Johann Christian Poggendorff: Physicist, Instrument Maker and Editor

Johann Christian Poggendorff (1796 to 1877) is one of those figures whose name turns up in many corners of science without most people knowing who he was. Physics students meet the Poggendorff compensation method, the ancestor of the laboratory potentiometer. Historians of electricity meet him as an early builder of galvanometers and the man who introduced mirror reading of their deflections. Battery collectors know the Poggendorff cell. Psychologists know the Poggendorff illusion, a line that seems to jump sideways when it passes behind a bar. And every historian of science has used the reference work still known simply as "the Poggendorff."

Above all, for 52 years he edited the Annalen der Physik und Chemie, which under his hand became one of the leading scientific journals in Europe. That role gave him enormous influence over what counted as physics in the German-speaking world, and it also produced some of the most famous editorial misjudgements in the history of science, including the rejection of Julius Robert Mayer's first paper on the conservation of energy.

This article covers his life, his instruments and how they worked, his long editorship and its controversies, the illusion that bears his name, and his biographical dictionary, along with common confusions about what he did and did not invent.

Quick facts

Early life: from apothecary to Berlin

Poggendorff was born in Hamburg on 29 December 1796. According to the German Wikipedia biography, his father, Johann Hinrich Poggendorff (1744 to 1817), was a prosperous sugar manufacturer who lost his fortune during the French occupation of Hamburg in the Napoleonic period. The 1911 Encyclopaedia Britannica tells the same story: the collapse of the family's finances during the French siege left the young Poggendorff without the means for a conventional academic education.

He was apprenticed to an apothecary in Hamburg, beginning at about the age of 16, and at 22 he was working as an apothecary's assistant in Itzehoe, a town in Holstein. Pharmacy was a common route into chemistry and natural science for young men of modest means in this period, since it demanded careful laboratory work and a knowledge of chemical substances. The German biography credits a former schoolmate, the chemist Friedlieb Ferdinand Runge, with encouraging his interest in science.

In 1820 Poggendorff moved to Berlin and entered the university there, now Humboldt University. In 1823 he was appointed meteorological observer to the Berlin Academy of Sciences. It was a modest post, but it placed him close to the Academy and its members just as electricity and magnetism were becoming the most exciting field in physics.

Electricity in the 1820s

To understand Poggendorff's instruments, it helps to know how new the field was when he started.

In 1820, the Danish physicist Hans Christian Ørsted reported that an electric current in a wire deflects a nearby compass needle. This was the first clear demonstration that electricity and magnetism are linked, and it set off a wave of experiments across Europe. Within weeks, investigators realised that the deflection could be used to detect and compare currents.

On 16 September 1820, Johann Schweigger at the University of Halle reported the earliest galvanometer. His key idea was to wind the wire many times around the compass, so that each turn added its effect on the needle. Because of the many turns, early instruments of this kind were called "multipliers." André-Marie Ampère gave Ørsted's discovery mathematical form and helped develop the instrument. In 1825 Leopoldo Nobili built the astatic galvanometer, with two opposed needles arranged so that the Earth's magnetic field cancels out, making the instrument much more sensitive to small currents.

Poggendorff was working on the same problem in the same year as Schweigger. The German Wikipedia biography credits him with building a usable galvanometer, or multiplier, in 1820, and his name is often coupled with Schweigger's in nineteenth-century accounts of the instrument's origins. The exact priority between their designs is a matter for specialist historians. What is clear is that Poggendorff was one of the first people in Germany to turn Ørsted's effect into a measuring instrument, and that instrument design remained his strongest talent throughout his career.

A short timeline of the galvanometer

The table below places Poggendorff's contributions among the main steps in the instrument's development, as given in Wikipedia's history of the galvanometer.

Read across the rows, the pattern is a steady search for sensitivity: more turns of wire, cancellation of the Earth's field, an optical pointer with no mass, and finally a moving coil in a strong fixed magnet. Poggendorff's mirror sits right in the middle of that progression.

The mirror reading: a weightless pointer

What Poggendorff introduced

In 1826, Poggendorff introduced the use of a small mirror attached to the moving magnet of a galvanometer, so that its deflection could be read optically instead of by a pointer on a dial. English Wikipedia describes this as an early mirror galvanometer. The German Wikipedia biography associates the development of this mirror reading with Carl Friedrich Gauss as well. Either way, reading a deflection with a mirror and a distant scale went on to become standard practice in sensitive galvanometers.

How it works

A conventional needle galvanometer has a physical limit. To see a small deflection, you need a long pointer, but a long pointer has mass and inertia, which makes the instrument sluggish and less sensitive. The mirror method avoids this.

1. A very light mirror is fixed to the suspended magnet, which hangs on a fine fibre inside the coil.
2. A beam of light, or in early versions the image of a scale seen through a telescope, is reflected by the mirror.
3. When current flows and the magnet turns, the mirror turns with it, and the reflected beam sweeps across a scale some distance away.

Wikipedia's galvanometer article describes the reflected beam as "a long mass-less pointer." The magnification comes from geometry. By the law of reflection, if a mirror turns through an angle θ, the reflected beam turns through 2θ. A scale placed at distance D therefore sees the spot move by about 2Dθ for small angles.

Worked example (hypothetical)

Suppose a current turns the magnet and mirror through 0.001 radian, about 0.057 degrees. A pointer 5 centimetres long would move its tip only 0.05 millimetres, far too little to read by eye. With a scale 1 metre from the mirror, the reflected spot moves about 2 × 1,000 mm × 0.001 = 2 millimetres, easily visible. Put the scale 3 metres away and the movement becomes about 6 millimetres. The pointer has effectively become metres long without adding any weight to the moving system.

What came later

The mirror method's later fame came through William Thomson, later Lord Kelvin, who patented a much more sensitive mirror galvanometer in 1858. Mirror galvanometers served as receivers on the first transatlantic submarine telegraph cables, where the signals arriving after thousands of kilometres of cable were extremely weak. Later still, in 1882, Jacques-Arsène d'Arsonval and Marcel Deprez reversed the arrangement, using a stationary permanent magnet and a moving coil, and Edward Weston produced a commercial portable version by 1888. That moving-coil design is the basis of the analogue meters used for most of the twentieth century.

Poggendorff did not invent all of this, but the step of reading a deflection with a mirror and a distant scale is one of the quiet foundations of precision electrical measurement, and he was among the first to use it.

The editor of the Annalen

Taking over Gilbert's journal

The journal Poggendorff edited has a long lineage. Its predecessors were the Journal der Physik (1790 to 1794) and the Neues Journal der Physik (1795 to 1797), edited by Friedrich Albrecht Carl Gren. The Annalen der Physik itself dates from 1799, and from then until 1824 it was edited by Ludwig Wilhelm Gilbert, so that it was widely known as Gilberts Annalen.

Gilbert died suddenly in 1824. Poggendorff, who had already been thinking about founding a journal of physics and chemistry, approached the Leipzig publishing house of Barth that year and took over the journal. Under him it was titled Annalen der Physik und Chemie. He edited it from 1824 to 1876, a span of 52 years, which is among the longest editorships of any major scientific journal. The 1911 Britannica says that under his management it became one of the leading scientific journals of Europe.

Scholars still often refer to the volumes of this period as "Poggendorff's Annalen," and citations of the form "Pogg. Ann." appear throughout nineteenth-century physics and chemistry.

What made him a good editor

The Britannica entry gives a fair summary of his strengths. He had an extraordinarily wide memory for scientific knowledge, a measured judgement that favoured experimental facts over speculation, and an orderly, methodical approach to work. A journal edited for half a century by such a person becomes a reliable record: careful, factual, and generally sceptical of grand theory.

The same entry places Poggendorff as a physicist in the upper tier but not the very highest. He was a skilled experimenter and an inventive designer of apparatus, but he was not especially strong in mathematics or in theoretical generalisation. That combination of strengths and weaknesses explains a good deal about his editorship.

The rejections

Poggendorff's caution sometimes worked against him. Three cases are regularly cited.

Julius Robert Mayer, 1841. Mayer, a ship's doctor from Heilbronn, sent Poggendorff a paper in 1841 proposing a law of conservation of "force," what we now call energy. According to the English Wikipedia biography of Mayer, the paper contained fundamental mistakes and was not published. The German account adds that Mayer received no reply, and that the manuscript was reportedly found among Poggendorff's papers after his death. Mayer revised his ideas, and in May 1842 his paper on the forces of inorganic nature appeared in Justus von Liebig's Annalen der Chemie und Pharmacie, including the first numerical estimate of the mechanical equivalent of heat. James Joule's experimental determination followed in 1843, and much of the credit went to Joule, leading to a bitter priority dispute. Mayer's recognition came late: an honorary doctorate from Tübingen in 1859, John Tyndall's championing of his work at the Royal Institution in London in 1862, the Prix Poncelet in 1869 and the Copley Medal in 1871.

Hermann von Helmholtz, 1847. According to the German Wikipedia biography of Poggendorff, he also declined a paper by the young Helmholtz on the same subject in 1847. Helmholtz published his treatise Über die Erhaltung der Kraft (On the Conservation of Force) as a separate book that year. It became one of the founding texts of the energy principle.

Philipp Reis, 1859 and 1862. Reis, a schoolteacher, sent Poggendorff a paper titled "On the Radiation of Electricity" in 1859, which was rejected. In 1862 he offered an account of his "telephone," a device he had demonstrated in 1861 that could transmit musical tones and some words, though not clear speech. That was also rejected. Reis blamed his low standing, calling himself "only a poor schoolmaster." According to the English Wikipedia article on Reis, after a lecture in Giessen, Poggendorff invited him to submit a description after all, but Reis reportedly declined, saying it was too late. The German biography of Poggendorff says he regarded the device as a novelty and left Reis out of his biographical handbook, and argues this helped prevent the telephone from being taken seriously in Germany.

What happened to the rejected ideas

The later fates of the three rejected authors are worth setting side by side, because they show how differently history can treat an editorial "no."

Mayer and Helmholtz found other outlets quickly, and the delay mainly cost them priority and recognition. Reis, isolated as a provincial schoolteacher, never had his work taken up in the way it might have been. The 1947 test results suggest his apparatus was more capable than his contemporaries realised.

How to judge these decisions

It is easy to read these cases as an old editor blocking new ideas. The reality is more mixed.

• Mayer's 1841 paper did contain errors, and the version that made his reputation was the revised one published a year later. A careful editor rejecting a flawed manuscript is doing his job, even if he failed to see the promising idea inside it.
• Theoretical arguments about a universal conservation law were exactly the kind of speculation Poggendorff's whole editorial philosophy distrusted. In the 1840s, energy conservation was not yet established, and the arguments for it mixed physics, physiology and philosophy.
• Reis's telephone was genuinely limited. The case against Poggendorff is less that he was wrong about its performance and more that he dismissed a direction of research that turned out to be very important.

What the cases show is the cost of a single editor with very great influence. When one person controls the main outlet in a field for half a century, his blind spots become the field's blind spots.

Volumes as a historical record

Because the journal ran continuously under one editor for so long, its volumes now serve as a running record of how physics and chemistry developed in the German-speaking world across the middle of the nineteenth century. Two references from Poggendorff's own era illustrate how it is cited today. Zöllner's paper on the illusion that bears his name appears in modern bibliographies as Annalen der Physik volume 186, issue 7, pages 500 to 525 (1860), in the continuous volume numbering used by modern databases. A paper by Poggendorff himself is cited in the old series numbering as Annalen der Physik, second series, volume 139, pages 513 to 546 (1870). Anyone tracing a nineteenth-century reference needs to know that both numbering systems exist.

After Poggendorff

On Poggendorff's death the journal passed to Gustav Heinrich Wiedemann, who edited it from 1877 to 1899, still as Annalen der Physik und Chemie. From 1900 it was edited by Paul Drude as Annalen der Physik, and then by Wilhelm Wien and Max Planck. In 1905 it published Albert Einstein's four "miracle year" papers on the photon hypothesis, Brownian motion, special relativity and mass-energy equivalence. The journal is still published today, monthly, by Wiley-VCH.

The compensation method: measuring without drawing current

The problem

In the early nineteenth century, measuring the "electromotive force" of a battery, what we would now call its open-circuit voltage, was surprisingly hard. If you connect a meter across a cell, current flows through the meter. That current passes through the cell's own internal resistance and loses some voltage on the way, so the meter reads less than the cell's true electromotive force. The weaker or more resistive the cell, the bigger the error.

Poggendorff's solution, 1841

In 1841 Poggendorff described a method that measures electromotive force while drawing no current at all from the cell being tested. The German sources call it the Poggendorff compensation circuit; in English the general technique is the potentiometer, a word that later also came to mean a variable resistor.

The method is a null method. Instead of reading the unknown voltage directly, you balance it against a known, adjustable voltage until a sensitive detector shows zero.

1. A steady current is driven through a long uniform resistance wire by a working battery. Because the wire is uniform, the voltage along it rises in direct proportion to length.
2. The unknown cell is connected, through a galvanometer, between one end of the wire and a sliding contact.
3. The contact is moved until the galvanometer reads zero. At that point the voltage along that length of wire exactly equals the unknown electromotive force, and no current flows through the unknown cell.
4. The wire is calibrated by doing the same balance with a reference cell of known voltage.

Because no current flows from the cell under test at balance, its internal resistance does not matter. The arrangement behaves like a voltmeter of infinite resistance. And because the galvanometer only has to show zero, it does not need to be calibrated; it only needs to be sensitive.

Worked example (hypothetical)

Suppose the slide wire is 100 cm long and the working battery drives a steady current through it.

• A reference cell of 1.0183 volts balances at 61.0 cm.
• An unknown cell balances at 89.5 cm.

Since voltage is proportional to length along the wire, the unknown electromotive force is

E = 1.0183 V × 89.5 / 61.0 ≈ 1.494 V.

Compare that with a simple meter. Imagine the same cell has an internal resistance of 2 ohms and is measured with a meter whose resistance is 100 ohms. Current flows through a total of 102 ohms, and the meter sees only 100/102 of the electromotive force: 1.494 × 100/102 ≈ 1.465 V, an error of about 2 percent. The potentiometer avoids that error entirely.

The figure of 1.0183 volts is the classic value of the Weston standard cell, which came decades after Poggendorff's work; it is used here only because it is the textbook calibration example. In Poggendorff's time, the compensation principle was applied with whatever reference sources were available, and the units of voltage themselves were not yet standardised.

Why it mattered

The compensation method became a standard laboratory technique. Its descendants dominated precision voltage measurement until electronic meters with very high input resistance became common in the twentieth century. The same principle is used in measuring thermocouple voltages, in potentiometric chemical analysis, where cell voltages are related to concentrations through the Nernst equation, and in school "metre bridge" experiments. Its accuracy is limited mainly by the uniformity of the wire, contact resistance, thermoelectric voltages at junctions, and the sensitivity of the null detector.

The Poggendorff cell

A chromic acid battery

In 1842, according to the German Wikipedia biography, Poggendorff introduced a galvanic cell now known by his name. It is a form of chromic acid cell, also called a bichromate cell, a primary (non-rechargeable) battery.

Why a depolariser

Simple acid cells with zinc and another conductor suffer from polarisation: hydrogen gas forms at the positive electrode, coats it, and quickly reduces the cell's output. A depolariser is a strong oxidising agent that removes this hydrogen as it forms. Chromic acid does this very effectively, which is why bichromate cells delivered a relatively high voltage and strong current, useful for laboratory work and demonstrations in the nineteenth century.

The bottle form

In the single-fluid version associated with Poggendorff, a zinc plate is mounted between two carbon plates in a long-necked glass bottle containing the acid mixture. The chromic acid mixture would dissolve the zinc even when no current was being drawn, so the zinc plate was mounted on a rod and lifted up into the neck of the bottle when the cell was not in use. A two-fluid version, associated with Fuller, separated the chromic acid and carbon from the dilute acid and zinc using a porous pot, and added a little mercury to amalgamate the zinc and reduce wasteful "local action."

Wikipedia describes these cells as now "only of historical interest." Modern batteries are far more convenient and avoid handling strong chromic acid mixtures. But in their day bichromate cells were valued sources of strong current, and the Poggendorff name is the reason he appears in catalogues of battery inventors.

An electrostatic machine

Poggendorff also built an electrostatic motor, a device that uses electrostatic forces rather than magnetism to produce rotation, which Wikipedia describes as analogous to Wilhelm Holtz's electrostatic machine. Holtz, a German physicist who later became professor of physics at Greifswald, invented an influence machine in 1865, an electrostatic generator that turns mechanical work into high-voltage electricity starting from a small initial charge. A machine of that kind can, in principle, also be run in reverse as a motor. Poggendorff's interest fits his lifelong pattern: careful apparatus that made electrical effects visible and measurable.

The Poggendorff illusion

What it is

The Poggendorff illusion is a geometrical-optical illusion. A straight diagonal line is interrupted by a vertical bar or rectangle. The two visible segments of the line are in fact exactly aligned, but most viewers see them as offset: the segment on the far side of the bar appears to continue above or below where it really does.

How it was found

The illusion is linked to the astrophysicist Johann Karl Friedrich Zöllner, who in 1860 described a related effect now called the Zöllner illusion, in which parallel lines crossed by short slanted strokes appear not to be parallel. Zöllner sent his discovery to Poggendorff as editor of the Annalen. Poggendorff noticed a second illusion in Zöllner's drawing, the apparent misalignment of oblique lines interrupted by the figure, and that effect bears his name. Zöllner's paper appeared in the Annalen der Physik in 1860. The German biography gives 1850 for Poggendorff's description, but the connection to Zöllner's 1860 figures is the version given in most sources.

Why it happens

There is still no single agreed explanation. The main proposals, as summarised in the Wikipedia article on the illusion, include:

The illusion is strongest when the line is oblique and weakens or disappears when the line is horizontal or vertical. Its strength depends on the width of the bar and the angle of the line. Research summarised on Wikipedia has found that susceptibility declines from about age 6 to 14 and stabilises in early adulthood, and a 2023 study by Higashiyama and colleagues reported that patients with cerebellar stroke were less affected than healthy people.

The illusion is a useful reminder that perception is not a simple copy of the image on the retina. It is a working estimate that the brain builds using expectations about how objects and edges usually behave.

The biographical dictionary

A reference work that outlived its author

Poggendorff's most durable scholarly contribution may be a book rather than an instrument. His Biographisch-literarisches Handwörterbuch zur Geschichte der exacten Wissenschaften (Biographical-literary concise dictionary of the history of the exact sciences) collected brief biographies and publication lists for mathematicians, astronomers, physicists and chemists of all eras and nations.

The work was continued by later editors long after Poggendorff's death, and in German-speaking scholarship it is still simply called "the Poggendorff." For a historian trying to find out when an obscure nineteenth-century chemist was born, where he worked and what he published, it remains a standard first stop.

Its compilation fits the rest of Poggendorff's career. As editor of the Annalen for half a century, he had an unrivalled view of who was publishing what across European science, and the Britannica emphasises his enormous memory for scientific literature.

Other historical writings

Poggendorff also published Lebenslinien zur Geschichte der exacten Wissenschaften (Berlin, 1853), a chronological chart of the lives of scientists, and his lectures on the history of physics were published after his death as Geschichte der Physik (Leipzig, 1879). A biography of him by Emil Frommel appeared in Berlin in 1877, the year he died.

A life in outline

The c. dates for the apprenticeship years are calculated from the ages given in the biographies, not separately recorded dates.

Honours and positions

He declined several offers of ordinary, that is full, professorships, preferring to devote himself to the Annalen and his own research. This is unusual. A full chair was the summit of a German academic career, and turning it down meant giving up prestige and power. It shows how central the journal was to his sense of his own work.

Family and death

Wikipedia records that his daughter Marie, born on 12 August 1838, married the classical scholar Valentin Rose in 1872. Poggendorff died in Berlin on 24 January 1877, having handed over the editorship of the Annalen only the year before. According to the German biography he was buried in the St. Marien and St. Nikolai cemetery in Prenzlauer Berg, Berlin. A plant genus in the passionflower family, Poggendorffia, was named in his honour.

Assessing his legacy

Instrument maker

Poggendorff's instruments share a theme: making small electrical effects measurable with precision. The multiplier galvanometer, mirror reading and the compensation method all extend the reach of measurement, and they were exactly what electrical science needed in the decades when quantities like current, resistance and electromotive force were first being defined. Later quantitative electrical work, including the telegraph engineering that relied on mirror galvanometers, built on instruments of this family.

Editor and gatekeeper

As editor, he built a journal that set standards of reliability for half a century and that later published some of the most important papers in the history of physics. He also kept out some important ideas, most notably Mayer's first formulation of energy conservation. Both facts belong in any honest account. His preference for experiment over speculation was, on balance, a strength for a journal of record, and a weakness when theory was moving faster than experiment.

Historian

His dictionary made him, in a sense, the first great bibliographer of modern science, and the work that still carries his name is used more today than any of his instruments.

Common misconceptions

"Poggendorff invented the galvanometer." Not alone. Schweigger reported a galvanometer, the "multiplier," in September 1820, and several people improved the design in the following years. Poggendorff was among the earliest builders and introduced the mirror-reading technique.

"The mirror galvanometer was Kelvin's invention." Kelvin patented a much more sensitive version in 1858 and made it famous on the transatlantic telegraph cables. Mirror reading itself goes back to Poggendorff in 1826.

"Poggendorff drew the Poggendorff illusion." The figure in which he noticed it was Zöllner's. Poggendorff's contribution was spotting the second effect.

"He was a theorist of electricity." His own contemporaries and the 1911 Britannica describe him as primarily an experimenter and apparatus designer who was not strong in mathematics or theory.

"The Poggendorff cell is just another name for any early acid battery." No. It is specifically a chromic acid, or bichromate, cell, defined by its use of chromic acid as the depolariser with zinc and carbon electrodes.

"A potentiometer is just a volume knob." Today the word usually means a variable resistor, but it originally meant an instrument for measuring potential, or voltage, by the null method Poggendorff described in 1841. The variable resistor took the name because it contains the same kind of sliding contact on a resistive track.

Frequently asked questions

What is Poggendorff best known for?

Among physicists, the compensation (potentiometer) method and his 52-year editorship of the Annalen der Physik und Chemie. Among psychologists, the Poggendorff illusion. Among historians, the biographical dictionary still called "the Poggendorff."

Why did he reject Mayer's paper?

According to Mayer's English Wikipedia biography, the 1841 manuscript contained fundamental mistakes. The German account adds that Poggendorff never replied. Mayer's corrected version appeared in Liebig's journal in 1842. The rejection is often cited as an example of a major idea being missed by an editor, but the paper as submitted was genuinely flawed.

How does a potentiometer measure voltage without drawing current?

It compares the unknown voltage with an adjustable known voltage and adjusts until a galvanometer between them reads zero. At that point no current flows from the source being measured, so its internal resistance causes no voltage drop and no error.

What voltage does a Poggendorff cell give?

About 2 volts per cell, according to the Wikipedia article on chromic acid cells.

What are the practical limits of the compensation method?

The Wikipedia article on the measuring potentiometer lists the main ones: the slide wire must be uniform in cross-section and resistivity, contact resistance and thermoelectric voltages at junctions introduce errors, and the precision of the balance depends on how sensitive the null detector is. Even so, specialised designs remained useful down to the microvolt range, around 1,000 nanovolts.

Who edited the Annalen after him?

Gustav Heinrich Wiedemann, from 1877 to 1899, followed by Paul Drude from 1900 and then Wilhelm Wien and Max Planck.

Is the Poggendorff illusion fully explained?

No. Researchers have proposed angle misperception, depth interpretation, learned scene statistics, neural orientation effects and other explanations. Several probably contribute, and the relative importance of each is still debated.

Did he hold a full professorship?

No. He was a royal professor and later an extraordinary professor at Berlin, but he declined several offers of full professorships to focus on the Annalen and his research.

Why is the reference work called "the Poggendorff"?

Because Poggendorff compiled the original volumes, published in 1863. Later editors continued the work, but his name stuck to it.

Summary

Johann Christian Poggendorff rose from a ruined merchant family and an apothecary's apprenticeship to become one of the central figures of nineteenth-century German science. As an experimenter he helped turn Ørsted's discovery into practical instruments, introduced the mirror method of reading small deflections, and in 1841 described the compensation method that became the potentiometer, still the conceptual basis of precise voltage measurement. His chromic acid cell, his electrostatic apparatus and the illusion he spotted in Zöllner's drawing all carry his name. As editor of the Annalen der Physik und Chemie from 1824 to 1876 he built one of Europe's most important journals, while also rejecting work by Mayer, Helmholtz and Reis that later proved significant. And as a historian he left a biographical dictionary that scholars still consult nearly a century and a half after his death.

Sources

• Johann Christian Poggendorff (English Wikipedia)
• Johann Christian Poggendorff (German Wikipedia)
• 1911 Encyclopædia Britannica: Poggendorff, Johann Christian (Wikisource)
• Annalen der Physik (Wikipedia)
• Galvanometer (Wikipedia)
• Potentiometer (measuring instrument) (Wikipedia))
• Chromic acid cell (Wikipedia)
• Poggendorff illusion (Wikipedia)
• Zöllner illusion (Wikipedia)
• Julius Robert von Mayer (Wikipedia)
• Philipp Reis (Wikipedia)
• Wilhelm Holtz (Wikipedia)