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Musical argon is most accurate thermometer ever
Plainpicture
THIS WEEK
Sweet argon music helps redefine temperature A NOISY ball of gas is now the most accurate thermometer ever. That removes one hurdle to redefining the scientific unit of temperature, the kelvin, in terms of a fundamental constant. At the moment, the kelvin is defined in terms of the temperature at which ice, liquid water and water vapour can coexist in equilibrium – 273.16 K or 0.01 °C. Other temperatures are measured by comparison with this “triple point”, but the further
away you get from it, the more inaccurate the result. “When you’re measuring at 1500 °C, it is actually slightly bonkers to have to compare back to the triple point of water,” says Michael de Podesta of the National Physical Laboratory in Teddington, UK. The International Committee for Weights and Measures (CIPM) in Paris, France, suggests defining the kelvin in terms of the Boltzmann constant, which relates the energy of particles to
The beat that Earth’s heart skipped
term patterns, some well known, some not. Richard Holme of the University of Liverpool, UK, looked at 50 years of GPS and astronomical data to see how day length varied during that time. The analysis threw up a wellknown cycle due to slow changes at the Earth’s core, which lengthen days by a few milliseconds over roughly a decade, then shrink them down again. There’s also a 5.9-year cycle, due to a persistent wobble between the fluid outer core and surrounding mantle, which changes day length by
THREE times in the last decade Earth’s spin has missed a beat. These seemingly random blips cause days to temporarily stretch and shrink. They have emerged from the clearest ever view of how long a day is. Earth’s spin fluctuates as the oceans and the atmosphere push and tug on the planet’s spin. But these small daily variations hide longer10 | NewScientist | 13 July 2013
into a value for the Boltzmann constant (Metrologia, DOI: 10.1088/0026-1394/50/4/354). Joachim Fischer of the German National Metrology Institute in Berlin, who heads the CIPM task group on redefining the kelvin, says the margin of error in the measurement, a record-breaking 0.71 parts per million, is potentially low enough to officially redefine the kelvin. Some details need to be ironed out first. A team at the LNE, the French National Laboratory of Metrology and Testing in Paris, calculated the constant via the same method but came up with a different value. Their error is slightly higher, at 1.24 ppm, but –Wanted: kelvin for extremes– the kelvin can’t be redefined until the discrepancy is understood. their temperature. That would Redefinition also won’t happen be accurate even at extremes. The until a slew of other units that are tricky part is setting an accurate due to be pinned to fundamental value for the constant. constants are agreed – and there De Podesta and his colleagues is argument over the kilogram. filled a copper vessel with a “The game is not quite over,” known volume of argon gas. says Michael Moldover at the US A gas’s energy is related to the National Institute of Standards average speed of its atoms, which and Technology in Gaithersburg, is linked to the speed of sound. Maryland, who created the argon By playing notes through the method in 1988. vessel at many frequencies, the For now, de Podesta needs a team determined the speed of rest. Smashing the Boltzmann sound in the argon, and the record was painstaking, including energy of its atoms. sculpting the argon vessel to a The team knew the temperature precise volume. “This has been of the argon, which was at the occupying my mind for six years triple point of water, so could and I’m absolutely exhausted,” convert the energy measurement he says. Jacob Aron n
fractions of milliseconds a year. When Holme stripped away both of these regular cycles, sudden unexpected jumps in day length emerged from the calculations. Three times in recent years – in 2003, 2004, and 2007 – our planet’s spin has stuttered. The jumps interrupt the longer-term changes by a fraction of a millisecond, and last several months before going back to normal (Nature,
“The planet’s magnetic field also undergoes sudden jerks that coincide with jumps in Earth’s spin”
DOI: 10.1038/nature12282). Satellite readings of the planet’s magnetic field over the last 20 years show that the field also undergoes sudden jerks, and Holmes found that they coincide with the jumps in the Earth’s spin. He says the sudden changes probably occur when a patch of molten outer core temporarily sticks to the mantle, causing a step change in angular velocity. Jon Mound of the University of Leeds, UK, says we need to rethink the dynamics of the Earth’s core in the light of these findings.
Mark Viney n
THIS WEEK
Sweet argon music helps redefine temperature A NOISY ball of gas is now the most accurate thermometer ever. That removes one hurdle to redefining the scientific unit of temperature, the kelvin, in terms of a fundamental constant. At the moment, the kelvin is defined in terms of the temperature at which ice, liquid water and water vapour can coexist in equilibrium – 273.16 K or 0.01 °C. Other temperatures are measured by comparison with this “triple point”, but the further
away you get from it, the more inaccurate the result. “When you’re measuring at 1500 °C, it is actually slightly bonkers to have to compare back to the triple point of water,” says Michael de Podesta of the National Physical Laboratory in Teddington, UK. The International Committee for Weights and Measures (CIPM) in Paris, France, suggests defining the kelvin in terms of the Boltzmann constant, which relates the energy of particles to
The beat that Earth’s heart skipped
term patterns, some well known, some not. Richard Holme of the University of Liverpool, UK, looked at 50 years of GPS and astronomical data to see how day length varied during that time. The analysis threw up a wellknown cycle due to slow changes at the Earth’s core, which lengthen days by a few milliseconds over roughly a decade, then shrink them down again. There’s also a 5.9-year cycle, due to a persistent wobble between the fluid outer core and surrounding mantle, which changes day length by
THREE times in the last decade Earth’s spin has missed a beat. These seemingly random blips cause days to temporarily stretch and shrink. They have emerged from the clearest ever view of how long a day is. Earth’s spin fluctuates as the oceans and the atmosphere push and tug on the planet’s spin. But these small daily variations hide longer10 | NewScientist | 13 July 2013
into a value for the Boltzmann constant (Metrologia, DOI: 10.1088/0026-1394/50/4/354). Joachim Fischer of the German National Metrology Institute in Berlin, who heads the CIPM task group on redefining the kelvin, says the margin of error in the measurement, a record-breaking 0.71 parts per million, is potentially low enough to officially redefine the kelvin. Some details need to be ironed out first. A team at the LNE, the French National Laboratory of Metrology and Testing in Paris, calculated the constant via the same method but came up with a different value. Their error is slightly higher, at 1.24 ppm, but –Wanted: kelvin for extremes– the kelvin can’t be redefined until the discrepancy is understood. their temperature. That would Redefinition also won’t happen be accurate even at extremes. The until a slew of other units that are tricky part is setting an accurate due to be pinned to fundamental value for the constant. constants are agreed – and there De Podesta and his colleagues is argument over the kilogram. filled a copper vessel with a “The game is not quite over,” known volume of argon gas. says Michael Moldover at the US A gas’s energy is related to the National Institute of Standards average speed of its atoms, which and Technology in Gaithersburg, is linked to the speed of sound. Maryland, who created the argon By playing notes through the method in 1988. vessel at many frequencies, the For now, de Podesta needs a team determined the speed of rest. Smashing the Boltzmann sound in the argon, and the record was painstaking, including energy of its atoms. sculpting the argon vessel to a The team knew the temperature precise volume. “This has been of the argon, which was at the occupying my mind for six years triple point of water, so could and I’m absolutely exhausted,” convert the energy measurement he says. Jacob Aron n
fractions of milliseconds a year. When Holme stripped away both of these regular cycles, sudden unexpected jumps in day length emerged from the calculations. Three times in recent years – in 2003, 2004, and 2007 – our planet’s spin has stuttered. The jumps interrupt the longer-term changes by a fraction of a millisecond, and last several months before going back to normal (Nature,
“The planet’s magnetic field also undergoes sudden jerks that coincide with jumps in Earth’s spin”
DOI: 10.1038/nature12282). Satellite readings of the planet’s magnetic field over the last 20 years show that the field also undergoes sudden jerks, and Holmes found that they coincide with the jumps in the Earth’s spin. He says the sudden changes probably occur when a patch of molten outer core temporarily sticks to the mantle, causing a step change in angular velocity. Jon Mound of the University of Leeds, UK, says we need to rethink the dynamics of the Earth’s core in the light of these findings.
Mark Viney n