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Contributor Essays

Nineteenth-Century Acoustics and Its Instruments: On the Method of Reenactment

by Paolo Brenni
January 14, 2020

The scientific instruments used for research and teaching in the nineteenth and early twentieth century are beautiful and fascinating artifacts. However, they are not self-explanatory, and it is difficult to present and explain them to the public in museums and historical collections. Visitors would like to see them in operation, and are often frustrated because the historical instruments rest inanimate in their showcases.

To enhance understanding and exploitation of this scientific and technical heritage, for several years the Fondazione Scienza e Tecnica in Florence, which holds one of Europe’s finest collections of nineteenth-century physics apparatus, has been making a number of reenactments of typical nineteenth-century experiments in acoustics (as well as in the other branches of physics). Paolo Brenni is the experimenter, assisted by Anna Giatti (curator of the physics collection). As accurately as possible, they work with historical descriptions of the instruments, experiments, and experimental settings.

These experiments are reproduced only after carefully studying the physics treatises and textbooks of the time and learning how to manipulate the historical instruments. To perform the demonstrations and reproduce the nineteenth-century laboratory technology as closely as possible, we attend to every detail. For example, we use gas light to illuminate the scales of early spectroscopes, arc light for presenting optical demonstrations, mercury for certain electric contacts, and so on. We endeavor to avoid all anachronism. The experiments are accompanied by short comments describing the most important operations and the fundamental physical phenomena related to them. The aim of the videos is to present demonstrations that, though forgotten today, were extremely common in the physics lecture halls of the nineteenth and early twentieth century.

The reenactments are documented by filmmaker Antonio Chiavacci. The videos have short captions, giving just the information needed to understand the working of the instruments. We do not add any comments on the history of the devices or their relevance in the history of physics. Our aim is to leave the video open to all possible uses. Museum curators have presented the videos alongside static exhibits to show how they worked. Physics teachers use them to explain experiments that are no longer performed today because they are difficult to repeat or too dangerous in terms of current safety standards. Finally, our videos allow historians of science to see how physics experiments were performed in the nineteenth and early twentieth century.

Before starting to film, we consulted the most important and popular physics treatises of the nineteenth century and carefully studied the instruments to be shown and the experiments to be performed. However, nineteenth-century “brass and glass” instruments are hardly user-friendly. In the past, a professional préparateur had to prepare the experiments for the professor who wished to show them to his audience. To ensure the success of a demonstration, one must acquire tacit knowledge that cannot be found in written sources. This can take a long time, requiring excellent experimental practice and much trial and error. We had the opportunity to gain the essential skills for manipulating instruments during the lengthy and delicate work of restoring the collection’s apparatus.

All the demonstrations were carried out with original late nineteenth-century apparatus. Certainly, some curators would be horrified at the thought of using original historical artifacts to make videos. But the risk of damage can be greatly reduced if historical instruments are handled by a specialist who knows them well. Furthermore, our videos have proved extremely useful, allowing a range of audiences to better appreciate and understand laboratory techniques and lecture demonstrations of the past.

In our videos, we tried hard to avoid using anachronistic artifacts (contemporary light sources, modern cables, electronic apparatus, etc.). We had to adopt modern solutions only in a very few cases, when, for various reasons, we could not utilize historical apparatus.

Our videos illustrate several disciplines of classical physics. For the physics of sound, we decided to film some of the most common demonstrations of nineteenth-century acoustics. This discipline experienced rapid development during the nineteenth century, both theoretically and experimentally. A large number of research and demonstration devices were introduced during the second half of the century by Rudolph Koenig, a famous instrument maker and pioneer of acoustics. Most of these instruments, however, were abandoned in the first decades of the twentieth century, when more efficient electroacoustic apparatus (such as frequency generators, loudspeakers, and oscilloscopes) were invented.

The following are our videos of acoustics:

Here we show how sound is associated with vibrations (glass vase, violin string, etc.). We also show a peculiar musical instrument, forgotten today—the Pan’s harp, which was also used in concerts. In this harp, sounds were produced by the longitudinal vibrations of a series of thin wooden rods.

This video illustrates how to produce Chladni figures with metallic and glass plates and membranes. These figures were a powerful method for visualizing and studying two-dimensional vibrations.

Until the beginning of the twentieth century, tuning forks were used to generate sounds of known frequency. This video shows how tuning fork vibrations were recorded on a smoked glass plate and how the phenomenon of beats was produced and recorded with two tuning forks producing slightly different frequencies.

In the nineteenth century, sounds of desired frequencies were produced using various types of disk sirens. A jet of air was directed to a series of holes on the circumference of a rapidly rotating disk. The periodic interruption of the jet produces a sound whose frequency depends on the speed of the rotation and the number of holes.

In this case, we could not use the original bellows to produce the airflow that was necessary to operate the disk siren—they were too delicate and too fragile. We tried a compressor, but it was too noisy. Eventually we used scuba-diving compressed air cylinders, which we hired from local diving stores.

In this video, we present two different systems for producing Lissajous figures. These are generated by the combination of two perpendicular oscillations; their shape depends on the frequencies, amplitudes, and phases of the oscillations. In the first part of the video, the Lissajous figures are traced on a screen by a beam of light after being reflected by two mirrors fixed on perpendicular tuning forks. The beam is produced by an arc lamp. The second part shows a special mechanical apparatus combining the orthogonal movements of two moving axles.

Here, we show the functioning of a manometric capsule, which makes it possible to modulate the amplitude of a flame with a sound, the action of an acoustic resonator, and the harmonic sound analyzer. Using this instrument, surely one of the most ingenious of nineteenth-century acoustics, the frequencies composing a complex sound could be determined (purely mechanically). In practice, it made a Fourier analysis of sounds.

Working with the sound analyzer was particularly problematic. For example, it is very difficult to regulate the pressure of the gas arriving in the capsules. If the pressure is too high, the flames become too big and overlap; if it is too low, the flames are extinguished too easily. Aside from the technical problems, there are problems related to the experimenter. It is very difficult to simultaneously compare the shape and the size of the oscillating flames, and to properly tune the resonators, etc. It is not surprising that only a very skillful and well-trained experimenter could successfully use a sound analyzer.

This video (made in collaboration with Roland Wittje) illustrates two demonstrations from the early days of electroacoustics. In the first, a voice causes the diaphragm of a microphone to vibrate and modulates the current feeding an electric arc. By its oscillation, the arc reproduces the voice (speaking arc). The reverse was also possible: a sound disturbing the arc was reproduced by a special loudspeaker connected to it. The second experiment shows how the oscillations produced by an arc lamp in an electric circuit can be modulated by varying the capacity or impedance of the circuit itself. These experiments, which at the time remained laboratory demonstrations, show some first attempts to generate “electric sounds.”

The main problem in performing these experiments was that we needed a source of continuous current at about 80–100 V. We tried to use a commercial rectifier, but it was not able to completely eliminate a 50 Hz modulation produced by the electrical network. This generated a very disturbing noise in the loudspeaker of the speaking arc. Finally, we managed to solve the problem by using a series of ten car batteries, specially acquired for the experiment, that delivered pure DC.

By Paolo Brenni, CNR (Italian National Research Council), Florence

 
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Ganot, Alphonse. Traité de physique élémentaire. Paris: Hachette, 1884,
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