Over thousands of years, Kilogramme has been defined a number of times. A standard to define it is placed since 1889. Known as Le Grand K, a cylinder of platinum-iridium is kept inside a jar at the International Bureau of Weights and Measures (BIPM) in Sèvres, France. For close to 130 years now, this cylinder’s mass is considered as an international standard for the kilogramme.
However, all that could change now. A number of representatives will vote on Friday in Versailles to redefine the International System of Units or SI. As per reports, the new definition will be voted in at the meeting. The kilogramme is among four fundamental units that are being reconsidered.
Apart from kilogramme, others are ampere (current), the Kelvin (temperature) and the mole (amount of substance). The definition of the kilogramme will be based on a concept of physics known as Planck constant.
What is the need for fundamental units?
The step has become a necessity since scientists want to come out with a measurement system which is based on unchanging fundamental properties of nature. Le Grand K, the “international prototype kilogram”, is the last object which was used to define an SI unit.
It is difficult to be changed— gets dusty, also it is affected by the atmosphere. After being cleaned, it is vulnerable to change, but the minute that change is. On the other hand, Planck constant, is just a constant, if a complex one — it is a quantity that connects a light particle’s energy to its density. It is defined in a unit that has the kilogramme built in it.
It is to be noted that there are seven fundamental units. Every unit of measurement can be gathered from one or more of these seven units: For example, the unit for speed factors in the units for time and distance.
Even as four of the fundamental units, which include kilogramme, are on their way to being redefined, the other three are based on unchanging properties of nature. These are second (time), the metre (distance), and the candela (luminous intensity, a measure for light’s brightness).
New concepts
The second has been defined as the time since the year 1967. It requires some amount of energy to be released as radiation from atoms of Caesium-133. This went on to become the basis of all measures of time and is used in atomic clocks.
After the second was defined, the metre fell in place. This was on yet another universal constant: speed of light. The metre is described as the distance travelled by light in vacuum in 1/299,792,458 of a second (which is already defined).
The Planck constant, which it is based on, is measured in joule seconds, however, this can also be expressed as kilogramme square metres per second, physicist Kevin Pimbblet says in an article in The Conversation. “We know what a second and a metre is from the other definitions. So by adding these measurements, along with an exact knowledge of Planck’s constant, we can get a new, very precise definition of the kilogram,” Pimbblet writes.