Biography of Friedrich Wilhelm Georg Kohlrausch

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Friedrich Wilhelm Georg Kohlrausch was German physicist who investigated the electroconductive properties of electrolytes and contributed to the understanding of their behaviour. His research work covered also investigations of elasticity, thermoelasticity, and thermal conduction as well as magnetic and electrical precision measurements.

Today, Friedrich Kohlrausch is classed as one of the most important experimental physicists. His early work helped to extend the absolute system of Carl Friedrich GauB and Wilhelm Weber to include electrical and magnetic measuring units. Friedrich Wilhelm Georg Kohlrausch was born on October 14, 1840, in Rinteln, Germany. After studying physics at Erlangen and Gottingen, Friedrich Kohlrausch completed his doctorate in Gottingen. After a two-year work as a lecturer at the Univesity of Frankfurt/Main, Kohlrausch was appointed a professor of physics at the Universitiy of Gottingen (1866-70).

In the Universitiy of Gottingen he performed the practical experiments that resulted in the book “Leitfaden der praktischen Physik” (Guidelines to Practical Physics) which was published in 1870 as the first book of this type in Germany. It contained not only descriptions of experiments, experimental setups and measuring techniques, but also tables of physical quantities.

To this very day, the textbook “Praktische Physik” (Practical Physics), which originated in Friedrich Kohlrausch’s “Leitfaden der praktischenPhysik”, is standard reading for physicists and engineers in Germany. This is attributable, above all, to the detailed descriptions provided of the precision measuring methods that form the basis of technical and experimental applications in a wide diversity of fields in physics.

In 1870 Kohlrausch became a professor at the Technical University of Zurich in Switzerland. One year later, he moved to the Technical University of Darmstadt in Germany. In 1874 he demonstrated that an electrolyte has a definite and constant amount of electrical resistance. By observing the dependence of conductivity upon dilution, he could determine the transfer velocities of the ions (charged atoms or molecules) in solution.

Kohlrausch’s research was centered on determining how electricity was conducted in solutions. In his investigations to establish whether Ohm’s law also applied to second class conductors, he was the first scientist to utilize alternating current. In this way, he prevented the deposition of decomposition products on the electrodes and obtained highly precise results in his measurements. He used a telephone to monitor the process.

The result was that Ohm’s law also found to apply to dissolvedelectrolytes. From 1875 to 1879, he examined numerous salt solutions, acids and solutions of other materials. His efforts resulted in the law of the independent migration of ions, that is, each type of migrating ion has a specific resistance no matter what its original molecular combination may have been, and therefore that a solution’s electrical resistance was due only to the migrating ions of a given substances. Kohlrausch suggested that the more dilute a solution, the greater its conductivity.

Kohlrausch measuring conductivity of solutions using a telephone to monitor the process

In 1875 he responded to a call from the University of WUrzburg in southern Germany, where he subsequently conducted his experiments on electrical quantity determination and the conductivity of electrolytes. From 1888 he researched and taught at Strasbourg University. He turned down a professorship at the Humboldt University in Berlin in 1894. In 1895 he succeeded Hermann von Helmholtz as President of the Physikalisch-Technische Reichsanstalt (PTR – Imperial Physical Technical Institute), an office which he held until 1905.

Here, as in the past, his activities were focused on experimental and instrumental physics: he constructed instruments and devised new measuring techniques to examine electrolytic conduction in solutions. He concluded the setup of the PTR, a task which had not yet been completed on the death of its first president. He introduced fixed regulations, work schedules and working hours for the institute. From 1900 he was also a professor at Humboldt University.

Standing, from the left: Heydweiller, Rasch, Svante Arrhenius, Walther Nernst, sitting, from the left: Palazzo, Friedrich Kohlrausch, Sheldon. Physical institute, University of WUrzburg, winter semester 1886/1887

The idea of creating the Physikalisch-Technischen Reichsanstalt (PTR) dated back to Schellbach’s expose entitled “Uber die GrUndung eines Museums fUr die exakten Wissenschaften” (On the Foundation of a Museum for the Exact Sciences) published in 1872. However, it was the endeavors of the industrialist Werner von Siemens from 1882 onwards – and the gift of a building site in particular – that ultimately led to the foundation of the “Physikalisch – Technische Reichsanstalt fUr die experimentelle Forderung der exakten Naturwissenschaften und der Prazisionstechnik” (Imperial Physical Technical Institute for the Experimental Advancement of the Exact Sciences and Precision Technology).

The institute was founded by a resolution passed by the German Imperial Parliament (“Reichstag”) on March 28, 1887. Organizationally, it consisted of a Physics Department and a Technical Department which, however, worked together on research projects. The former included laboratories for heat, electricity and optics, while the latter encompassed labs for precision metrology, heat and pressure, and the Optical Laboratory (in 1914 the departments were assigned to individual subject areas).

On October 1, 1887 the Technical Department commenced its work on temporary premises at Charlottenburg Technical University in Berlin. The first President of the PTR was Hermann von Helmholtz. A committee comprising personalities from the field of science, commerce and government served as a supervisory board for the institute’s scientific and technical activities. In the early years, the members included Ernst Abbe, Rudolf Clausius, Wilhelm Conrad Rontgen and Werner von Siemens.

Under direction of Kohlrausch “Physikalisch-Technische Reichsanstalt” (the Imperial Physical Technical Institute in Germany) created numerous standards and calibration standards which were also used internationally outside Germany.

Basic Research Focused on Metrology Right from the outset, the institute was characterized by a totally new approach: studies in the fields of physics and technology, and for large-scale projects in particular, were conducted in teams. Nevertheless, the individual accomplishments of numerous research scientists left their unmistakable mark on the development of the PTR. A large number of remarkable milestones punctuate the history of the metrology institute. It performed basic research focused on the field of measuring technology: it tested the accuracy and reliability of measuring instruments.

From 1898 onwards, it was given the legal task of creating units of electricity and monitoring instruments for measuring electrical quantities. In the same year it commenced the certification of clinical thermometers. In 1923 the Imperial Institute for Weights and Measures was integrated. This meant the PTR was then responsible for all legal measuring units, making it the supreme supervisory body for monitoring calibration and testing offices.

The founding of the Physikalisch-Technische Reichsanstalt resulted from an awareness that a national measuring authority was an essential part of the infrastructure of an industrialized country. Its successor, the Physikalisch-Technische Bundesanstalt (PTB – the Federal Physical technical Institute), follows the same mission of its predecessor: the advancement of science and technology, and the promotion of industry.

Kohlrausch was intent on creating optimum working conditions in the laboratories and to shield the labs from disturbing external influences. For six years, for instance, he fought against a streetcar line which was due to be laid near the PTR. However, before the streetcar was to make its first journey, the institute succeeded in developing an astatic torsion magnetometer which was uninfluenced by disturbing electromagnetic fields. The use of this instrument and the shielded wire galvanometer developed by du Bois and Rubens meant that precision electrical and magnetic work continued to be possible.

Over the years, Kohlrausch added experiments which met the needs of physical chemistry and electrical technology in particular. He improved precision measuring instruments and developed numerous measuring methods in almost all of the fields of physics known during his lifetime, including a reflectivity meter, a tangent galvanometer, and various types of magnetometers and dynamometers. The Kohlrausch bridge, which he invented at that time for the purpose of measuring conductivity, is still well known today.

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