Question

Research photos of the official metric reference standards for the meter and kilogram and describe the reference prototypes.

Short answer

Metric reference prototypes include a platinum-iridium bar for the meter and cylinder for the kilogram.

Step-by-step solution

Understanding the Metric System

The metric system is an internationally recognized system of measurement used for specifying quantities like length and mass. The meter and kilogram are fundamental units of this system.

Metric Reference for the Meter

The original reference for the meter was a platinum-iridium bar created and kept in France. This bar was defined as the standard meter in 1889. Its dimensions were controlled at the melting point of ice to ensure accuracy. Although the definition has since changed to rely on the speed of light, the bar served as the prototype for many years.

Metric Reference for the Kilogram

Like the meter, the kilogram was also defined by a platinum-iridium cylinder, known as the International Prototype of the Kilogram (IPK). It was created in 1889 and remains locked in a vault at the International Bureau of Weights and Measures (BIPM) in France. It was the definitive reference for the kilogram until 2019, when the kilogram was redefined based on the Planck constant.

Key concepts

The Meter

The meter is a core unit within the metric system, representing a fundamental measure of length. Originally defined in 1793, it was intended to be one ten-millionth of the distance from the equator to the North Pole. However, producing an accurate physical representation of such a vast measurement was a significant challenge.

In 1889, a more practical solution was implemented with the construction of the prototype meter, a bar made of 90% platinum and 10% iridium. This bar was stored under specific conditions to maintain its precise length as the definitive standard for what a meter should be. This physical object was sheltered at the Bureau International des Poids et Mesures (BIPM) in France. Over time, however, advances in technology rendered the platinum-iridium bar obsolete as a measure of a meter.

Today, the meter is defined by the speed of light. In 1983, scientists agreed to define one meter as the distance light travels in a vacuum during 1/299,792,458 of a second. This change ensures that the definition of a meter is based on a constant of nature, rather than a man-made artifact subject to wear and tear.

The Kilogram

The kilogram is another essential unit in the metric system, specifically representing mass. Like the meter, its original standardized measurement was also established in 1889 in the form of a physical object, the International Prototype of the Kilogram (IPK).

This prototype is a cylinder made out of the same platinum-iridium alloy as the meter prototype, providing stability and precision. It was also stored and periodically carefully checked to ensure it remained unchanged. Although meticulously maintained, the reliance on a single physical object for such a critical measurement posed challenges. It was known to have been slowly gaining mass due to surface contaminations and degradation.

In 2019, a historic shift took place when the definition of the kilogram was changed to rely on the Planck constant. This constant is a key component of quantum mechanics and allows the kilogram to be precisely calculated using unchanging physical laws instead of a physical prototype. This transition was significant for ensuring accurate mass measurement worldwide.

The International Prototype

Historically, the international prototypes for both the meter and the kilogram were pivotal for ensuring uniformity and standardization in measurements across the globe. Crafted from an extremely stable and enduring platinum-iridium alloy in 1889, these objects were carefully sheltered at BIPM in France and served as the benchmark for their respective measurements for over a century.

These prototypes underscore the importance of having consistent and reliable standards in scientific endeavors and daily commerce. They represent the lengths humanity has gone to achieve precision and uniformity in measurements, which are fundamental to technical development and international trade.

Despite their historical significance, reliance on such physical prototypes had certain drawbacks, such as wear over time or alterations due to environmental factors. These challenges prompted the scientific community to redefine these units based on universal constants.

The transition to using the speed of light for the meter and the Planck constant for the kilogram manifests the pursuit of precision and adaptability in scientific advancements.