Editing Hermann von Helmholtz (section)

== Research ==

[[File:Helmholtz 1848.jpg|thumb|upright|Helmholtz in 1848]]

===Mechanics===

His first important scientific achievement, an 1847 treatise on the [[conservation of energy]], was written in the context of his medical studies and philosophical background. His work on energy conservation came about while studying [[muscle]] [[metabolism]]. He tried to demonstrate that no energy is lost in muscle movement, motivated by the implication that there were no ''vital forces'' necessary to move a muscle. This was a rejection of the speculative tradition of ''[[Naturphilosophie]]'' and [[vitalism]] which was at that time a dominant philosophical paradigm in German physiology. He was working against the argument, promoted by some vitalists, that "living force" can power a machine indefinitely.<ref name="Patton">[https://plato.stanford.edu/entries/hermann-helmholtz/?simple=True Patton, Lydia. "Hermann von Helmholtz." (2008), Stanford Encyclopedia of Philosophy.]</ref>

Drawing on the earlier work of [[Nicolas Léonard Sadi Carnot|Sadi Carnot]], [[Benoît Paul Émile Clapeyron]] and [[James Prescott Joule]], he postulated a relationship between [[mechanics]], [[heat]], [[light]], [[electricity]] and [[magnetism]] by treating them all as manifestations of a single ''force'', or [[energy]] in today's terminology. He published his theories in his book ''Über die Erhaltung der Kraft'' (''On the Conservation of Force'', 1847).<ref>English translation published in ''Scientific memoirs, selected from the transactions of foreign academies of science, and from foreign journals: Natural philosophy'' (1853), p. 114; trans. by John Tyndall. [https://books.google.com/books?id=C1i4AAAAIAAJ&pg=PA114 Google Books], [http://hdl.handle.net/2027/uc1.b4252190?urlappend=%3Bseq=124 HathiTrust]</ref>

In the 1850s and 60s, building on the publications of [[William Thomson, 1st Baron Kelvin|William Thomson]], Helmholtz and [[William Rankine]] helped popularize the idea of the [[heat death of the universe]].

In fluid dynamics, Helmholtz made several contributions, including [[Helmholtz's theorems]] for vortex dynamics in inviscid fluids.<gallery>
File:Helmholtz-1.jpg|1889 copy of Helmholtz's "Über die Erhaltung der Kraft", no. 1
File:Helmholtz-2.jpg|Title page of "Über die Erhaltung der Kraft", no. 1
File:Helmholtz-3.jpg|First page of "Über die Erhaltung der Kraft", no. 1
</gallery>

===Sensory physiology===

Helmholtz was a pioneer in the scientific study of human vision and audition. Inspired by [[psychophysics]], he was interested in the relationships between measurable physical stimuli and their correspondent human perceptions. For example, the amplitude of a sound wave can be varied, causing the sound to appear louder or softer, but a linear step in sound pressure amplitude does not result in a linear step in perceived loudness. The physical sound needs to be increased exponentially in order for equal steps to seem linear, a fact that is used in current electronic devices to control volume. Helmholtz paved the way in experimental studies on the relationship between the physical energy (physics) and its appreciation (psychology), with the goal in mind to develop "psychophysical laws".

The sensory physiology of Helmholtz was the basis of the work of [[Wilhelm Wundt]], Helmholtz's student, who is considered one of the founders of experimental [[psychology]]. More explicitly than Helmholtz, Wundt described his research as a form of empirical philosophy and as a study of the mind as something separate. Helmholtz had, in his early repudiation of [[Naturphilosophie]], stressed the importance of [[materialism]], and was focusing more on the unity of "mind" and body.<ref>{{Cite book| title = Making Modern Science: A Historical Survey | author = Peter J. Bowler and Iwan Rhys Morus | publisher = University of Chicago Press | year = 2005 | page = 177 | isbn = 978-0-226-06861-9 | url = https://books.google.com/books?id=LEl3s-wYg10C&q=Helmholtz+materialism++sensory+nineteenth-century+Naturphilosophie&pg=PA177 }}</ref>

===Ophthalmic optics===

In 1851, Helmholtz revolutionized the field of [[ophthalmology]] with the invention of the [[ophthalmoscope]]; an instrument used to examine the inside of the [[human eye]]. This made him world-famous overnight. Helmholtz's interests at that time were increasingly focused on the physiology of the senses. His main publication, titled ''Handbuch der Physiologischen Optik'' (''Handbook of Physiological Optics'' or ''Treatise on Physiological Optics''; English translation of the 3rd volume [https://web.archive.org/web/20180320133752/http://poseidon.sunyopt.edu/BackusLab/Helmholtz/ here]), provided empirical theories on [[depth perception]], [[colour vision]], and [[motion perception]], and became the fundamental reference work in his field during the second half of the nineteenth century. In the third and final volume, published in 1867, Helmholtz described the importance of [[unconscious inference]]s for perception. The ''Handbuch'' was first translated into English under the editorship of [[James P. C. Southall]] on behalf of the [[Optical Society of America]] in 1924–5. His theory of [[accommodation reflex|accommodation]] went unchallenged until the final decade of the 20th century.

Helmholtz continued to work for several decades on several editions of the handbook, frequently updating his work because of his dispute with [[Ewald Hering]] who held opposite views on spatial and colour vision. This dispute divided the discipline of physiology during the second half of the 1800s.

===Nerve physiology===

In 1849, while at Königsberg, Helmholtz measured the speed at which the signal is carried along a nerve fibre. At that time most people believed that nerve signals passed along nerves immeasurably fast.<ref name="glynn">{{cite book |last1=Glynn |first1=Ian |title=Elegance in Science|year=2010 |publisher= Oxford University Press|location=Oxford|isbn= 978-0-19-957862-7|pages=147–150 }}</ref> He used a recently dissected sciatic nerve of a frog and the calf muscle to which it attached. He used a [[galvanometer]] as a sensitive timing device, attaching a mirror to the needle to reflect a light beam across the room to a scale which gave much greater sensitivity.<ref name="glynn"/> Helmholtz reported<ref>Helmholtz, Hermann von (1850).''Vorläufiger Bericht über die Fortpflanzungs-Geschwindigkeit der Nervenreizung''. In: Archiv für Anatomie, Physiologie und wissenschaftliche Medicin. Veit & Comp., pp. 71–73. [http://vlp.mpiwg-berlin.mpg.de/library/data/lit29168 MPIWG Berlin]</ref><ref>Helmholtz, Hermann von (1850). ''Messungen über den zeitlichen Verlauf der Zuckung animalischer Muskeln und die Fortpflanzungsgeschwindigkeit der Reizung in den Nerven''. In: Archiv für Anatomie, Physiologie und wissenschaftliche Medicin. Veit & Comp., pp. 276–364. [http://vlp.mpiwg-berlin.mpg.de/library/data/lit1862 MPIWG Berlin]</ref> transmission speeds in the range of 24.6 – 38.4 meters per second.<ref name="glynn"/>

===Acoustics and aesthetics===
[[File:Hermann von Helmholtz-2.jpg|thumb|Last photograph of von Helmholtz, taken three days before his final illness]] [[File:Helmholtz resonator 2.jpg|right|thumb|The Helmholtz resonator (''i'') and instrumentation]]

In 1863, Helmholtz published ''[[Sensations of Tone]]'', once again demonstrating his interest in the physics of perception. This book influenced musicologists into the twentieth century. Helmholtz invented the [[Helmholtz resonance|Helmholtz resonator]] to identify the various [[audio frequency|frequencies]] or [[Pitch (music)|pitches]] of the pure [[sine wave]] components of [[Fourier analysis#Applications in signal processing|complex sounds containing multiple tones]].<ref name="Helmholtz1885">{{cite book |url=https://archive.org/details/onsensationston00unkngoog |last=von Helmholtz |first=Hermann |year=1885 |title=On the sensations of tone as a physiological basis for the theory of music |edition=Second English |translator-first1=Alexander J. |translator-last1= Ellis |location=London |publisher=Longmans, Green, and Co. |page=[https://archive.org/details/onsensationston00unkngoog/page/n69 44] |access-date=12 October 2010 }}</ref>

Helmholtz showed that different combinations of resonators could mimic [[vowel]] sounds: [[Alexander Graham Bell]] in particular was interested in this but, not being able to read German, misconstrued Helmholtz's diagrams as meaning that Helmholtz had transmitted multiple frequencies by wire—which would allow multiplexing of telegraph signals—whereas, in reality, electrical power was used only to keep the resonators in motion. Bell failed to reproduce what he thought Helmholtz had done but later said that, had he been able to read German, he would not have gone on to invent the telephone on the [[harmonic telegraph]] principle.<ref>{{cite web | title = PBS, American Experience: The Telephone – More About Bell | website = [[PBS]]| url = https://www.pbs.org/wgbh/amex/telephone/peopleevents/mabell.html}}</ref><ref name="MacKenzie2008">MacKenzie 2003, p. 41.</ref><ref>Groundwater 2005, p. 31.</ref><ref>Shulman 2008, pp. 46–48.</ref>

[[File:Ludwig Knaus - Der Physiker Hermann von Helmholtz (1881).jpg|thumb|left|upright|Helmholtz in 1881, portrait by [[Ludwig Knaus]]]]

The translation by [[Alexander J. Ellis]] was first published in 1875 (the first English edition was from the 1870 third German edition; Ellis's second English edition from the 1877 fourth German edition was published in 1885; the 1895 and 1912 third and fourth English editions were reprints of the second).<ref>{{cite book |title=On the Sensations of Tone as a Physiological Basis for the Theory of Music |author=Hermann L. F. Helmholtz, M.D. |year=1912 |edition=Fourth |publisher=Longmans, Green, and Co |isbn=9781419178931 |url=https://archive.org/details/onsensationston01helmgoog}}</ref>

===Electromagnetism===

Helmholtz studied electrical oscillations from 1869 to 1871, and in a lecture delivered to the Naturhistorisch-medizinischen Verein zu Heidelberg (Natural History and Medical Association of Heidelberg) on 30 April 1869, titled ''On Electrical Oscillations'', he indicated that the perceptible damped electrical oscillations in a coil connected to a [[Leyden jar]] were about {{frac|1|50}} second in duration.<ref>{{cite book|url=https://books.google.com/books?id=JCNWAAAAMAAJ&q=electrical+oscillation+helmholtz&pg=PA268|title=Hermann von Helmholtz|first=Leo|last=Koenigsberger|date=28 March 2018|publisher=Clarendon press|isbn=978-0-486-21517-4|access-date=28 March 2018|via=Google Books}}</ref>

In 1871, Helmholtz moved from Heidelberg to Berlin to become a professor of physics. He became interested in [[electromagnetism]], and the [[Helmholtz equation]] is named for him. Although he made no major contributions to this field, his student [[Heinrich Rudolf Hertz]] became famous as the first to demonstrate [[electromagnetic radiation]]. [[Oliver Heaviside]] criticised Helmholtz's electromagnetic theory because it allowed the existence of [[longitudinal wave]]s. Based on work on [[Maxwell's equations]], Heaviside pronounced that longitudinal waves could not exist in a vacuum or a homogeneous medium. Heaviside did not note, however, that longitudinal electromagnetic waves can exist at a boundary or in an enclosed space.<ref>John D. Jackson, Classical Electrodynamics, {{ISBN|0-471-30932-X}}.</ref>