Die Helmholtz Kurven: auf der Spur der verlorenen Zeit (The Helmholtz Curves: In Search of Lost Time)

  • Henning Schmidgen
Merve: 2009. 270 pp. €20 9783883962795 | ISBN: 978-3-8839-6279-5

Hermann von Helmholtz (1821–1894) was a towering figure of the European Enlightenment, a physiologist and accomplished draftsman with the soul of a Prussian physicist. He conducted his research with rigorous mathematical precision, investigating his biological preparations by adapting whichever industrial-revolution technologies he saw fit.

Hermann von Helmholtz's rediscovered curves show how frog muscle contracts and relaxes after nerve stimulation (time runs right to left). Credit: ACADÉMIE DES SCIENCES/INSTITUT DE FRANCE

His formidable set of skills, combined with an equally formidable intelligence, enabled him in 1850 to work out the speed of signal propagation in nerves — then a fundamental problem in the highly competitive field of nerve and muscle physiology. However, his contemporaries did not believe him.

Recognition was as important for scientists then as it is today.

Using the physiologist's favourite preparation of a large frog muscle still attached to its long nerve, he had declared the speed of conductance to be around 27 metres per second. This seemed improbably slow to a sceptical scientific community by now familiar with the speeds of light and sound. So Helmholtz developed yet another skill: science communication. He decided to generate visible proof of his claim, using curves that were drawn by the contracting frog muscle itself after electrical stimulation of its nerve.

Those original curves were never published and were believed to have been lost. But last summer, German science historian Henning Schmidgen found them hidden in the archives of the Paris Academy of Sciences in France. His discovery prompted him to tell the story of Helmholtz's ingenious experiments to both measure and demonstrate the speed of nerve conductance. Schmidgen's account, Die Helmholtz Kurven, shows how recognition was as important for scientists then as it is today.

In his earliest 'frog drawing machine', Helmholtz suspended the frog muscle and attached a weight to it by a thread. He attached a stylus to the thread and placed a rotating glass disk coated in soot directly in front of it. When he stimulated the nerve, the muscle contracted, pulling the stylus across the disk. The scratched curve showed the asymmetric form of the muscle contraction, with its slow build up and fade. However, he was reluctant to use the system to study the speed of nerve conduction, worried that friction would distort the results.

Instead, he designed an almost friction-free system, adapting an electromagnetic approach used in ballistics to measure short time intervals. He used a galvanometer — a type of ammeter that detects and measures electric current — to transform the duration of muscle contractions into the deflection of a needle through electromagnetic force. To increase resolution, he measured the extent of the deflection through a telescope placed a few metres away.

He wrote up his results and rushed the paper to the Paris Academy of Sciences in February 1850, after it had been translated into French by his friend and colleague, the physiologist Emil du Bois-Reymond. The paper comprised three pages of abstraction and spectacularly failed to convince his contemporaries. A 90-page elaboration, full of numbers and written in German, also failed to hit the public nerve, as it were.

Helmholtz wondered whether perhaps his nerve–muscle preparation should do the communicating directly, and so returned to his frog drawing machine. He replaced the glass disk with a scaled-down drum made from a champagne glass with a smoked surface, and spun it fast enough for the stylus to scratch the shape and time course of a full muscle contraction into the soot. He compared the curves that were produced when the nerve was stimulated either close to, or distant from, the muscle. From the curve's displacement, he calculated the speed of signal propagation in the nerve — and got the same value he had calculated using his electromagnetic method.

He made ingenious permanent records of the scratched images, capturing them using a material called isinglass — a sticky collagen film made from the dried swim bladders of fish. Among other things, it was used as a clarifying agent for wines and in plasters and glues. He transferred the smoky images onto squares of isinglass and sent them off with a second explanatory manuscript to the Paris Academy of Sciences in September 1851. With this, Helmholtz won the recognition he desired. Ironically, the published manuscript did not include the images.

Last year, Schmidgen was studying correspondence between Helmholtz, Du Bois-Reymond and the Paris Academy of Sciences when it occurred to him that the paragraph that referred to the curves in the draft manuscript was missing from the printed paper. There was no way of knowing whether Helmholtz had decided not to send the curves after all, or whether the academy had simply not printed them. Unless, by chance, the publishers had simply kept the curves in the file. Schmidgen flew to Paris to see. And this is how he made the kind of discovery of original material that science historians dream of.