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Musée des Arts et Métiers, Paris, France

48° 51′ 58″ N, 2° 21′ 19″ E

Science Before and After the French Revolution

Britain and Germany may contest the title of best science museum with the London Science Museum and Munich Deutsches Museum (see Chapters 77 and 19, respectively), but France’s Musée des Arts et Métiers (Museum of Arts and Trades) boasts of being the oldest (it was founded in 1794), and has a superb collection of devices dating from before 1750 to the present day.

The museum, situated in an 800-year-old priory, has a collection covering scientific instruments, materials, construction, communications, energy, mechanics, and transport.

The scientific instrument collection recounts the history of the creation of the metric system (with the meter, liter, and gram). The meter was to be one ten-millionth of the distance along the meridian from the North Pole to the Equator (running through Paris, of course), and when calculations were made in 1793 the meter was defined (with an error of 5 millimeters; see Chapter 8 for more on the meridian and meter). The gram was the weight of a cubic centimeter of water, and a liter was the volume of a cube with 10-centimeter sides.

Also in the scientific instrument collection is Foucault’s device for measuring the speed of light. In 1862, Foucault measured the speed of light using simple apparatus to estimate the speed of light at 298,000 kps (he was off by less than 1%).

Pascal’s calculating machines are also on display. His 1642 arithmetic machine performs addition by adding from right to left, carrying digits mechanically as the addition calculation continues. Despite its age, the Pascalina machine looks as though it might still work today.

The materials collection includes machines for spinning cotton, weaving (there’s a Jacquard loom; see Chapter 12), and making paper, glass, porcelain, steel, and aluminum foil.

The communications collection has printing presses, Daguerre’s cameras for making Daguerreotype photographs, and early telegraph equipment (including the entirely mechanical line-of-sight semaphore system used in France up until 1860 for long-distance communication). Cinema plays an important part in the collection, too, with the Lumière brothers’ equipment.

Émile Baudot’s telegraph equipment is also present in the collection. Baudot invented the predecessor of all computer codes used to represent characters. His Baudot code of 5 on-or-off bits allowed 32 different characters to be transmitted. He also invented time division multiplexing, whereby multiple telegraph signals could share the same line, transmitting up to five different messages at the same time.

Since much of the museum is in French, the best way for an English-speaking visitor to enjoy it is by renting a portable audio guide. The guide has over six hours of commentary and covers 175 of the major objects on display.

Once you are done exploring the museum, it’s a short Paris Metro trip to Le Panthéon to see Foucault’s Pendulum (see Chapter 13).

Practical Information

Information about the museum (in English and French) is available from http://www.artsetmetiers.net/. It’s very easy to get to—the Paris Metro lines 3 and 11 both have a stop called Arts et Métiers.

Measuring the Speed of Light

Foucault’s 1862 calculation of the speed of light relied on precision machinery made for that purpose, and a supply of air from an organ-maker’s bellows. In his experiment, sunlight was focused into a beam and directed toward a plate onto which parallel transparent lines had been cut 0.1 millimeter apart, creating a horizontal scale. The light from this screen was split, with some of the light going directly into an objective lens where Foucault could view the scale. The rest of the light was directed toward a rotating mirror.

As the mirror turned, it reflected light against a series of mirrors. The light would bounce through the mirrors and back again onto the rotating mirror. From the rotating mirror, it bounced back toward the objective, where Foucault could compare its image of the scale with the image coming directly from the scale itself.

The key to measuring the speed of light was the insight that while the light was bouncing around Foucault’s series of mirrors (see Figure 17-1), the rotating mirror was still turning. By the time the light returned to it, the rotating mirror had moved a little, and this movement meant that the image in the objective of the bounced light was shifted relative to the original scale. The amount of shift was proportional to the speed of light.

Figure 17-1. Foucault’s speed of light equipment;
courtesy of Service Interétablissements de Coopération Documentaire des universités de Strasbourg. Département du Patrimoine (34, boulevard de la Victoire 67000 Strasbourg, http://www-sicd.u-strasbg.fr/)

In Foucault’s experiment, the mirror rotated at 400 revolutions per second; this was verified using a clockwork mechanism that rotated a slotted disk. When the slots in the disk appeared stationary when viewed in the light reflected from the rotating mirror, Foucault knew that it was rotating at precisely 400 revolutions per second.

The rotating mirror was powered by a small air turbine, which received air at a constant pressure from hand-pumped organ bellows. The bellows were filled with air, and the air pressure reaching the turbine was adjusted to achieve the precise rotation rate required.

The series of mirrors made the reflected light travel 20 meters before returning. Foucault then measured the difference between the two images of the scale. From this he knew the angle through which the mirror had turned while the light was being reflected through the series of mirrors. From that he was also able to calculate the length of time that the light had been bouncing around the 20 meters of mirrors, since he knew the rotation rate of the mirror very precisely.

Given the time and the distance, Foucault was then able to calculate the speed of light.