While we will be providing pictures you can use to follow our instructions, we recommend getting out your own ruler or measuring tape so you can follow along in real time. Nomination of the STRUVE GEODETIC ARC for inscription on the WORLD HERITAGE LIST, p. 40 Struve Geodetic Arc Nevertheless, the infrared light from a methane-stabilised laser was inconvenient for use in practical interferometry. It was not until 1983 that the chain of frequency measurements reached the 633nm line of the helium–neon laser, stabilised using molecular iodine. [183] [184] That same year, the 17thCGPM adopted a definition of the metre, in terms of the 1975 conventional value for the speed of light: [185] The metre is the length of the path travelled by light in vacuum during a time interval of 1⁄ 299,792,458 of a second. Example: You’re trying to measure the length of your pointer finger. The ruler comes to the seventh line past 3 inches. This would mean that your finger is 3 7/16 inches long. Progress in science finally allowed the definition of the metre to be dematerialized; thus in 1960 a new definition based on a specific number of wavelengths of light from a specific transition in krypton-86 allowed the standard to be universally available by measurement. In 1983 this was updated to a length defined in terms of the speed of light; this definition was reworded in 2019: [3] The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m⋅s −1, where the second is defined in terms of the caesium frequency Δ ν Cs.

The Roman foot was divided into 4 palms, 12 inches or 16 fingers. A Roman cubit was equivalent to 1.5 feet, a pace to 5 feet. A Roman mile contained 1000 paces or 5000 feet. A Roman league comprised 7500 Roman feet. The Romans imposed Roman units of measurement throughout their empire. During the Middle Ages, new feet of different lengths appeared in Europe. They all derived more or less directly from the Roman foot. These feet were divided into 12 inches, themselves divided into 12 lines of 6 points each. Multiples of these feet became the length standards in various European cities. For example, the Paris toise included six Paris feet, while the English yard measured three London feet. [8] [9] [10] [11] The Meridian room of the Paris Observatory (or Cassini room): the Paris meridian is drawn on the ground. Now, notice the lines between each inch, with some longer and some shorter than others. Each of these tiny lines represents a fraction of an inch. There are five different lengths of lines in total.An early definition of the metre was one ten-millionth of the Earth quadrant, the distance from the North Pole to the Equator, measured along a meridian through Paris. The Metre Convention ( Convention du Mètre) of 1875 mandated the establishment of a permanent International Bureau of Weights and Measures (BIPM: Bureau International des Poids et Mesures) to be located in Sèvres, France. This new organisation was to construct and preserve a prototype metre bar, distribute national metric prototypes, and maintain comparisons between them and non-metric measurement standards. The organisation distributed such bars in 1889 at the first General Conference on Weights and Measures (CGPM: Conférence Générale des Poids et Mesures), establishing the International Prototype Metre as the distance between two lines on a standard bar composed of an alloy of 90% platinum and 10% iridium, measured at the melting point of ice. [148]

The third-biggest lines on a ruler are the 1/4 inch lines, which appear midway between the 1/2 inch and whole inch lines: Example: Say you’re measuring a piece of cloth and the ruler ends at the fourth line after the 10-inch mark. This would mean that the cloth is 10 1/4 (10.25) inches long. Example: You take out a ruler to measure the width of your fingernail. The ruler stops at 1 cm, meaning that your nail is precisely 1 cm wide.

How to Read a Ruler in Centimeters

Delambre used a baseline of about 10km (6,075.90 toises) in length along a straight road between Melun and Lieusaint. In an operation taking six weeks, the baseline was accurately measured using four platinum rods, each of length two toises (a toise being about 1.949m). [73] Thereafter he used, where possible, the triangulation points used by Cassini in his 1744 survey of France. Méchain's baseline, of a similar length (6,006.25 toises), and also on a straight section of road between Vernet (in the Perpignan area) and Salces (now Salses-le-Chateau). [74] Although Méchain's sector was half the length of Delambre, it included the Pyrenees and hitherto unsurveyed parts of Spain. Example: Say you're measuring the width of your smartphone, and it comes up to the fifth line after 4 cm on your ruler. This would mean that the phone is 4.5 cm (45 mm) wide. Developments in electronics also made it possible for the first time to measure the frequency of light in or near the visible region of the spectrum, [ further explanation needed] instead of inferring the frequency from the wavelength and the speed of light. Although visible and infrared frequencies were still too high to be directly measured, it was possible to construct a "chain" of laser frequencies that, by suitable multiplication, differ from each other by only a directly measurable frequency in the microwave region. The frequency of the light from the methane-stabilised laser was found to be 88.376 181 627(50)THz. [179] [181]