On May 20th, 2019, an important change took place. Not only was it the official 144th birthday of the Metre Convention, but it was the day where four the International System of Units (SI units) were redefined, signifying the first major change to the system since 1960.

**The International System of Units**

The International System of Units (SI) is used as the modern form of the internationally recognized system of measurement, otherwise known as the metric system. It consists of seven units of measurement and is built on the following seven units – the second, meter, kilogram, ampere, kelvin, mole, and candela. These are all based on the set constants of nature that can be observed and measured.

How can you be sure the units are reliable? That depends on two things. First, reliability depends heavily on the units’ precise measurement of standards for the various physical constants of nature. Secondly, as more units continue to be found the definition of those constants must change as well, as the system should continue to evolve as better measurement technologies become available.

*Can you name the base units for the International System of Units (SI)? Take this **quiz** and find out.*

**The Redefined SI Units System**

In May 2019, a total of four of the seven base SI units were redefined – the ampere, Kelvin, mole, and kilogram. Rather than redefining the values of the units themselves, the system improved by setting exact numerical values for the constants these base units are associated with. These new values were then measured by a series of experiments that followed.

**Ampere**

The Ampere (A) has been redefined to a more simple definition – and for good reason! The previous definition was unnecessarily wordy and difficult to understand. Now, amperes (A) have been redefined to measure electrical current or define electrical charge at 1.602176634×10^{−19} coulombs.

**Kelvin**

The Kelvin (K) also went through a significant change. This base unit measures temperature and is defined as a change in energy of 1.3806505×10^{−23} joules, otherwise known as the Boltzmann constant. The Boltzmann constant is based on energy and time and is now used to measure Kelvins. Previously, the Kelvin was defined in terms of the triple point of water.

**Mole**

Previously, the mole was linked to the number of atoms in 0.012 kg of carbon-12. Now, it measures the amount, or the number of units or entities of a substance, using Avogadro’s constant. One mole is equal to Avogadro’s constant, or 6.02214076×10^{23} entities.

**Kilogram**

What used to be defined as the mass of the International Prototype of the Kilogram is now related to the Planck constant. The kilogram (kg) measures a photon’s equivalent mass of energy, given its frequency. The Planck constant is 6.62607015×10^{−34} kg⋅m^{2}⋅s^{−1}. It has taken decades to redefine the kilogram because the Planck constant is so small and difficult to accurately measure.

While the first four units were redefined more dramatically with changes in the values of their constants, the remaining three also underwent minor changes.

**Second**

The second (s) measures units of time and although the new definition is very similar to the previous, it has been more clearly defined as new conditions have been set in place. It’s now defined in terms of the fixed interval of Caesium frequencies and the frequency of the Caesium-133 atom expressed in Hertz.

**Meter**

The only change to the meter (m) is the added complexity to its definition. What used to be just a measure of length traveled by light per second is the definition of the second, which is now measured in terms of Caesium frequencies.

**Candela**

Last but not least, the candela (cd) definition remains relatively the same, except for the added parts to the definition of the second and meter. The candela is defined as the luminous intensity in a specific direction, broadcasted by a source. The new definition, however, states that a candela is defined “by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540×10^{12} Hz,K_{cd}, to be 683 when expressed in the unit lm W^{−1}, which is equal to cd sr W^{−1}, or cd sr kg^{−1}m^{−2}s^{3}, where the kilogram, metre and second are defined in terms of h, c and ∆νCs”.

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