The International Committee of Weights and Measures has approved the change the definition for the kilogram, as well as other basic units. After more than 129 years, the current definition of the kilogram will go down in history. As of May 2019, the new definition will take over. The definition of the basic units of the ampere, kelvin and mol will also be changed.
In the 26th conference of this committee, the 60 Member States voted unanimously in favor of these new definitions. For ordinary people, it will not really mean any change in their lives, but speaking of science, history is being made. As the experts say, this will be told in the textbooks. And it’s a breakthrough for physicists and scientists.
According to the committee, these redefinitions improve the accuracy with which current measurements are made. Thus the new definition of the ampere will improve the measurement of the electrical temperature and the kelvin, the radiometric one. The new definition of the mole will be much simpler than the current one. The members of the committee comment that these redefinitions were necessary to adapt to the technology of this century.
The new definition of how to measure a kilogram will no longer depend on a material object. Instead, it will depend of a value based on a physical constant, the Planck constant.
The previous definition of Kilogram
Until now, the kilogram definition was based on a physical object. A metallic cylinder called big K – kilogram prototype. This cylinder is composed of an alloy of platinum and iridium. So the kilogram is defined as the mass of that international prototype, which is stored in the International Office of Peas and Measures in Sévres, near Paris.
For 130 years this cylinder stored under two glass bells has served as a basis for mass measurements. From time to time it is taken out again to calibrate similar prototypes around the world. Like the one found in the United States, which is what they use as a standard measure in that country.
Before this definition there was another that specified that a kilogram was the mass of a cubic decimeter (one liter) of distilled water at an atmosphere of pressure and 3.98 º C. This temperature is one in which water has the highest density in the normal atmospheric pressure. The problem with this definition was that the density of the water depended slightly on the pressure, whose units include mass as a factor, thus creating a circular dependence on the definition.
The new definition of Kilogram
In the new definition physicists have dispensed with the Grand K. As of May 2019 the new definition will be linked to the Planck constant . This constant will be defined as 6.62607015 × 10 -34 kg⋅m 2 ⋅s -1 . Therefore, the kilogram will be defined from two other basic units of the International System of Units, such as the meter and the second .
So to calibrate the different patterns distributed around the world, from now on, a Watt scale and the value of the Planck constant will be used. The Watt scale is an electromechanical weight measurement instrument. It measures the weight by the force of an electric current and a voltage.