Heinrich Rudolf Hertz (/hɜːrts/; German: [ˈhaɪ̯nʁɪç ˈhɛɐ̯ts]; 22 February 1857 – 1 January 1894) was a German physicist who first conclusively proved the existence of the electromagnetic waves theorized by James Clerk Maxwell's electromagnetic theory of light. The unit of frequency – cycle per second – was named the "hertz" in his honor.
Heinrich Rudolf Hertz
22 February 1857
|Died||1 January 1894 (aged 36)|
|Alma mater||University of Munich|
University of Berlin
|Known for||Electromagnetic radiation|
Hertz's principle of least curvature
|Awards||Matteucci Medal (1888)|
Rumford Medal (1890)
|Institutions||University of Kiel|
University of Karlsruhe
University of Bonn
|Doctoral advisor||Hermann von Helmholtz|
|Doctoral students||Vilhelm Bjerknes|
Heinrich Rudolf Hertz was born in 1857 in Hamburg, then a sovereign state of the German Confederation, into a prosperous and cultured Hanseatic family. His father was Gustav Ferdinand Hertz. His mother was Anna Elisabeth Pfefferkorn.
While studying at the Gelehrtenschule des Johanneums in Hamburg, Hertz showed an aptitude for sciences as well as languages, learning Arabic and Sanskrit. He studied sciences and engineering in the German cities of Dresden, Munich and Berlin, where he studied under Gustav R. Kirchhoff and Hermann von Helmholtz. In 1880, Hertz obtained his PhD from the University of Berlin, and for the next three years remained for post-doctoral study under Helmholtz, serving as his assistant. In 1883, Hertz took a post as a lecturer in theoretical physics at the University of Kiel. In 1885, Hertz became a full professor at the University of Karlsruhe.
In 1886, Hertz married Elisabeth Doll, the daughter of Dr. Max Doll, a lecturer in geometry at Karlsruhe. They had two daughters: Johanna, born on 20 October 1887 and Mathilde, born on 14 January 1891, who went on to become a notable biologist. During this time Hertz conducted his landmark research into electromagnetic waves.
Hertz took a position of Professor of Physics and Director of the Physics Institute in Bonn on 3 April 1889, a position he held until January 1894. During this time he worked on theoretical mechanics with his work published in the book Die Prinzipien der Mechanik in neuem Zusammenhange dargestellt (The Principles of Mechanics Presented in a New Form), published posthumously in 1894.
In 1892, Hertz was diagnosed with an infection (after a bout of severe migraines) and underwent operations to treat the illness. He died of granulomatosis with polyangiitis at the age of 36 in Bonn, Germany in 1894, and was buried in the Ohlsdorf Cemetery in Hamburg.
Hertz's wife, Elisabeth Hertz née Doll (1864–1941), did not remarry. Hertz left two daughters, Johanna (1887–1967) and Mathilde (1891–1975). Hertz's daughters never married and he has no descendants.
Hertz always had a deep interest in meteorology, probably derived from his contacts with Wilhelm von Bezold (who was his professor in a laboratory course at the Munich Polytechnic in the summer of 1878). However, Hertz did not contribute much to the field himself except some early articles as an assistant to Helmholtz in Berlin, including research on the evaporation of liquids, a new kind of hygrometer, and a graphical means of determining the properties of moist air when subjected to adiabatic changes.
In 1886–1889, Hertz published two articles on what was to become known as the field of contact mechanics. Hertz is well known for his contributions to the field of electrodynamics (see below); however, most papers that look into the fundamental nature of contact cite his two papers as a source for some important ideas. Joseph Valentin Boussinesq published some critically important observations on Hertz's work, nevertheless establishing this work on contact mechanics to be of immense importance. His work basically summarises how two axi-symmetric objects placed in contact will behave under loading, he obtained results based upon the classical theory of elasticity and continuum mechanics. The most significant failure of his theory was the neglect of any nature of adhesion between the two solids, which proves to be important as the materials composing the solids start to assume high elasticity. It was natural to neglect adhesion in that age as there were no experimental methods of testing for it.
To develop his theory Hertz used his observation of elliptical Newton's rings formed upon placing a glass sphere upon a lens as the basis of assuming that the pressure exerted by the sphere follows an elliptical distribution. He used the formation of Newton's rings again while validating his theory with experiments in calculating the displacement which the sphere has into the lens. K. L. Johnson, K. Kendall and A. D. Roberts (JKR) used this theory as a basis while calculating the theoretical displacement or indentation depth in the presence of adhesion in 1971. Hertz's theory is recovered from their formulation if the adhesion of the materials is assumed to be zero. Similar to this theory, however using different assumptions, B. V. Derjaguin, V. M. Muller and Y. P. Toporov published another theory in 1975, which came to be known as the DMT theory in the research community, which also recovered Hertz's formulations under the assumption of zero adhesion. This DMT theory proved to be rather premature and needed several revisions before it came to be accepted as another material contact theory in addition to the JKR theory. Both the DMT and the JKR theories form the basis of contact mechanics upon which all transition contact models are based and used in material parameter prediction in nanoindentation and atomic force microscopy. So Hertz's research from his days as a lecturer, preceding his great work on electromagnetism, which he himself considered with his characteristic soberness to be trivial, has come down to the age of nanotechnology.
During Hertz's studies in 1879 Helmholtz suggested that Hertz's doctoral dissertation be on testing Maxwell's theory of electromagnetism, published in 1865, which predicted the existence of electromagnetic waves moving at the speed of light, and predicted that light itself was just such a wave. Helmholtz had also proposed the "Berlin Prize" problem that year at the Prussian Academy of Sciences for anyone who could experimentally prove an electromagnetic effect in the polarization and depolarization of insulators, something predicted by Maxwell's theory. Helmholtz was sure Hertz was the most likely candidate to win it. Not seeing any way to build an apparatus to experimentally test this, Hertz thought it was too difficult, and worked on electromagnetic induction instead. Hertz did produce an analysis of Maxwell's equations during his time at Kiel, showing they did have more validity than the then prevalent "action at a distance" theories.
After Hertz received his professorship at Karlsruhe he was experimenting with a pair of Riess spirals in the autumn of 1886 when he noticed that discharging a Leyden jar into one of these coils would produce a spark in the other coil. With an idea on how to build an apparatus, Hertz now had a way to proceed with the "Berlin Prize" problem of 1879 on proving Maxwell's theory (although the actual prize had expired uncollected in 1882). He used a Ruhmkorff coil-driven spark gap and one-meter wire pair as a radiator. Capacity spheres were present at the ends for circuit resonance adjustments. His receiver was a simple half-wave dipole antenna with a micrometer spark gap between the elements. This experiment produced and received what are now called radio waves in the very high frequency range.
Between 1886 and 1889 Hertz would conduct a series of experiments that would prove the effects he was observing were results of Maxwell's predicted electromagnetic waves. Starting in November 1887 with his paper "On Electromagnetic Effects Produced by Electrical Disturbances in Insulators", Hertz would send a series of papers to Helmholtz at the Berlin Academy, including papers in 1888 that showed transverse free space electromagnetic waves traveling at a finite speed over a distance. In the apparatus Hertz used, the electric and magnetic fields would radiate away from the wires as transverse waves. Hertz had positioned the oscillator about 12 meters from a zinc reflecting plate to produce standing waves. Each wave was about 4 meters long. Using the ring detector, he recorded how the wave's magnitude and component direction varied. Hertz measured Maxwell's waves and demonstrated that the velocity of these waves was equal to the velocity of light. The electric field intensity, polarization and reflection of the waves were also measured by Hertz. These experiments established that light and these waves were both a form of electromagnetic radiation obeying the Maxwell equations. Hertz also described the "Hertzian cone", a type of wave-front propagation through various media.
Hertz helped establish the photoelectric effect (which was later explained by Albert Einstein) when he noticed that a charged object loses its charge more readily when illuminated by ultraviolet radiation (UV). In 1887, he made observations of the photoelectric effect and of the production and reception of electromagnetic (EM) waves, published in the journal Annalen der Physik. His receiver consisted of a coil with a spark gap, whereby a spark would be seen upon detection of EM waves. He placed the apparatus in a darkened box to see the spark better. He observed that the maximum spark length was reduced when in the box. A glass panel placed between the source of EM waves and the receiver absorbed UV that assisted the electrons in jumping across the gap. When removed, the spark length would increase. He observed no decrease in spark length when he substituted quartz for glass, as quartz does not absorb UV radiation. Hertz concluded his months of investigation and reported the results obtained. He did not further pursue investigation of this effect, nor did he make any attempt at explaining how the observed phenomenon was brought about.
Hertz's proof of the existence of airborne electromagnetic waves led to an explosion of experimentation with this new form of electromagnetic radiation, which was called "Hertzian waves" until around 1910 when the term "radio waves" became current. Within 10 years researchers such as Oliver Lodge, Ferdinand Braun, and Guglielmo Marconi employed radio waves in the first wireless telegraphy radio communication systems, leading to radio broadcasting, and later television. In 1909, Braun and Marconi received the Nobel Prize in physics for their "contributions to the development of wireless telegraphy". Today radio is an essential technology in global telecommunication networks, and the transmission medium underlying modern wireless devices.
In 1892, Hertz began experimenting and demonstrated that cathode rays could penetrate very thin metal foil (such as aluminium). Philipp Lenard, a student of Heinrich Hertz, further researched this "ray effect". He developed a version of the cathode tube and studied the penetration by X-rays of various materials. Philipp Lenard, though, did not realize that he was producing X-rays. Hermann von Helmholtz formulated mathematical equations for X-rays. He postulated a dispersion theory before Röntgen made his discovery and announcement. It was formed on the basis of the electromagnetic theory of light (Wiedmann's Annalen, Vol. XLVIII). However, he did not work with actual X-rays.
Heinrich Hertz was a Lutheran throughout his life and would not have considered himself Jewish, as his father's family had all converted to Lutheranism when his father was still in his childhood (aged seven) in 1834.
Nevertheless, when the Nazi regime gained power decades after Hertz's death, his portrait was removed by them from its prominent position of honor in Hamburg's City Hall (Rathaus) because of his partly Jewish ethnic ancestry. (The painting has since been returned to public display.)
Hertz's widow and daughters left Germany in the 1930s and went to England.
Heinrich Hertz's nephew Gustav Ludwig Hertz was a Nobel Prize winner, and Gustav's son Carl Helmut Hertz invented medical ultrasonography. His daughter Mathilde Carmen Hertz was a well-known biologist and comparative psychologist. Hertz's grandnephew Hermann Gerhard Hertz, professor at the University of Karlsruhe, was a pioneer of NMR-spectroscopy and in 1995 published Hertz's laboratory notes.
The SI unit hertz (Hz) was established in his honor by the International Electrotechnical Commission in 1930 for frequency, an expression of the number of times that a repeated event occurs per second. It was adopted by the CGPM (Conférence générale des poids et mesures) in 1960, officially replacing the previous name, "cycles per second" (cps).
In 1928 the Heinrich-Hertz Institute for Oscillation Research was founded in Berlin. Today known as the Fraunhofer Institute for Telecommunications, Heinrich Hertz Institute, HHI.
In 1969 (East Germany), a Heinrich Hertz memorial medal was cast. The IEEE Heinrich Hertz Medal, established in 1987, is "for outstanding achievements in Hertzian waves [...] presented annually to an individual for achievements which are theoretical or experimental in nature".
In 1980, in Italy a High School called "Istituto Tecnico Industriale Statale Heinrich Hertz" was founded in the neighborhood of Cinecittà Est, in Rome.
A crater that lies on the far side of the Moon, just behind the eastern limb, is named in his honor. The Hertz market for radio electronics products in Nizhny Novgorod, Russia, is named after him. The Heinrich-Hertz-Turm radio telecommunication tower in Hamburg is named after the city's famous son.
Heinrich Hertz has been honored by a number of countries around the world in their postage issues, and in post-World War II times has appeared on various German stamp issues as well.
Carl Hellmuth Hertz (also written Carl Helmut Hertz, October 15, 1920 – April 29, 1990) was a German physicist known primarily for being involved in the development of inkjet technology and ultrasound technology. He was the son of Gustav Ludwig Hertz and great nephew of Heinrich Hertz.Fraunhofer Institute for Telecommunications
The Fraunhofer Institute for Telecommunications, Heinrich Hertz Institute, HHI, also known as Fraunhofer HHI or Fraunhofer Heinrich Hertz Institute, is an organization of the Fraunhofer Society based in Berlin. The institute engages in applied research and development in the fields of physics, electrical engineering and computer sciences.Gustav Ferdinand Hertz
Gustav Ferdinand Hertz (born August 2, 1827 as David Gustav Hertz in Hamburg, died September 8, 1914) was a German lawyer and senator of the Free Imperial City of Hamburg. He was the father of the pioneering physicist Heinrich Hertz.Heinrich Hertz Submillimeter Telescope
The Submillimeter Telescope (SMT), formerly known as the Heinrich Hertz Submillimeter Telescope, is a submillimeter wavelength radio telescope located on Mount Graham, Arizona. It is a 10-meter-wide parabolic dish inside a building to protect it from bad weather. The building front doors and roof are opened when the telescope is in use. The telescope's construction was finished in 1993. Along with the 12 Meter Telescope on Kitt Peak, this telescope is maintained by the Arizona Radio Observatory, a division of Steward Observatory at the University of Arizona.
The dryness of the air around and above Mt. Graham is particularly vital for EHF (extremely short wavelength radio) and far-infrared observations - a region of the spectrum where the electromagnetic waves are strongly attenuated by any water vapor or clouds in the air.
This telescope is used nine-to-ten months of the year, and it is stowed only when there is too much water vapor in the atmosphere, primarily during the summertime. This telescope is one of the telescopes that makes up Mount Graham International Observatory.Heinrich Hertz Tower
The Heinrich Hertz Tower (German: Heinrich-Hertz-Turm) is a landmark radio telecommunication tower in the city of Hamburg, Germany.
Designed by architect Fritz Trautwein, in co-operation with civil engineers Jörg Schlaich, Rudolf Bergermann and Fritz Leonhardt, the tower was built between 1965–1968 for the former Deutsche Bundespost (German Federal Post and Telecommunications Agency, now Deutsche Telekom 's subsidiary Deutsche Funkturm GmbH) near Planten un Blomen park.
With an overall height of 279.2 m (916 ft) it is Hamburg's tallest structure, consisting of a 204 m (670 ft) steel-reinforced concrete lower section topped by a 45 m (148 ft) steel-lattice tower and a three-segmented cylinder of about 30 m (98 ft), which supports various antennas. There are eight concentric platforms stacked one above the other: starting at 128 m (420 ft) with the two-story observation (lower floor) and restaurant (upper floor) platform, served by two high-speed elevators. Above that at 150 m (492 ft) is the operations platform housing the workforce and equipment, and further up six differentially sized, smaller open platforms in same distances, populated with high-gain directional microwave radio relay antennas ("parabolic mirrors"). Number nine was added at 25 m height in July 2005.
After the observation platform and restaurant were closed due to asbestos decontamination, former stuntman Jochen Schweizer had a bungee jumping base installed. The restaurant will not open again due to new fire escape regulations, and the bungee platform was closed at the end of 2001.
The tower has been home to the VFDB Hamburg section's radio amateur club station "DF0HHT". It also housed a DGPS transmitting station serving the city of Hamburg's Surveying Agency.
The tower is named after the Hamburg-born German physicist Heinrich Hertz. A memorial plaque in his honour on the tower's wall reads: "Heinrich Hertz – Dem Sohn der Stadt Hamburg" ("Heinrich Hertz - Son of the City of Hamburg").Henrik Hertz
Not to be confused with Heinrich Hertz, a German physicistHenrik Hertz (25 August 1797 – 25 February 1870) was a Danish poet.Hertz
The hertz (symbol: Hz) is the derived unit of frequency in the International System of Units (SI) and is defined as one cycle per second. It is named for Heinrich Rudolf Hertz, the first person to provide conclusive proof of the existence of electromagnetic waves. Hertz are commonly expressed in multiples: kilohertz (103 Hz, kHz), megahertz (106 Hz, MHz), gigahertz (109 Hz, GHz), terahertz (1012 Hz, THz), petahertz (1015 Hz, PHz), and exahertz (1018 Hz, EHz).
Some of the unit's most common uses are in the description of sine waves and musical tones, particularly those used in radio- and audio-related applications. It is also used to describe the speeds at which computers and other electronics are driven.Hertzian cone
A Hertzian cone is the cone produced when an object passes through a solid, such as a bullet through glass. More technically, it is a cone of force that propagates through a brittle, amorphous or cryptocrystalline solid material from a point of impact. This force eventually removes a full or partial cone in the material. This is the physical principle that explains the form and characteristics of the flakes removed from a core of tool stone during the process of lithic reduction.
This phenomenon is named after the German physicist Heinrich Rudolf Hertz, who first described this type of wave-front propagation through various media.
Although it might not be agreed by all, natural phenomena which have been grouped with the Hertzian cone phenomena include the crescentic "chatter marks" made on smoothed bedrock by glacial ice dragging along boulders at its base, the numerous crescentic impact marks sometimes seen on pebbles and cobbles, and the shatter cones found at bolide impact sites. James Byous, working independently (at privately funded Dowd Research, Savannah, Georgia USA) has made a protracted study of Hertzian cones. Some of his work may be found via sharing points or directly at Dowd Research. He has produced a comprehensive glossary on Hertzian fractures and related terms. A Hertzian cone is often 104 degrees when created by an indenter. Smaller cones may be produced due to lack of size of the material, or irregularities in the structure of the material. However, in ballistics the faster the projectile the steeper the edges and angle of the cone.Hertz–Knudsen equation
In surface chemistry, the Hertz–Knudsen equation, also known as Knudsen-Langmuir equation describes evaporation rates, named after Heinrich Hertz and Martin Knudsen.History of quantum mechanics
The history of quantum mechanics is a fundamental part of the history of modern physics. Quantum mechanics' history, as it interlaces with the history of quantum chemistry, began essentially with a number of different scientific discoveries: the 1838 discovery of cathode rays by Michael Faraday; the 1859–60 winter statement of the black-body radiation problem by Gustav Kirchhoff; the 1877 suggestion by Ludwig Boltzmann that the energy states of a physical system could be discrete; the discovery of the photoelectric effect by Heinrich Hertz in 1887; and the 1900 quantum hypothesis by Max Planck that any energy-radiating atomic system can theoretically be divided into a number of discrete "energy elements" ε (epsilon) such that each of these energy elements is proportional to the frequency ν with which each of them individually radiate energy, as defined by the following formula:
where h is a numerical value called Planck's constant.
Then, Albert Einstein in 1905, in order to explain the photoelectric effect previously reported by Heinrich Hertz in 1887, postulated consistently with Max Planck's quantum hypothesis that light itself is made of individual quantum particles, which in 1926 came to be called photons by Gilbert N. Lewis. The photoelectric effect was observed upon shining light of particular wavelengths on certain materials, such as metals, which caused electrons to be ejected from those materials only if the light quantum energy was greater than the work function of the metal's surface.
The phrase "quantum mechanics" was coined (in German, Quantenmechanik) by the group of physicists including Max Born, Werner Heisenberg, and Wolfgang Pauli, at the University of Göttingen in the early 1920s, and was first used in Born's 1924 paper "Zur Quantenmechanik". In the years to follow, this theoretical basis slowly began to be applied to chemical structure, reactivity, and bonding.IEEE Heinrich Hertz Medal
The IEEE Heinrich Hertz Medal was a science award presented by the IEEE for outstanding achievements in the field of electromagnetic waves. The medal was named in honour of German physicist Heinrich Hertz, and was first proposed in 1986 by IEEE Region 8 (Germany) as a centennial recognition of Hertz's work on electromagnetic radiation theory from 1886 to 1891. The medal was first awarded in 1988, and was presented annually until 2001.Microwave engineering
Microwave engineering pertains to the study and design of microwave circuits, components, and systems. Fundamental principles are applied to analysis, design and measurement techniques in this field. The short wavelengths involved distinguish this discipline from Electronic engineering. This is because there are different interactions with circuits, transmissions and propagation characteristics at microwave frequencies.
Some theories and devices that pertain to this field are antennas, radar, transmission lines, space based systems (remote sensing), measurements, microwave radiation hazards and safety measures.
During World War II microwave engineering played a significant role in developing radar that could accurately locate enemy ships and planes with a focused beam of EM radiation. The foundations of this discipline are found in Maxwell's equations and the work of Heinrich Hertz, William Thomson's waveguide theory, J.C. Bose, the klystron from Russel and Varian Bross, as well as contributions from Perry Spencer, and others.Mount Graham International Observatory
Mount Graham International Observatory (MGIO) is a division of Steward Observatory, the research arm for the Department of Astronomy at The University of Arizona, in the United States. It is located in southeastern Arizona's Pinaleño Mountains near Mount Graham.
Construction of MGIO began in 1989. MGIO currently operates and maintains facilities for three scientific organizations. The first two telescopes, the Vatican Advanced Technology Telescope and the Heinrich Hertz Submillimeter Telescope began operations in 1993. The Large Binocular Telescope, one of the world's largest and most powerful optical telescopes, began operations using mirrors independently in 2004, with joint operations between the two mirrors beginning in 2008.Public tours of the MGIO are conducted by the Eastern Arizona College's (EAC) Discovery Park Campus between mid-April and mid-October (weather permitting and subject to reservations).Musée de Radio France
The Musée de Radio France was a museum operated by Radio France and located in the Maison de Radio-France, near the Pont de Grenelle in the XVIe arrondissement at 116, avenue du Président Kennedy, Paris, France. The museum was established in 1966, and contained a remarkable collection of radios and televisions from their origins to the present day, including the 1793 telegraph by Claude Chappe and early crystal radios. The museum's 2000 objects include prototypes and commercial devices, archival documents, photographs, and manuscripts, replicas of early radio laboratories and studios, and exhibits featuring research by Edouard Branly, Lee de Forest, Heinrich Hertz, Guglielmo Marconi, James Clerk Maxwell, and Alexander Stepanovich Popov. In 2007, the museum was closed to the public due to the renovation of the Maison de Radio France.Photoelectric effect
The photoelectric effect is the emission of electrons or other free carriers when light shines on a material. Electrons emitted in this manner can be called photo electrons. This phenomenon is commonly studied in electronic physics, as well as in fields of chemistry, such as quantum chemistry or electrochemistry.
According to classical electromagnetic theory, this effect can be attributed to the transfer of energy from the light to an electron. From this perspective, an alteration in the intensity of light would induce changes in the kinetic energy of the electrons emitted from the metal. Furthermore, according to this theory, a sufficiently dim light would be expected to show a time lag between the initial shining of its light and the subsequent emission of an electron. However, the experimental results did not correlate with either of the two predictions made by classical theory.Instead, electrons are dislodged only by the impingement of photons when those photons reach or exceed a threshold frequency (energy). Below that threshold, no electrons are emitted from the material regardless of the light intensity or the length of time of exposure to the light. (Rarely, an electron will escape by absorbing two or more quanta. However, this is extremely rare because by the time it absorbs enough quanta to escape, the electron will probably have emitted the rest of the quanta.) To make sense of the fact that light can eject electrons even if its intensity is low, Albert Einstein proposed that a beam of light is not a wave propagating through space, but rather a collection of discrete wave packets (photons), each with energy hν. This shed light on Max Planck's previous discovery of the Planck relation (E = hν) linking energy (E) and frequency (ν) as arising from quantization of energy. The factor h is known as the Planck constant.In 1887, Heinrich Hertz discovered that electrodes illuminated with ultraviolet light create electric sparks more easily. In 1900, while studying black-body radiation, the German physicist Max Planck suggested that the energy carried by electromagnetic waves could only be released in "packets" of energy. In 1905, Albert Einstein published a paper advancing the hypothesis that light energy is carried in discrete quantized packets to explain experimental data from the photoelectric effect. This model contributed to the development of quantum mechanics. In 1914, Millikan's experiment supported Einstein's model of the photoelectric effect. Einstein was awarded the Nobel Prize in 1921 for "his discovery of the law of the photoelectric effect", and Robert Millikan was awarded the Nobel Prize in 1923 for "his work on the elementary charge of electricity and on the photoelectric effect".The photoelectric effect requires photons with energies approaching zero (in the case of negative electron affinity) to over 1 MeV for core electrons in elements with a high atomic number. Emission of conduction electrons from typical metals usually requires a few electron-volts, corresponding to short-wavelength visible or ultraviolet light. Study of the photoelectric effect led to important steps in understanding the quantum nature of light and electrons and influenced the formation of the concept of wave–particle duality. Other phenomena where light affects the movement of electric charges include the photoconductive effect (also known as photoconductivity or photoresistivity), the photovoltaic effect, and the photoelectrochemical effect.
Photoemission can occur from any material, but it is most easily observable from metals or other conductors because the process produces a charge imbalance, and if this charge imbalance is not neutralized by current flow (enabled by conductivity), the potential barrier to emission increases until the emission current ceases. It is also usual to have the emitting surface in a vacuum, since gases impede the flow of photoelectrons and make them difficult to observe. Additionally, the energy barrier to photoemission is usually increased by thin oxide layers on metal surfaces if the metal has been exposed to oxygen, so most practical experiments and devices based on the photoelectric effect use clean metal surfaces in a vacuum.
When the photoelectron is emitted into a solid rather than into a vacuum, the term internal photoemission is often used, and emission into a vacuum distinguished as external photoemission.Riess spiral
Riess spirals, or Knochenhauer spirals, are a pair of spirally wound conductors with metal balls at their ends. Placing one above the other forms an induction coil. Heinrich Hertz used them in his discovery of radio waves. They are named for German physicist Peter Theophil Riess.Wilhelm Hort
Wilhelm Karl Konrad Siegmund Adam Hort (20 March 1878 in Madelungen, now part of Eisenach – 2 June 1938 in Berlin) was a German physicist.
He studied mathematics and physics at the University of Jena, mechanical and electrical engineering at the Technical University of Braunschweig, then completed his studies at the University of Göttingen, where he received a doctorate in physics (1904). In 1917 he received his habilitation at the Technical University of Berlin (TU Berlin) and in 1923 obtained the title of professor.In 1928 he became head of the department of mechanics at the Heinrich-Hertz-Institut für Schwingungsforschung (Heinrich Hertz Institute of Oscillation Research) in Berlin, and in 1931 returned to TU Berlin as chair of Mechanische Schwingungslehre (mechanical oscillations theory). In 1919, with Georg Gehlhoff, he founded the Deutsche Gesellschaft für technische Physik (German Society for Technical Physics).