An Introduction to the New SI

• Sandra Knotts ^[2] ; Peter J. Mohr ^[1] ; William D. Phillips ^[3]
  1. [1] National Institute of Standards and Technology

     National Institute of Standards and Technology

     Estados Unidos

     
  2. [2] Perkiomen Valley High School
  3. [3] University of Maryland

  Mostrar afiliaciones +
• Localización: The Physics Teacher, ISSN 0031-921X, Vol. 55, Nº. 1, 2017, págs. 16-21
• Idioma: inglés
• Texto completo no disponible (Saber más ...)
• Resumen

  • Plans are under way to redefine the International System of Units (SI) around 2018. The new SI specifies the values of certain physical constants to define units. This article explains the new SI in order to provide a resource for high school teachers as well as for advanced students already familiar with the pre-2018 SI.

    Physical science is based on measurements, and the results of measurements are expressed in terms of units. For example, someone can measure the length of a table and report that it is 1.4 m long, where m stands for meters. This statement provides useful information to other people, because there is agreement about what a meter is. In fact, there is a treaty among 58 nations, including the United States, that codifies agreement on exactly what the meter is. The treaty is called the Convention du Mètre or Treaty of the Meter, and it also specifies other units.1 According to the treaty, Le Systèm International d’Unités (SI), or the International System of Units, is agreed to be the standard by which all participating member states set their units. Even though a majority of people in the United States still use units such as inches and pounds, the official standards for these units are linked to the SI. For example, the U.S. definition of the inch is that it is exactly 0.0254 m or 2.54 cm, where the c stands for 10−2.

    Although the practice of establishing standards for measurements dates back thousands of years, the Treaty of the Meter was only established in 1875 with 17 nations initially signing on, including the United States. Incidentally, the anniversary of the signing, May 20th, is now known as World Metrology Day. The SI, established in 1960, is still more recent and continues to evolve.2,3 In fact, it is anticipated that the definitions of measurement standards specified by the SI will undergo a substantial revision in 2018.4–6 In the new SI, units are defined by assigning specific values to a set of physical constants.7,8 Despite the fact that the definitions will be changed, the effect on everyday measurements will be imperceptible. This paper describes the new SI and shows how units will be based on assigned values of the constants.