1780s: All influence political and astronomers in French were calling to solve the unit of length. Jean Picard, Olaus Romer and other astronomers had suggested that a unit of length be defined as the length of a pendulum with a period of 2 seconds.1790s: The Bishop of Autun suggested to the Constituent Assembly on the state of French weights and measures a new measure of length based on the length of the seconds pendulum at the latitude of Paris, 45°N and also suggested the Academy of Sciences in Paris collaborate with the Royal Society of London in defining the new unit. The Assembly and subsequently Louis XVI approved this proposal, but nothing came of it.End of 1790s: The Academy had placed the matter in the hands of as illustrious a scientific commission as has ever existed: Lagrange, Laplace, Borda, Monge, and Condorcet.March 19, 1791: They suggested the new unit of length be one ten-millionth of the distance at sea level from the pole to the equator. Enormous meridian measuring projects were to the science of the as space programs or the construction of large particle accelerators have been to ours. They challenged the limits of the day's technology and tested the predictions of the new physics—in the late 18th century18th century, Newtonian predictions that the earth was not a sphere. Preeminence in such projects was a matter of national pride, at least among “natural philosophers.” Borda, for example, a member of the commission, had constructed extremely precise graduated circles for measuring angles, just what would be needed for this sort of work.March 26, 1791: The Assembly approved the proposed unit and work began on realizing it. To replace the hated “royal foot” until the results of the survey were in, a provisional meter was defined. The survey was put in the hands of P. F. A. Méchain and J. B. J. Delambre.In the summer of 1792: Delambre began working his way south from the coast near Dunkirk, while Méchain started north from the Mediterranean. They would meet at Rodez, 300 miles south of Paris. Méchain's share was shorter, but more difficult, for it crossed the Pyrenees Mountains that separate Spain and France.September 1792: The Republic was declared.Within a few months France was at war with Great Britain, Austria, Prussia, Holland and Spain; Louis XVI had been executed, and Parisian mobs were massacring various groups. he flags on their survey poles were white—the color of the royalists! They were from Paris. All they had going for them was that their story—we are measuring the distance from Dunkirk to Barcelona—was so unbelievable in the midst of war and revolution that no real spy would have used it. On August 8, 1793: The National Convention abolished the Academy of Sciences as unrepublican. The Committee of Public Safety, however, remained intent on doing away with the old feudal measures and needed the help of the Academicians to do it, so it persuaded the Convention to create a new, independent temporary commission (Commission temporaire des poids et mesures républicains) with the same members.November 1793: Lavoisier was arrested; the commission asked for his release; the Committee of Public Safety responded by kicking five more members off the commission, including Delambre. Seeing which way the wind blew, the commission then devoted itself to preparing revolutionary denunciations of the old weights and measures. Delambre thought they should kill the whole meridian-measuring project and just accept the provisional meter.April 7, 1795: War required map, a military cartographer who was also a Jacobin was put in charge of map-making. Needing trained staff, he brought Delambre and Méchain back to Paris. An order establishing the names now in use (meter, liter, gram) also reestablished the commission and ordered resumption of the survey. Fall of 1797: Delambre finished his portionb ut Méchain had yet to reach Rodez. Sick, with winter coming, he wrote to his colleague, “I will sacrifice everything, give up everything, rather than return without completing my part.” And so the survey stalled. But Méchain recovered and resumed work.
September 1798: He reached Rodez. To this point, except for the sides of two triangles, only angles had been measured, the angles of contiguous triangles stretching all the way from Dunkirk to Barcelona. If any side of only one of these triangles were known, the dimensions of all the others could be calculated, and from them the distance along the meridian. While Mechain labored in the south, Delambre measured one of the baselines with a special ruler. It took him 33 days.
November 28, 1798: The French convened an international meeting of experts from friendly powers and puppet states. One of the meeting's committees consisted of four persons, each of whom independently calculated the length of the meter from the measurements made by Delambre and Méchain and from certain assumptions about the shape of the earth. Their calculations agreed. The meter was established at 0.144 lignes of the toise de Perou shorter than than the provisional meter.
December 10, 1799: The metric system itself was legalized. The Mètre des Archives was, by definition, a meter long, from end to end. Metrologists call such a standard an end measure. End measure standards are not a good idea, because any simple way of measuring their lengths requires touching the ends, which causes wear and shortens the standard. A much better form for a standard of a unit of length is a pair of scratches on a metal bar, because the lines' locations can be determined visually. Such a standard is called a line measure.
1870 and 1872: A discussion of international standardization of the meter. The attendees favored replacing the Mètre des Archives with a new prototype which would be a line measure and made of a harder, platinum-iridium alloy. They also suggested that the meter be taken as the length of the Mètre des Archives, “in the state in which it is found,” without reference to the quadrant of the earth.
In 1875: Twenty countries attended the third conference. Eighteen subscribed to a treaty (the Convention du Mètre), which set up the Bureau International des Poids et Mésures. Production of the meter standard, however, proved very difficult. Besides having an extremely high melting point (2,443°C), iridium had not yet been produced in purities greater than 50%.
1877s: The bars from the first casting of the alloy, in 1874, were rejected and the problem was turned over to the London firm of Johnson, Matthey and Co. They succeeded and one of the resulting bars was made the provisional standard, even though it was 0.006 mm shorter than the Mètre des Archives.
1882s: France ordered thirty more bars, one of which (No. 6) turned out to be, as nearly as could be ascertained, exactly the length of the Mètre des Archives.
1889s: This bar is the standard which was declared to be the International Prototype of the Meter by the First General Conference on Weights and Measures, “This prototype, at the temperature of melting ice, shall henceforth represent the metric unit of length.” The International Prototype continues to be preserved by the BIPM. As a way of distributing this standard to the countries signing the treaty, “national Meters” were made, which were copies of the International Prototype plus or minus 0.01 millimeter, supplied with a correction factor obtained by comparing that particular national meter with the International Prototype.
1892-3: A. A. Michelson and J. R. Benoit succeeded in measuring the meter in terms of the wavelength of red light given off by excited cadmium atoms.
1907: The International Solar Union (which is now the IAU) defined the international angstroms , a unit of distance to be used in measuring wavelengths, by making 6438.4696 international angstroms equal to the wavelength of the red line of cadmium. This value was taken from Benoit's experiments, and was chosen so that one angstrom was approximately 10-10 meter.
1948s: How the meter might eventually be defined in terms of light from such an isotope. Three isotopes were intensively investigated to see which would be most suitable as the basis for a standard of length: krypton-86 (36 protons), mercury-198 (80 protons), and cadmium-114 (48 protons). The committee in charge of following these developments recommended that any new definition be stated in terms of the wavelength in a vacuum instead of in air, and that the length of the wavelength should be specified by comparing it with the already determined wavelength of the red line of cadmium, not with the International Prototype of the Meter.
1954s: The 10th CGPM accepted these recommendations, in effect making the angstrom exactly equal to 10-10 meter and defining the meter in terms of light, although this was not formally acknowledged until 1960.
1983s: The 17th CGPM (Resolution 1) redefined the meter in terms of the speed of light in a vacuum. The value for the speed of light, 299,792,458 meters per second, had already been recommended in 1975 by the 15th CGPM, (Resolution 2). Its use in the meter's definition made the speed of light fall within the limits of uncertainty of the best existing measurements.
Thus the second rejected as too arbitrary in 1791 has become the basis of the meter. We have probably not seen the last redefinition of the meter; the current definition may need tuning if even more accuracy becomes necessary. For example, the speed of light is affected by the strength of the gravitational field, and the 1983 definition does not take such factors into account.