B}{n e d}.

This expression implies that V_H is independent of the width w and inversely proportional to carrier density n, allowing inference of n from measured V_H if other parameters are known; it also reveals the sign of e from the voltage polarity, distinguishing electron-like from hole-like conduction. In Hall's experiments, the small V_H values underscored the effect's subtlety, requiring precise apparatus to detect.^[8] Hall detailed his findings in the paper "On a New Action of the Magnet on Electric Currents," published in the American Journal of Mathematics in September 1879. The work was recognized as a novel contribution to electromagnetism, demonstrating a new magneto-electric phenomenon, though initial reception included some skepticism regarding possible artifacts like thermo-electric effects or uneven contacts; it did not immediately transform the field, as the underlying nature of charge carriers remained unclear until later discoveries in atomic physics. A follow-up publication in the American Journal of Science in 1880 expanded on measurements with additional metals, solidifying the effect's reproducibility.^[6]^[7]^[1]

Research on Thermoelectricity and Thermal Conductivity

Following his discovery of the Hall effect, which secured his position at Harvard University, Edwin Hall turned to investigations of thermal and electrical interactions in metals during the 1880s and 1890s. His studies on thermoelectric action primarily involved precise measurements of the Seebeck and Peltier effects, where temperature differences or currents at metal junctions generate voltages or heat absorption. Hall employed specialized apparatus to induce and quantify temperature gradients along metal samples, such as bismuth and iron, allowing him to record electromotive forces with high accuracy under controlled conditions. For instance, in his 1893 work, he described a thermo-electric method using cylindrical conductors to study heat flow and associated voltage generation, enabling isolation of thermoelectric phenomena from other conductive losses. Hall's measurements revealed consistent patterns in the Seebeck coefficient, which quantifies voltage per unit temperature difference, across various metals, and he extended this to Peltier heat effects at junctions. In a 1921 analysis, he calculated Peltier coefficients for junctions involving bismuth and other metals at 0°C and 100°C, showing how heat absorption relates to electron transport without invoking magnetic influences. These experiments integrated empirical data with broader electromagnetic principles, positing that thermoelectric voltages arise from differential electron mobilities in response to thermal gradients. Hall's apparatus innovations, including the use of compensated thermocouples to minimize extraneous voltage errors, enhanced measurement precision and became a methodological standard for subsequent thermoelectric research. Parallel to thermoelectric studies, Hall conducted extensive experiments on thermal conductivity, focusing on heat flow through wires and thin foils of conductors under steady-state conditions. He quantified thermal conductivity coefficients by applying known heat inputs and measuring temperature profiles along samples, often using platinum resistance thermometers for calibration. His 1892 publication detailed measurements on cast iron and nickel wires, reporting thermal conductivities of approximately 0.12 cal/cm·s·°C for cast iron and 0.05 for nickel at room temperature, highlighting variations due to impurities and microstructure.^[9] Hall's work uncovered anomalies in thermal conductivity for platinum and iron, where values deviated from expected linear temperature dependence. For platinum foils, he found conductivity increasing slightly from 0.164 cal/cm·s·°C at 0°C to 0.173 at 100°C, contrary to simpler metallic models, while soft iron exhibited a decrease from 0.163 to 0.151 cal/cm·s·°C over the same range, attributed to lattice imperfections and electron scattering. In his 1900 study on soft iron, Hall used elongated wire samples to probe these effects, correlating thermal data with electrical resistivity to test conduction theories. These findings, published in 1921 as a comprehensive summary, linked thermal conduction to a dual electron model—free and bound carriers—without magnetic fields, influencing later interpretations of metallic heat transport.^[10]

Publications and Later Life

Key Publications

Edwin Hall's most seminal publication was his 1879 paper, "On a New Action of the Magnet on Electric Currents," published in the American Journal of Mathematics, which detailed his experimental discovery of the Hall effect.^[5] This work, originating from his doctoral thesis at Johns Hopkins University, demonstrated the transverse voltage generated by a magnetic field perpendicular to an electric current in a conductor, laying foundational groundwork for later developments in electromagnetism and solid-state physics.^[5] Hall co-authored several influential textbooks that shaped physics education in the United States. His A Text-Book of Physics, written with Joseph Y. Bergen and published by Henry Holt and Company in 1891 (with subsequent editions in 1897 and 1903), emphasized an experimental approach to fundamental principles, integrating laboratory exercises to verify theoretical concepts.^[5] Another key pedagogical text, The Teaching of Chemistry and Physics in the Secondary School (1902, co-edited with Alexander Smith), included Hall's chapter on physics instruction, advocating for inductive methods and practical demonstrations in high school curricula.^[5] Hall also produced laboratory-oriented works. Additionally, Elementary Lessons in Physics (1894, with a second edition in 1900, Henry Holt and Company) targeted secondary education, offering simplified explanations and exercises to build conceptual understanding through observation.^[5] Beyond these, Hall authored numerous papers on thermoelectricity in journals such as the Proceedings of the American Academy of Arts and Sciences during the 1880s and 1900s, exploring thermal conductivity, the Thomson effect, and related phenomena in metals like iron and nickel.^[5] Notable examples include "On the Thermal Conductivity of Cast Iron and of Cast Nickel" (1892, Proceedings of the American Academy of Arts and Sciences) and a series from 1900–1907 on thermoelectric measurements in soft iron, which advanced quantitative understanding of coupled thermal-electrical processes.^[5] In later life, Hall resumed research on the Hall effect, publishing "The Four Transverse Effects and Their Relations in Certain Metals" in the Proceedings of the National Academy of Sciences in 1925.^[5] His final work, A Dual Theory of Conduction in Metals (1938, Murray Printing Company), summarized his theories on metallic conduction.^[5] Hall's publications had lasting impact on physics education, with his textbooks and manuals widely adopted in American universities and secondary schools, promoting an emphasis on experimental verification over purely theoretical instruction.^[5] For instance, his Descriptive List of Elementary Exercises in Physics (multiple editions from 1886 to 1912) influenced national standards, including those of the National Educational Association, by standardizing laboratory-based preparation for college admission.^[5]

Retirement, Recognition, and Death

Hall retired from his position as Rumford Professor of Physics at Harvard University in 1921 at the age of 66.^[3] Following retirement, he remained in Cambridge, Massachusetts, where he continued informal research on thermoelectricity and the Hall effect, as well as engaged in writing, while also pursuing personal interests such as reading classics in French, Italian, and Spanish, as well as newspapers and periodicals.^[11]^[1] In recognition of his longstanding contributions to the teaching of physics, Hall was awarded the Oersted Medal by the American Association of Physics Teachers in 1937.^[12] Hall married Caroline Eliza Bottum in 1882; the couple had two children, Constance Huntington Hall and Frederic Hilborn Hall, the latter of whom died while a senior at Harvard.^[11] Post-retirement, Hall led a relatively private life centered in Cambridge. Hall died on November 20, 1938, in Cambridge at the age of 83, from heart failure following a brief illness.^[13] He was buried at Mount Auburn Cemetery in Cambridge.^[13]