Ferdinand Braun

How to use this course

Ferdinand Braun shared the 1909 Nobel Prize in Physics with Guglielmo Marconi “in recognition of their contributions to the development of wireless telegraphy”. Marconi is a household name; Braun is not. Yet three of Braun’s contributions underlie everyday electronics. In 1874 he found that some crystals conduct better in one direction than the other, the first observation of a semiconductor rectifier. In 1897 he built the cathode-ray tube that German speakers still call the Braun tube. And from 1898 he reorganized the radio transmitter around a closed, tuned circuit coupled to the antenna, then built some of the first directional antenna arrays.

Braun was a university physicist all his life, a careful experimenter and a gifted teacher, who also founded companies that became part of Telefunken. He died in Brooklyn in 1918, an enemy alien stranded in the United States by the First World War.

Each technical lesson opens with a paragraph for the general reader, then gives precise statements, derivations and exercises with folded solutions. Lessons 08 and 09 pair with Lessons 04 and 07 of the Marconi course, which treat the spark oscillator and syntonic tuning from Marconi’s side; the notation is the same.

Timeline

Fulda, Marburg, Berlin, 1850–1872

Karl Ferdinand Braun was born in Fulda on 6 June 1850, the sixth of seven children of Konrad Braun, a clerk and court official in the service of the Electorate of Hesse, and his wife Franziska, née Göhring. He went to the cathedral grammar school, the Domgymnasium, in Fulda. Practical electricity ran in the family: his elder brother Wunibald later co-founded the Frankfurt instrument makers Hartmann & Braun with Eugen Hartmann in 1884.

In 1868 Braun began studying mathematics and natural science at the University of Marburg. In 1869 he moved to Berlin, where he worked in the private laboratory of Heinrich Gustav Magnus, one of the leading experimental physicists in Germany. Magnus died in the spring of 1870, and Braun continued under Georg Hermann Quincke, a specialist in capillarity and optics. He took his doctorate in Berlin in 1872 with a dissertation on how stiffness, fixing and amplitude affect the vibrations of strings. The topic was acoustic, but the careful study of oscillations it required became the thread of his career.

Würzburg, Leipzig and the rectifier, 1872–1877

In 1872 Quincke moved to Würzburg and took Braun with him as assistant. Assistantships led to professorships only slowly, so in 1873 Braun passed the state examination for secondary-school teachers, and in 1874 he became a teacher of mathematics and science at the Thomasschule in Leipzig, the old school of Johann Sebastian Bach. He kept up research in the evenings. In 1876 he published his only book, Der junge Mathematiker und Naturforscher, an introduction to mathematics and science for young readers.

A deviation from Ohm’s law

In Würzburg Braun had begun to study the electrical conduction of metal sulfides, natural crystals such as galena (lead sulfide) and pyrite. His paper “Ueber die Stromleitung durch Schwefelmetalle” was submitted to the Annalen der Physik und Chemie on 23 November 1874. He found that when a fine metal wire touched the crystal, the resistance depended on the direction and size of the current. Ohm’s law, with resistance independent of current, failed. This was the first recorded observation of rectification at a metal–semiconductor contact. Braun could not explain it, and nobody could until the quantum theory of solids; Walter Schottky and Nevill Mott gave the explanation in the late 1930s. Lesson 04 presents their theory.

Professor in four universities, 1877–1897

In 1877 Braun returned to Marburg as extraordinary professor of theoretical physics. In 1880 he moved to the same rank at the University of Strasbourg, then German, and in 1883 became full professor of physics at the Technische Hochschule in Karlsruhe, where in 1884 he developed an electrical pyrometer. In 1885 he accepted a chair at Tübingen, where he planned and built a new physics institute over the following years. Soon after arriving he married Amélie Bühler of Lahr; they had two sons and two daughters.

At Tübingen he worked on thermodynamics and electrochemistry. In 1887 he studied how pressure changes the solubility of solids and stated a general principle for how a system in equilibrium responds to a disturbance, close to the one stated by Henry Le Chatelier in 1884. He also designed the Braun electrometer, a robust instrument for measuring high voltages that remained standard in physics teaching laboratories for decades.

In 1895 he returned to Strasbourg as director of the Physics Institute, turning down an invitation to succeed Gustav Wiedemann at Leipzig. Strasbourg remained his base for the rest of his life. There, in early 1897, he built the cathode-ray tube (Lesson 06), and in 1898 he turned to wireless telegraphy.

Rectification at point contacts

For the general reader. A rectifier lets current through in one direction and blocks it in the other. Braun found the effect in 1874 by pressing a metal point against a crystal of galena. Twenty-five years later he and others used such contacts as detectors of radio waves, the “cat’s whisker” of early crystal sets. Today’s silicon diodes and the Schottky diodes in fast electronics work on the same principle, now understood through the quantum theory of electrons in solids.

The Schottky barrier

When a metal of work function \(\phi_m\) touches an n-type semiconductor of electron affinity \(\chi\), with \(\phi_m>\chi\), electrons flow from the semiconductor into the metal until the Fermi levels align. This leaves a layer of positive donor ions near the surface, the depletion region, and a barrier of height \(\phi_B\approx\phi_m-\chi\) (in the simplest model) that electrons in the metal must climb. A voltage \(V\) applied across the contact lowers or raises the barrier seen from the semiconductor side, but not from the metal side.

Derivation sketch. The current from semiconductor to metal is carried by electrons with enough energy to cross the barrier. Their number follows a Boltzmann factor, so this current is \(I_se^{eV/kT}\), growing as forward bias \(V\) lowers the barrier. The reverse flow from the metal faces the fixed height \(\phi_B\) and is \(I_s\), independent of \(V\). In equilibrium (\(V=0\)) the two balance. The net current is the difference.

Detection

Expanding for small \(v\), with \(n=1\), \(I(v)=I_s\big(v/V_T+v^2/2V_T^2+\cdots\big)\). A radio signal \(v=V_0\cos\omega t\) averages the linear term to zero and leaves a direct current

proportional to the received power. This is the square-law detection discussed in Lesson 05 of the Marconi course, now with an explicit nonlinearity. Braun said in his Nobel lecture that he recognized the value of crystal contacts as receivers in 1901 and that from 1905 his company developed them. Crystal detectors, which needed no battery and were far more sensitive than the coherer, dominated radio reception until vacuum tubes took over in the 1920s.

The Le Chatelier–Braun principle

For the general reader. Squeeze a system in equilibrium and it shifts in a way that partly relieves the squeeze. Heat it and it shifts in the way that absorbs heat. Henry Le Chatelier stated the rule in 1884 for chemical equilibria; Braun reached it independently in 1887 from his study of how pressure affects the solubility of solids. In modern thermodynamics the principle is a theorem about stability, with exact inequalities.

Consider a system with two pairs of conjugate variables, “forces” \(f_1,f_2\) and “displacements” \(x_1,x_2\), described by a potential \(G(f_1,f_2)\) with \(x_i=-\partial G/\partial f_i\). For example, \(f_1=P\), \(x_1=-V\) for a gas, and \(f_2=T\), \(x_2=S\). Stability of equilibrium makes the matrix \(\chi_{ij}=\partial x_i/\partial f_j=-\partial^2G/\partial f_i\partial f_j\) positive semidefinite. Its symmetry, \(\chi_{12}=\chi_{21}\), is a Maxwell relation.

Proof. Holding \(x_2\) fixed means \(dx_2=\chi_{21}df_1+\chi_{22}df_2=0\), so \(df_2=-(\chi_{21}/\chi_{22})df_1\). Then

Since \((\partial x_1/\partial f_1)_{f_2}=\chi_{11}\) and \(\chi_{12}^2/\chi_{22}\ge0\), the first inequality follows; positive semidefiniteness gives \(\det\chi\ge0\), the second. Two familiar cases: the isothermal compressibility exceeds the adiabatic one, \(\kappa_T\ge\kappa_S\), and \(C_P\ge C_V\).

Pressure and solubility

Braun’s problem was a solid in equilibrium with its saturated solution. At equilibrium the chemical potential of the solid equals that of the solute in solution. Differentiating with respect to pressure at constant temperature, for an ideal dilute solution of mole fraction \(x\),

where \(\Delta V_{\text{sol}}=\bar V_{\text{solute}}-V_{\text{solid}}\) is the change in volume per mole dissolved. If dissolving shrinks the total volume, pressure increases solubility; if it expands the volume, pressure decreases solubility. The system responds to compression by favouring the denser state, as the principle requires.

The Braun tube

For the general reader. In February 1897 Braun published a way to watch an electric current change in time. A narrow beam of cathode rays struck a fluorescent screen and made a bright spot. A coil carrying the current under study deflected the beam, and a rotating mirror spread the spot’s motion across time, so that the eye saw the shape of the current. The same year J. J. Thomson showed that cathode rays are electrons. The Braun tube became the oscilloscope, and later the picture tube of television.

Braun’s tube, described in “Ueber ein Verfahren zur Demonstration und zum Studium des zeitlichen Verlaufes variabler Ströme” (Annalen der Physik, 15 February 1897), was a cold-cathode discharge tube at moderate vacuum, driven by a very high voltage. An aperture selected a thin beam, which fell on a screen coated with a fluorescent material. The current to be studied passed through a coil outside the tube, deflecting the beam along one axis. A rotating mirror supplied the time axis. In 1899 his assistant Jonathan Zenneck added a second, sweeping deflection, giving a true two-dimensional trace on the screen.

Derivation. In the magnetic case, the transverse force \(evB\) acts for a time \(\ell/v\), giving a transverse velocity \(eB\ell/m\) and a deflection angle \(\theta=eB\ell/(mv)\). The beam then appears to come from the centre of the region, so \(D=L\theta\). In the electric case the transverse velocity is \((eV_d/md)(\ell/v)\) and \(\theta=eV_d\ell/(mdv^2)=V_d\ell/(2dV_a)\). The electric deflection does not depend on \(e/m\), which is why it was later preferred in oscilloscopes; the magnetic deflection does, which Thomson exploited, combining both, to measure \(e/m\).

Through water or through air

For the general reader. Braun came to wireless in 1898 by trying to telegraph through water. Electrodes dipped in a river or the sea drive currents that spread through the water, and a distant pair of electrodes can pick up a small part. He soon found that this method fades far too quickly with distance and turned to radio waves in the air. The contrast is instructive: a conducted field and a radiated field fall off with distance at very different rates.

Derivation. In a steady state \(\nabla\cdot\mathbf J=0\) and \(\mathbf J=-\sigma\nabla\phi\), so \(\phi\) satisfies Laplace’s equation away from the electrodes. A point electrode injecting current \(I\) gives \(\phi=I/(4\pi\sigma r)\), by symmetry and the total outward current \(I\). Superposing a source and a sink a distance \(d\) apart gives the dipole potential, the same mathematics as an electrostatic dipole with \(p/\varepsilon_0\) replaced by \(Id/\sigma\). On the axis \(E=-\partial\phi/\partial r=Id/(2\pi\sigma r^3)\). At a surface, such as the sea with air above, the method of images doubles the field but does not change the power law.

Skin depth in sea water

Alternating currents in a conductor are confined to a skin depth \(\delta=\sqrt{2/(\omega\mu\sigma)}\). Sea water has \(\sigma\approx4\) S/m, so high-frequency fields die out within metres. Braun’s Nobel lecture recalls that he set aside the water experiments of 1898 and turned to the air. His 1901 book Drahtlose Telegraphie durch Wasser und Luft (Wireless telegraphy through water and air) records both lines of work.

The coupled transmitter

For the general reader. In Marconi’s early transmitters the spark gap sat in the antenna itself. The antenna could hold only a little charge, and the spark damped the oscillation within a few cycles. Braun’s idea, patented in Germany in October 1898, was to store the energy in a large bank of Leyden jars, let it oscillate in a closed circuit containing the spark, and pass it to a separate, sparkless antenna through a transformer. More energy per spark and gentler damping meant stronger, purer waves. Marconi’s tuned system of 1900 used the same principle, and it became standard.

Energy

A spark gap breaks down at a voltage \(V_s\) fixed by its length. Charged to \(V_s\), a plain antenna of capacitance \(C_a\) stores \(\tfrac12C_aV_s^2\). A long wire antenna has \(C_a\) of order 1 nF at most. A closed circuit with Leyden jars of capacitance \(C_1\gg C_a\), charged to the same voltage, stores \(\tfrac12C_1V_s^2\), larger by the factor \(C_1/C_a\). The closed circuit radiates very little itself, so its damping is set mostly by the spark, and it hands its energy to the antenna over several cycles.

Coupled circuits

Let the closed circuit (\(L_1,C_1\)) and the antenna circuit (\(L_2,C_2\)) be tuned to the same \(\omega_0\) and coupled by mutual inductance \(M=k\sqrt{L_1L_2}\). As in Lesson 07 of the Marconi course, neglecting losses,

Proof. With \(q_j=a_je^{i\omega t}\) the equations have a nonzero solution when \((1/C_1-\omega^2L_1)(1/C_2-\omega^2L_2)=\omega^4M^2\). Divide by \(L_1L_2\) and use \(1/(L_jC_j)=\omega_0^2\): \((\omega_0^2-\omega^2)^2=k^2\omega^4\), so \(\omega^2(1\pm k)=\omega_0^2\). The two modes start in phase and drift apart at the rate \(\omega_--\omega_+\), which gives the beats. Braun explained this double-frequency behaviour in his Nobel lecture by analogy with a pair of coupled pendulums: tighter coupling separates the two frequencies more widely, and only loose coupling brings both close to the natural frequency.

Direct and inductive coupling

Braun tried three arrangements: direct coupling, with the antenna connected to a tap on the closed-circuit coil; inductive coupling through a transformer; and mixed forms. For mobile stations he replaced the earth connection with a counterpoise, a network of wires above the ground acting as the second plate of the antenna capacitor. Accounts of Marconi’s 1901 Poldhu transmitter, designed with John Ambrose Fleming, describe a coupled circuit of this general type.

Damping, selectivity and the quenched spark

For the general reader. A spark transmitter emits a burst of waves that dies away. The faster it dies, the wider the band of frequencies it occupies, and the harder it is for a receiver to pick one station out from others. Braun’s laboratory measured these damping rates and showed how loose coupling in the receiver made it more selective. Max Wien’s quenched spark of 1906 then cut the primary circuit out after one transfer of energy, leaving the antenna to ring on a single frequency.

A damped oscillation \(e^{-\alpha t}\cos\omega t\) loses amplitude by the factor \(e^{-\delta}\) per period, where \(\delta=2\pi\alpha/\omega=\pi/Q\) is the logarithmic decrement. Its power spectrum is a Lorentzian of full width at half maximum \(2\alpha\), that is \(\Delta\omega/\omega=\delta/\pi\). Braun quoted in his Nobel lecture the measurements of Vilhelm Bjerknes, who found decrements of about 0.26 to 0.40 for spark circuits, and recalled that Wilhelm Feddersen had photographed about 20 half-cycles of a Leyden-jar discharge in 1862.

Proof sketch. The received energy is the integral over frequency of the transmitter’s power spectrum times the receiver’s power response. Near resonance each is a Lorentzian, of half-widths \(\alpha_1\) and \(\alpha_2\). The integral of the product of two Lorentzians, as a function of the offset of their centres, is their convolution, again a Lorentzian, with half-width \(\alpha_1+\alpha_2\). Converting with \(\alpha=\delta\omega/2\pi\) gives the result. This is the basis of the Bjerknes method: measuring the resonance curve with a wavemeter of known \(\delta_2\) yields the transmitter’s \(\delta_1\). Eduard Dönitz built such a wavemeter, developed from the Strasbourg work, for Siemens & Halske.

Loose coupling and quenching

Coupling a resonant circuit to another circuit adds the latter’s losses to it, raising its effective decrement. In 1902 Braun found at two stations in the Strasbourg forts that loosening the coupling between the receiving antenna and its detector circuit made reception sharper. In the transmitter, the double frequency of Lesson 08 was the problem. Max Wien’s quenched spark, a gap made of many short sections between cooled metal plates, de-ionizes quickly: after the first transfer of energy to the antenna the primary current is near zero, the gap stops conducting, and the antenna rings on alone at its own frequency with its own small decrement. Telefunken’s “singing spark” stations of the following years used this principle.

Directional antennas and phased arrays

For the general reader. A single vertical antenna sends equally in all horizontal directions. In 1905 two of Braun’s assistants, on a parade ground near Strasbourg, set three antennas at the corners of a triangle and fed one of them a quarter cycle later than the other two. The waves added in one direction and partly cancelled in the opposite one. By switching which antenna was delayed, they turned the beam without moving anything. This was among the first phased arrays, the principle of modern radar and of the antennas in mobile networks.

The phase \(k\,\hat{\mathbf u}\cdot\mathbf r_n\) accounts for the shorter path from element \(n\) toward the distant observer. This is the same sum that gives the uniform linear array of Lesson 09 in the Marconi course.

Two elements: the cardioid

Place element 1 at the origin with current \(I\) and element 2 at distance \(d=\lambda/4\) along \(x\) with current \(Ie^{-i\pi/2}\). In the direction at angle \(\theta\) from the \(x\)-axis, \(AF=I[1+e^{i(\pi/2)(\cos\theta-1)}]\), so

This is \(2I\) for \(\theta=0\) and zero for \(\theta=\pi\): a heart-shaped pattern, the cardioid, with the beam toward the delayed element. The delayed element’s lag exactly compensates the quarter wavelength of travel in the forward direction, and doubles to a half cycle in the backward one.

Braun’s triangle

Braun’s Nobel lecture describes the 1905 arrangement: three antennas at the corners of an equilateral triangle whose height was a quarter wavelength, the two at the base fed in phase and the one at the apex a quarter cycle late. Exchanging the roles of the antennas rotated the preferred direction by \(120^\circ\) (or by \(60^\circ\) with other combinations), and measured field strengths agreed with calculation. In 1902 Braun had also found that an antenna inclined close to the horizontal acts as a directional receiver; in 1913 he used a loop antenna to determine the bearing of the Eiffel Tower station, a step toward radio direction finding.

Telefunken and the Nobel Prize, 1898–1914

Braun’s wireless work began in 1898 with experiments in Strasbourg, on the Rhine and near Cuxhaven at the mouth of the Elbe. His German patent 111578 on the coupled transmitter dates from 14 October 1898, and a British patent followed in 1899–1900. Late in 1898 the Cologne chocolate manufacturer Ludwig Stollwerck organized a consortium to exploit the patents. Field work at Cuxhaven from 1899 was carried out largely by his assistants Mathias Cantor and Jonathan Zenneck, and on 24 September 1900 a regular exchange of messages was established with the island of Heligoland, 62 km away. In 1901 Braun published his account of the work, Drahtlose Telegraphie durch Wasser und Luft, and on 27 July of that year the Gesellschaft für drahtlose Telegraphie System Prof. Braun und Siemens & Halske was founded in Berlin.

The Braun–Siemens group built equipment for the German army; a rival group at AEG under Adolf Slaby and Georg von Arco built it for the navy. Their patent disputes, and the threat posed by the Marconi company, led Emperor Wilhelm II to press for a merger. On 27 May 1903 Siemens & Halske and AEG founded the Gesellschaft für drahtlose Telegraphie, known by its telegraphic address, Telefunken, a name Braun had used since 1901. Braun was a scientific adviser, not a manager.

In Strasbourg he remained above all a teacher and laboratory director. He served as rector of the university in 1905–06. His assistants and students included Zenneck, who became a leading radio scientist, and the Russian physicists Leonid Mandelstam and Nikolai Papaleksi, who took their doctorates at Strasbourg and later built a school of radiophysics in the Soviet Union; Max Dieckmann was another doctoral student and assistant. In 1909 Braun shared the Nobel Prize in Physics with Marconi. In his lecture of 11 December 1909, “Electrical oscillations and wireless telegraphy”, he traced the coupled transmitter, loose coupling and directional transmission, and named his aim as sparkless telegraphy with continuous waves. In 1914 he was elected a corresponding member of the Prussian Academy of Sciences.

America, death and legacy, 1914–1918

After war broke out in 1914, the American Marconi company sued the Atlantic Communication Company, the Telefunken subsidiary that operated the wireless station at Sayville, Long Island, one of the few direct links between Germany and the United States. Braun was summoned as a witness for the defence. In December 1914 he and Zenneck sailed from Bergen, Norway, taking a northern route to avoid the British blockade. Braun, then 64, stayed in New York to support the station and its case. When the United States entered the war on 6 April 1917, the litigation lapsed and Braun became an enemy alien. He was briefly detained, then allowed to live freely in Brooklyn. In poor health and away from his laboratory, he could do no further scientific work.

Braun died on 20 April 1918 at his son’s home in Brooklyn, aged 67, months before the armistice. Strasbourg, where he had worked for over twenty years, became French after the war. In 1921 his son Konrad brought his ashes back to Fulda, where they were buried on 4 June 1921 with few people present.

Legacy

• Semiconductors: his 1874 observation of rectification at crystal contacts is the starting point of semiconductor electronics, from the crystal detector to the Schottky diode.
• Displays and measurement: the cathode-ray tube became the oscilloscope and, in Manfred von Ardenne’s electronic television of 1930 and for most of the twentieth century, the picture tube. In German it is still the Braunsche Röhre. The Society for Information Display has awarded the Karl Ferdinand Braun Prize since 1987.
• Radio: the coupled, tuned transmitter became universal practice, and his 1905 array is an ancestor of every phased-array antenna.
• Institutions: Telefunken, the Ferdinand-Braun-Institut in Berlin, and the scientific schools of Zenneck and of Mandelstam and Papaleksi.

Connections

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

Teachers and advisors

• Carl Ludwig Siegel 1937
• Georg Hermann Quincke 1870-1874

Students

• Jonathan Zenneck 1895-1905
• Leonid Mandelstam
• Max Dieckmann
• Nikolai Papaleksi

Influenced by

• Heinrich Hertz
• Heinrich Gustav Magnus 1869-1870

Influenced

• Manfred von Ardenne

Colleagues and collaborators

• J. J. Thomson 1897
• Nikola Tesla 1915-1917
• Henry Le Chatelier
• Mathias Cantor 1899-1900
• Max Wien

Rivals

• Guglielmo Marconi 1898-1914
• Adolf Slaby 1898-1903

Friends

• Emil Artin 1958-1962

Also mentioned in this life

• Ludwig Boltzmann
• Enrico Fermi
• Josiah Willard Gibbs
• Pierre-Simon Laplace
• James Clerk Maxwell
• Georg Simon Ohm

Fields

• experimental physics 16 lives
• solid-state physics 6 lives
• thermodynamics 12 lives

On CoursesHub

• Thermodynamics

Sources and further reading

Biography

1. Nobel Foundation, Ferdinand Braun: Biographical.
2. Karl Ferdinand Braun, Wikipedia (English), and Ferdinand Braun, Wikipedia (German).
3. J. Zenneck, Braun, Ferdinand, Neue Deutsche Biographie 2 (1955).
4. Ferdinand-Braun-Institut, Vita Ferdinand Braun.
5. Ferdinand Braun: a pioneer in wireless technology and electronics, IEEE Milestones documentation.
6. F. Kurylo and C. Susskind, Ferdinand Braun: A Life of the Nobel Prizewinner and Inventor of the Cathode-Ray Oscilloscope, MIT Press, 1981.

Physics and technology

1. F. Braun, Ueber die Stromleitung durch Schwefelmetalle, Annalen der Physik und Chemie 229 (1875).
2. F. Braun, Ueber ein Verfahren zur Demonstration und zum Studium des zeitlichen Verlaufes variabler Ströme, Annalen der Physik 296 (1897), 552–559.
3. F. Braun, Drahtlose Telegraphie durch Wasser und Luft, Veit, Leipzig, 1901.
4. F. Braun, Electrical oscillations and wireless telegraphy, Nobel lecture, 11 December 1909.
5. Nobel Foundation, The Nobel Prize in Physics 1909.
6. J. Zenneck, Elektromagnetische Schwingungen und drahtlose Telegraphie, Enke, Stuttgart, 1905.
7. S. M. Sze and K. K. Ng, Physics of Semiconductor Devices, 3rd ed., Wiley, 2007, chapter on metal–semiconductor contacts.
8. H. B. Callen, Thermodynamics and an Introduction to Thermostatistics, 2nd ed., Wiley, 1985, section on the Le Chatelier–Braun principle.
9. H. G. J. Aitken, Syntony and Spark: The Origins of Radio, Princeton University Press, 1985.