Christian Gottlieb Kratzenstein

Christian Gottlieb Kratzenstein. Copper etching based on drawing by Paul Ipsen, 1781. Christian Gottlieb Kratzenstein (30 January 1723, Wernigerode – 6 July 1795, Copenhagen) was a German-born medical doctor, physicist and engineer. From 1753 to the end of his life he was a professor at the University of Copenhagen where he served as rector four times. He is especially known for his investigations of the use of electricity in medicine and the first attempts at mechanical speech synthesis. As a teacher he wrote the first textbook on experimental physics in the united kingdom of Denmark-Norway.

Biography Kratzenstein was baptized on 2 February 1723 in Wernigerode, Sachsen-Anhalt, Holy Roman Empire and grew up there in an academic family together with three brothers. His father gave them a good upbringing and education. During the years 1733–1742 he attended the Latin school in the same city. Already at this age he was recognized for his interests in reading and learning. He was especially fascinated by the latest discoveries within the natural sciences and mechanics.

In 1742 Kratzenstein started to study physics and medicine at the university in Halle which at that time had a leading position in that region. His interests turned now to the investigations of electricity and in particular the effects on living organisms. After four years he received in 1746 doctoral degrees in physics and in medicine. He was then only 23 years old. After two years as a privatdozent he was in 1748 elected to the Academy of Sciences Leopoldina in the same city.

Kratzenstein became known in a short time as an engaging lecturer and attracted a large audience of both regular students and interested persons from the citizenry. He covered topics ranging from the newest insights about plants and animals, through geology, physiology to physics and chemistry.

His work at the university strengthened the academic standards and manifested itself in many ways. Thus it was Kratzenstein who seized the initiative before the important transits of Venus in 1761 and 1769 instead of the astronomers at the Rundetårn observatory. In order to spread the impact of his lectures he wrote a textbook in experimental physics. It was published in several editions and appeared in German, French and Latin in addition to the Danish version. As a result of all his efforts and engagements he served as rector at the university during four periods.

During his time in Copenhagen he maintained contacts with his former colleagues in Saint Petersburg. The science academy there announced in 1778 a prize competition concerning the mechanisms behind the vowels A, E, I, O and U in human speech. Euler had previously been interested in this problem and it is likely that he was behind the formulation of the task. Kratzenstein won the first prize in 1780 by constructing a «vowel organ» which could produce these special sounds. This represents one of the first contributions to modern speech synthesis.

By his wide range of engagements and temperament Kratzenstein would often end up in conflicts with his colleagues. At the big fire in Copenhagen 1795 he lost most of his possessions and scientific equipment. He moved out to the suburb Frederiksberg where he died a month later.

Body and soul

As a student in Halle Kratzenstein made his first step to fame by his pamphlet Beweis, dass die Seele ihren Körper baue in 1743. This was typical for the philosophical discourse at the university at that time.

Based on these ideas and investigations by Kratzenstein there have been speculations that he could have been a model for the fictitious doctor Frankenstein in the book of the same name written by Mary Shelley several decades later.

Two years later Kratzenstein wrote the more theoretical work Theoria electricitatis mores geometrica explicata on the nature of electricity. Around this time he made measurements to find out how the electric force between two charged objects varied with their separation. On the theoretical side he argued that the electric current was due to the motion of two fluids which today would correspond to the flow of positive and negative electric charges. The charges themselves should be due to vortices in these fluids. Around the same time Benjamin Franklin explained the same phenomena based on a one-fluid picture where negative charge was due to a lack of positive charge. It was this explanation that prevailed.

Together with a similar thesis about bodily fluids and their properties, Kratzenstein received in 1746 a doctor's degree both in physics and in medicine.

Navigation

During the five years at the science academy in Saint Petersburg Kratzenstein was to a large extent occupied by improving methods and equipment for navigation on the high seas. The magnetic compass was made more reliable, astronomical observations should be made more precise together with the development of more accurate clocks to be used on ships for the determination of geographical longitude.

These new instruments were tried out on the voyage from Arkhangelsk to Saint Petersburg in 1753 Kratzenstein discovered that the Norwegian coast was placed 150 km too far east on contemporary maps. This can seem unlikely due to the lack of accuracy of the watches being used.

Transits of Venus

After the first observed transit of Venus in 1619, Edmund Halley had stressed the importance of the two upcoming transits in 1761 and 1769. Also in the Nordic countries there was great interest in taking part in these observations. In Copenhagen it was the astronomers in the local observatory Rundetårn who formally were responsible for such activities. But in practice it was Kratzenstein who became the leader of this endeavour. In public lectures before the transit in 1761 he presented the theoretical background for this rare phenomenon and calculated transit times together with suggestions for suitable places of observation.

Kratzenstein had followed this discussion since he remained in contact with Euler and already from 1770 had been investigating the same problems. From his textbooks in experimental physics it is clear that he had a good understanding of the physics behind sound. It was therefore not so surprising that the academy in Saint Petersburg in 1778 announced a new prize problem exactly around these questions. The first part should investigate the tonal differences between the five vowels A, E, I, O and U, while the latter part asked for a device which could generate these sounds.

At the final evaluation by the academy in 1780, it was Kratzenstein's "vowel organ" that got the first prize. His contribution Tentamen resolvendi problema was published the year after. It consisted of a first part describing how the vowels could be produced in the vocal tract. His medical background was here of great help. The second part was the construction of a new kind of organ with pipes for each of the vowels. Each pipe had a characteristic resonant cavity which should emulate the vocal tract for the corresponding vowel. In order to excite these resonators he made use of free reeds which at that time were little known.

This instrument was demonstrated in Saint Petersburg to the full satisfaction of the academy, but was damaged and disappeared shortly afterwards. But the use of free reeds in musical instruments became later widespread and can today be found in the harmonica, accordion, harmonium and bandoneon. It is not known how Kratzenstein got the idea to use them, but they had for a long time been a central part of the Chinese musical instrument sheng.