Kirchhoff’s scientific activities were so innovative and groundbreaking that they still serve as the basis for research and industrial applications in physics, chemistry, and electrical engineering today.

In honor of the 200^th anniversary of Gustav Kirchhoff’s birth, the Kirchhoff-Institute for Physics, in collaboration with the Physics Institute and the University Museum, is presenting an exhibition to discover new things and reclassify old ones.

 

Gustav Robert Kirchhoff took over the chair of physics at the Ruprecht-Karls-Universität Heidelberg in 1854. At this time, he was only 30 years old and known only to a small circle of experts. Within a few years, his scientific achievements earned him great recognition among experts and popularity with the general public.

Gustav Kirchhoff and chemistry professor Robert Bunsen achieved world fame through their close, friendly collaboration at the University of Heidelberg. In 1859, they jointly developed spectral analysis, a pioneering method for examining the light of chemical elements. Based on Kirchhoff’s radiation law, it laid the foundation for modern spectroscopy and led to the discovery of new chemical elements such as caesium and rubidium.

The work in spectral analysis marked important milestones in the history of physics and contributed to the mathematical and scientific subjects in Heidelberg experiencing a period of unprecedented fame. This laid the foundations for later astrophysics and quantum physics. Kirchhoff’s legacy includes groundbreaking work in other areas of physics, including thermal radiation and thermodynamics.

Today, Kirchhoff’s rules are best known. They are among the fundamental laws of electrical engineering and are still used today.

1824 Gustav Robert Kirchhoff is born on March 12 in Königsberg, Prussia

1842 to 1847 Studies at the University of Königsberg under Friedrich Julius Richelot and Franz Ernst Neumann, whose students occupied almost all German chairs of physics

1845, while still a student, Kirchhoff publishes an essay “On the passage of an electric current through a plane...” His observations on current and voltage distribution are known today as “Kirchhoff’s rules”

1850 to 1854 Professor at the University of Breslau, first contacts with Bunsen

1854 Transfer to the University of Heidelberg as “Professor of Physics and Director of the Physics Cabinet”

1857 Kirchhoff marries Clara Richelot (* 1838; † 1869), a daughter of his Königsberg mathematics professor, with whom he has five children

1859 Bunsen and Kirchhoff develop the basics of spectral analysis

1860 Publication entitled: “Chemical analysis by spectral observations; by G. Kirchhoff and R. Bunsen” in: Annals of Physics and Chemistry, 1860 No. 6, Volume CX, the important German professional journal

1861 Discovery of new chemical elements, such as caesium and rubidium

1872 After the death of his first wife, Kirchhoff marries Luise Brömmel, head of the nursing service at an eye clinic in Heidelberg

1875 Kirchhoff accepted an appointment at the University of Berlin, where he became the first professor of theoretical physics in Germany. At the same time, he becomes an exceptionally remunerated memeber of the Prussian Academy of Sciences

1887 Kirchhoff dies highly respected in Berlin

The more careful the preparatory work, the more precise the measurements – a prerequisite for the breakthrough in spectral analysis was Bunsen’s improved gas burner and the pure substances he produced himself.

 

In 1854, Robert Bunsen, his student Henry Enfield Roscoe and the laboratory mechanic Peter Desaga (1812–1879) further developed a handy burner. A stream of gas (typically methane) mixed with air is ignited and produces a non-turbulent and non-luminous flame. It provides a heat source that is safe and, especially controllable by adjusting the size of the air supply opening and thus changing the air supply.

Desaga, who ran a store for optical and chemical apparatus in Heidelberg’s Hauptstraße, marketed the new model from 1855 as “Bunsen’s luminescent gas apparatus”, later known as the Bunsen burner.

Spectroscopic investigations had already been carried out earlier by important scientists such as William Hyde Wollaston (1766-1828), Joseph von Fraunhofer (1787-1826) and John Herschel (1792- 1871). However, they were misled into drawing false conclusions by natural impurities. In particular, the superposition of the characteristic, intense yellow lines of the sodium spectrum made it difficult to identify other elements.

Bunsen and Kirchhoff developed methods to minimize sodium impurities and make the spectra of other elements more clearly visible.

 

Bunsen prepared samples of Dürkheim mineral water and Saxon lepidolite using elaborate, careful separation procedures. Thanks to the high sensitivity of spectral analysis, spectral lines could now be observed that did not match any of the elements known at the time.

 

On May 3, 1860 and February 23, 1861, Kirchhoff and Bunsen informed the Academy of Sciences in Berlin of the discovery of two new elements: Caesium and Rubidium! The names are derived from the Latin color names for the colors of the spectral lines:

Caesium - from caesius ‘sky blue’ - has two blue spectral lines, and rubidium has two red - from rubidus ‘dark red’ - spectral lines. Bunsen and Kirchhoff subsequently published the naming, methods of isolation and an initial characterization together in the "Annalen der Physik un Chemie".

These discoveries represented a significant advance in analytical chemistry and demonstrated the effectiveness of spectral analysis as a method for discovering new elements.

Gustav Kirchhoff was appointed “full professor of physics and director of the Physics Cabinet” in Heidelberg in 1854 – what does that mean?

 

A cabinet can be a cupboard or a room in which a collection is kept. In 1752, the Elector gave a small collection of apparatus to Christian Mayer, the new Professor of Experimental and Mathematical Physics, for lecture purposes. Further donations and purchases followed. In 1850, the Institute of Physics was housed in the Haus zum Riesen: on the second floor, a large collection room, a lecture hall for physics, a room for the director and a working room as a physics laboratory were located close together. Kirchhoff’s predecessor, Philipp von Jolly, made it possible for students to conduct their own experiments in the laboratory here for the first time. With the international success of the natural sciences in Heidelberg, initiated by scholars such as Kirchhoff, Bunsen and Helmholtz, physics grew rapidly and so did the need for equipment. As director of the Physics Cabinet, Kirchhoff was allowed to spend 400 guilders a year on it.

In 1905, two institutes were built for the Nobel Prize winner Philipp Lenard at Philosophenweg 12, equipped with a large lecture hall, an adjoining room for preparing lectures in experimental physics and a separate room for storing the collection. With the construction of the new lecture hall building for physics “Im Neuenheimer Feld 308” in the 1970s, a large part of this collection was moved with it so that it could continue to be used for experimental physics lectures.The oldest objects – some from the time before Kirchhoff – have been housed in the building “Im Neuenheimer Feld 226” as the “Collection of Historical Instruments of the Institute of Physics” since 2012. They are occasionally shown in exhibitions.

In 1859, Kirchhoff worked on the solar spectrum and Bunsen experimented with chemical analysis using flame coloration. To bring the two together, they had the laboratory technician Peter Desaga assemble an improvised apparatus from existing parts.

 

The first spectral apparatus was a prism spectrograph that directed the light to be examined through a thin entrance slit. A collimator aligned the light in parallel before it hit a prism, which caused the light to be split according to frequency. Pivoting binoculars made it possible to measure the splitting of the light, whereby the angles between the rays were measured. As it is difficult to measure angles in a bundle of rays, a camera was often used instead of binoculars to measure the resulting distances on a photographic plate. Every luminous gas under low pressure emits a characteristic spectrum, which shows up as a line spectrum with lines appearing only when the substance in question is glowing. This applies to both emission and absorption spectra, which are characterized by dark lines in a continuous spectrum. The spectral lines provide information about what substances have surrounded the light source or have penetrated on the way to the examination apparatus. Spectral analysis made it possible to describe the glow of stellar atmospheres and marked the beginning of astrophysics. It was also a prerequisite for the development of quantum mechanics in the 20th century and founded an entire branch of analytics in chemistry.

Ever since Kirchhoff and Bunsen first recognized that light spectra serve as unique fingerprints for a material’s chemical composition, spectroscopy has found widespread applications across various domains, from research to industry.

 

The light emitted by stars and galaxies carries vital clues about their chemistry, helping astronomers in unraveling cosmic mysteries. By identifying light spectra typical of water, scientists can discover potentially habitable planets in the vastness of space.

 

In the pharmaceutical industry, spectroscopy is an important window into the molecular structure of drugs. It allows for realtime monitoring of stability, precise measurement of drug concentrations in the bloodstream, and detection of counterfeit medications, ensuring safety and efficacy. Spectroscopy is also used in medicine, particularly for cancer diagnosis. By analyzing tissue’s response to light, medical personnel gain insights into its cellular composition, which leads to early detection and personalized treatment strategies.

 

Lastly, the food industry relies on spectroscopy to quantify the precise nutritional levels of raw materials, such as the water content. This is possible by measuring the spectrum of the light reflected by the raw ingredients and identifying the presence of specific fingerprints. In this way, it is possible to select the best potatoes for your chips.

Gustav Robert Kirchhoff is probably most often associated with the circuit laws on the distribution of currents in a conductor system named after him, which he defined and published in 1845.

 

These can be found in one of his first papers, which Kirchhoff published while still a fourth-year student in Königsberg. He investigated the passage of electric current through a circular plane. He calculated the exact positions of the equipotential lines in this plane, which he was then able to confirm experimentally with a round copper plate. As an appendix to this research work, he formulated the previously known relationships between currents and voltages in electrical networks into generally applicable laws. These laws are still important today in electrical engineering for circuit design and network analysis.

For the protagonists of Romanticism, empathizing with nature and living and working symbiotically with like-minded people was a rebellion against social confinement.

Kirchhoff was more than a generation younger. Enjoying nature in ideal landscapes and cultivating close friendships served him and his bourgeois surroundings as a source of relaxation after strenuous work, but reading circles and social gatherings could also become exhausting for such a popular and conscientious person as Kirchhoff. In December 1854, he wrote: “... I don't like the many parties to which I am invited as a new arrival and from which people don't go home until 1 a.m., sometimes not until 3 a.m.. ... Even if I have a good time in society, I am still tired for the day and reading [giving lectures] makes me sick.” Nevertheless, the connections were so close that the godparents of the professor's children often came from the Bunsen, Kirchhoff or Helmholtz families.