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NPL ahead in race towards new temperature definition

14 July 2013

Scientists at the National Physical Laboratory (NPL) have performed the most accurate measurement yet of the Boltzmann constant.

The acoustic resonator consists of two copper hemispheres and is used to help redefine the kelvin in terms of the Boltzmann constant
The acoustic resonator consists of two copper hemispheres and is used to help redefine the kelvin in terms of the Boltzmann constant

The constant states how much energy at the individual particle level corresponds to each degree of temperature. This measurement could revolutionise the way we define temperature, replacing the standard method that has been used for over 50 years.

The new measurement result is 1.380 651 56 (98) × 10-23 joules per kelvin. The '(98)' shows the uncertainty in the last two digits, which amounts to 0.7 parts per million - almost half the previous lowest uncertainty. This result has been published in the journal Metrologia.

Scientists currently define the SI unit of temperature, the kelvin, and the degree Celsius using the temperature of the triple point of water - the point at which liquid water, solid ice and water vapour can all exist in equilibrium. This 'standard temperature' has been defined as 273.16 K exactly.

All of the temperature measurements we make in everyday life are an assessment of how much hotter or colder an object is compared to this value. As temperature measurements need to be made with increasing accuracy across a wide range of disciplines, fixing a single temperature as a standard becomes problematic, especially if measuring extremely hot or cold temperatures.

The solution is to redefine the kelvin using a fixed constant of nature and the suggested method is to use the Boltzmann constant.

NPL's Michael de Podesta, who led the study, said: "It is fascinating that humans worked out how to measure temperature long before we knew what temperature actually was. Now we understand that the temperature of an object is related to the energy of motion of its constituent atoms and molecules.

When you touch an object and it feels 'hot' you are literally sensing the 'buzzing' of the atomic vibrations. The new definition directly links the unit of temperature to this basic physical reality."

NPL scientists, in collaboration with Cranfield University and the Scottish Universities Environmental Research Centre (SUERC), used acoustic thermometry to make the measurement by building an acoustic resonator and making amazingly precise measurements of the speed of sound in argon gas.

These measurements allowed them to calculate the average speed of the argon molecules and hence the average amount of kinetic energy that they had - from this they were able to calculate the Boltzmann constant with an extremely high accuracy.

Downloaded a free copy of the paper: A low-uncertainty measurement of the Boltzmann constant.

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