The purpose of storing the international standard Kilogram under double bell jars in Paris is primarily to protect it from contaminants and external influences that could affect its mass. Let's break down why the jars are used and why there are two of them.
1. Protection from contaminants: The double bell jars help create a controlled environment around the Kilogram to shield it from dust, gases, and other pollutants that could potentially affect its mass. The jars act as a barrier against these contaminants, preventing any interaction between the standard and its surroundings.
2. Vacuum sealing: One of the key reasons for using the double bell jars is to create a vacuum. By removing the air inside the jars, the standard is isolated from the oxygen in the atmosphere. This prevents oxidation, as you mentioned, which could potentially alter the mass of the Kilogram. Oxidation can lead to the formation of a thin layer on the surface of the standard, affecting its weight.
3. Stability and redundancy: Having two jars serves as a safety measure to ensure the stability and accuracy of the standard Kilogram. In case one of the jars becomes compromised or contaminated, the second jar provides redundancy and continuing protection for the standard. This redundancy helps minimize any potential impact on the mass of the Kilogram due to external factors.
It is important to note that the double bell jars are not the only means used to prevent the standard Kilogram from gaining mass. The jars are part of a comprehensive system that includes periodic cleaning and calibration processes to maintain the accuracy and integrity of the standard over time.