Feeds

Glorious silicon globes could hold key to elusive PERFECT kilogram

El Reg drills into why we need an ultra-accurate mass

Beginner's guide to SSL certificates

Splitting a speck of dust

No one knows why Robinson's balance in Canada and the NIST balance in the US can't agree.

"Most of the checks have been carried out - everybody has done the straight forward stuff, it's now down to some very subtle things in the apparatus," Robinson said.

The differences between the results are infinitesimal yet signficant. Earlier this year Robinson achieved a value of 6.626071 x 10-34 joule seconds using the balance now at INMS. The NIST value, from 2007, is 6.626068 x 10-34 joule seconds.

John Pratt, group leader of NIST's fundamental electrical measurement group, said the gap between the two devices translates to a difference in the measured mass of 250 parts per billion; the CGPM committee wants 20 parts per billion. To give an idea of scale, the kilogram masses in Paris have changed by 50 parts per billion in just over 100 years.

Pratt, who succeeded NIST physicist Richard Steiner on the project, told us 40 parts per billion would be "fine", adding: "At 20 parts per billion, it becomes more difficult to transport the test mass around the world and have it be reliable. It's a dust speck on the mass at that level."

NIST Watt Balance copy right Robert Rathe, © Robert Rathe; courtesy NIST

Steiner adjusting the NIST's watt balance. Credit: Robert Rathe, NIST

In a watt balance, a coil of wire is suspended in a magnetic field, and electric current is passed through the coil - this causes the coil to move downwards with a force proportional to the current. The coil is attached to one end of a balancing beam and the mass under test is attached to the opposite end, so that the coil's downward force acts as a counterweight to the normal gravitational pull on the test object. When the force, plus the weight of the coil, matches the weight of the object, the current can be recorded.

In the second phase of the experiment, the mass is removed and the coil glides through the same magnetic field at a constant speed, and the voltage generated in the coil by this movement is measured. Planck's constant can be calculated from these two electrical values.

You can get more details on the maths involved from the NPL website here and the NIST website here.

There, though, the similarities between the devices end. Robinson's balance used wires to support the coil while the other has solid arms; the NIST machine is huge - spanning two floors - while the NPL-designed machine sits neatly on a work surface, two metres tall and long, and one meter wide; NIST employs a superconducting solenoid to generate its magnetic field while the NPL-designed balance uses a permanent magnet made of neodymium-boron - a type of rare earth used in electrical motors and hard-drive heads. Also, Robinson's machine, now at INMS, measured a 0.5kg mass not 1kg, supposedly for greater accuracy.

"The trouble is if anybody actually knew what the problem is, they could fix it. The problem is quite deeply buried in the apparatus," Robinson said. "That means they have to make as many checks as possible to see where the problem might lie."

Robinson, who led NPL's watt balance work, said that, for example, he identified small unwanted motions of the balance and its support as the test mass was placed in and removed from its holding pan; these are the sorts of tiny mechanical problems that the scientists must mitigate or remove completely.

Building a watt balance is an expensive process; one will set you back $1m according to NIST, and Robinson made two with NPL colleague Bryan Kibble, who first proposed the watt balance system in 1975. A Mark-III model of the measurement machine was designed by NPL between 2003 and 2006, but it was not built as the lab closed its experiment in 2007 and sold the equipment to the Canadian institute in 2009.

INMS has since implemented changes suggested by Robinson: the balance arm was tilted and shortened to change the way the mass is raised and lowered. Also, rods replaced the wires to suspend the coil.

NIST has pressed on: boffins removed superfluous wiring and inspected connections to track down and close holes where current may leak. New team members were also brought in a year ago. "This experiment is a point where it needed fresh eyes," Pratt said. "Extremely talented and experienced fresh eyes."

Internet Security Threat Report 2014

More from The Register

next story
FORGET the CLIMATE: FATTIES are a MUCH BIGGER problem - study
Fat guy? Drink or smoke? You're worse than a TERRORIST
Renewable energy 'simply WON'T WORK': Top Google engineers
Windmills, solar, tidal - all a 'false hope', say Stanford PhDs
Rosetta probot drilling DENIED: Philae has its 'LEG in the AIR'
NOT best position for scientific fulfillment
SEX BEAST SEALS may be egging each other on to ATTACK PENGUINS
Boffin: 'I think the behaviour is increasing in frequency'
HUMAN DNA 'will be FOUND ON MOON' – rocking boffin Brian Cox
Crowdfund plan to stimulate Blighty's space programme
Post-pub nosh neckfiller: The MIGHTY Scotch egg
Off to the boozer? This delicacy might help mitigate the effects
I'M SO SORRY, sobs Rosetta Brit boffin in 'sexist' sexy shirt storm
'He is just being himself' says proud mum of larger-than-life physicist
NASA launches new climate model at SC14
75 days of supercomputing later ...
LIFE, JIM? Comet probot lander found 'ORGANICS' on far-off iceball
That's it for God, then – if Comet 67P has got complex molecules
prev story

Whitepapers

Why cloud backup?
Combining the latest advancements in disk-based backup with secure, integrated, cloud technologies offer organizations fast and assured recovery of their critical enterprise data.
A strategic approach to identity relationship management
ForgeRock commissioned Forrester to evaluate companies’ IAM practices and requirements when it comes to customer-facing scenarios versus employee-facing ones.
High Performance for All
While HPC is not new, it has traditionally been seen as a specialist area – is it now geared up to meet more mainstream requirements?
Managing SSL certificates with ease
The lack of operational efficiencies and compliance pitfalls associated with poor SSL certificate management, and how the right SSL certificate management tool can help.
Top 5 reasons to deploy VMware with Tegile
Data demand and the rise of virtualization is challenging IT teams to deliver storage performance, scalability and capacity that can keep up, while maximizing efficiency.