MODERN METROLOGY

The Industrial Revolution to the 20th Century

The Evolution of Metrology

The Industrial Revolution marked a pivotal era in the advancement of metrology, as the demand for standardized measurements grew alongside mass production and global trade. The need for uniformity in manufacturing, engineering, and commerce led to the development of national and international measurement systems.

By the 20th century, metrology had become a cornerstone of scientific progress, industrial efficiency, and technological innovation. The establishment of organizations such as the International Bureau of Weights and Measures (BIPM) and the widespread adoption of the metric system further reinforced the critical role of measurement in ensuring precision, quality control, and consistency across industries worldwide.

In the early 19th Century, the United States lacked standardized units of measurement. Each state and region often had its system of units, leading to confusion and inefficiencies, particularly in science, engineering, and commerce. To address this issue, the United States needed a uniform and accurate system of measurement.

In 1807, Congress decided to survey the US coast and formed the United States Coast Survey (USCS) under the Treasury Department. Ferdinand Rudolph Hassler’s involvement in the standard unit of length is primarily associated with his work in the USCS, where he played a significant role in establishing the standard unit of length for the United States.

Hassler recognized the importance of standardizing length units, especially for conducting accurate surveys and mapping. As the head of the United States Coast Survey, he understood that a consistent and reliable unit of length was essential for the success of their surveying efforts.

To establish a standard unit of length, Hassler employed a two-fold approach:

  1. Bringing the metric system to the United States: Hassler advocated the metric system, a decimal-based system of measurement introduced in France during the French Revolution. He recognized the benefits of its simplicity and ease of use in scientific and engineering applications. Therefore, he sought to introduce the metric system to the United States to promote uniformity and compatibility with international scientific work.
  2. Determining the length of a standardized measure requires implementing a standard unit of length based on the metric system, necessitating an accurate reference measure. Hassler and his team worked diligently to measure the length of a specific distance that could be used as the basis for the standard unit. They conducted precise measurements of various baselines using triangulation methods as part of the Great Trigonometric Survey.

Their efforts resulted in the determination of the “US Coast Survey Foot,” which was defined as exactly 12/39.37 meters (approximately 0.3048006 meters) in 1832. This was a close approximation to the British imperial foot used at that time but was based on the metric system and more precisely defined.

Hassler chose a standard unit of length: a bronze bar made by Edward Troughton, an English instrument maker. Hassler understood this specific bar had not been compared to the Parliamentary Yard, which was the absolute authority of length at the time. Hassler planned on comparing the two bars at the completion of his survey. However, the original Parliamentary Yard was destroyed by fire in 1834.

The Parliamentary Yard was restored, and reliable copies were made for safekeeping and distribution among treaty nations. France sent two of the Parliamentary Yard bars to the United States. When Bronze No. 11 was compared to Hassler’s Troughton bar, it was found to be 0.00087 inches shorter and was replaced by Bronze No. 11 as the accepted standard of length in the United States.

 While not gaining widespread acceptance in the United States, the complete metric system was embraced throughout Europe.

The Treaty of the Meter

The Meter Convention, also known as the Treaty of the Meter, was an international treaty signed on May 20, 1875, in Paris, France. Its purpose was to establish a formal international agreement to create a universal system of measurement based on the metric system.

The origins of the Meter Convention date back to the French Revolution when the metric system was first introduced in France in the 1790s. The metric system was designed to be decimal-based, providing a coherent and standardized way to measure length, mass, volume, and other physical quantities.

Key provisions of the Meter Convention:
  1. The International Bureau of Weights and Measures (BIPM): The Meter Convention established the International Bureau of Weights and Measures (BIPM) as the custodian and guardian of the international prototype of the meter and kilogram. The BIPM, located in Sèvres, France, is responsible for maintaining and disseminating the International System of Units (SI) and ensuring the worldwide uniformity of measurements.
  2. International Prototype Meter and Kilogram: The treaty defined the International Prototype Meter as the standard of length and the International Prototype Kilogram as the standard of mass. These physical artifacts made of platinum-iridium alloy served as the primary references for the meter and kilogram, respectively.
  3. Definition of the Meter: The treaty defined the meter as one ten-millionth of the distance from the North Pole to the Equator, along a meridian passing through Paris. This definition remained in use until 1960, when it was redefined based on the wavelength of a specific spectral line krypton-86.
  4. Metric System Adoption: The treaty’s signatory nations agreed to adopt the metric system as their official measurement system and promote its use in international trade and commerce.

The Meter Convention resulted in committees being appointed to construct permanent standards technically superior to those made during the French Revolution. The treaty also provided an International Bureau of Weights and Measures to be established on neutral ground in Sevres, France.

In 1889, the finest specimens of meter bars and kilogram mass were selected as the ultimate authority to represent the meter and kilogram reference standards and placed securely in an underground vault as provided by the treaty.

In 1893, Meter Bar No. 27 and Kilogram No. 20 were presented during a ceremony in the office of President Benjamin Harrison. Later, the United States Congress recognized them as our national standards for the Meter and Kilogram.

Over time, the Meter Convention evolved as advances in science and technology led to more precise methods for defining the fundamental units of measurement. In 1960, the meter was redefined based on the wavelength of a specific spectral line, krypton-86. Since then, the meter has undergone further redefinition based on the speed of light in a vacuum, leading to the current definition of the speed of light.

World Metrology Day, May 20th

The Meter Convention remains a significant treaty that laid the foundation for the modern metric system and the International System of Units (SI), providing a unified and coherent system of measurement used internationally in science, industry, trade, and daily life. Today, we celebrate “World Metrology Day” on May 20.

Modern Era

Technological Advancements: The 20th and 21st centuries have seen remarkable advancements in measurement technologies, from atomic clocks for time measurement to quantum standards for electrical units.

Redefinition of SI Units: In 2019, the SI units were redefined based on fundamental constants of nature, such as the Planck constant and the speed of light, ensuring their universality and stability.

References
  1. NIST Special Publication 330, 2019, The International System of Units (SI), Retrieved from: https://www.nist.gov/pml/special-publication-330
  2. BIPM, Bureau International des Poids st Mesures, Retrieved from: https://www.bipm.org/en/

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