Glorious silicon globes could hold key to elusive PERFECT kilogram

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

Top 5 reasons to deploy VMware with Tegile

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."

Secure remote control for conventional and virtual desktops

More from The Register

next story
Bond villains lament as Wicked Lasers withdraw death ray
Want to arm that shark? Better get in there quick
Renewable energy 'simply WON'T WORK': Top Google engineers
Windmills, solar, tidal - all a 'false hope', say Stanford PhDs
The next big thing in medical science: POO TRANSPLANTS
Your brother's gonna die, kid, unless we can give him your, well ...
SEX BEAST SEALS may be egging each other on to ATTACK PENGUINS
Boffin: 'I think the behaviour is increasing in frequency'
Reuse the Force, Luke: SpaceX's Elon Musk reveals X-WING designs
And a floating carrier for recyclable rockets
NASA launches new climate model at SC14
75 days of supercomputing later ...
Britain's HUMAN DNA-strewing Moon mission rakes in £200k
3 days, and Kickstarter moves lander 37% nearer takeoff
Simon's says quantum computing will work
Boffins blast algorithm with half a dozen qubits
prev story


Why and how to choose the right cloud vendor
The benefits of cloud-based storage in your processes. Eliminate onsite, disk-based backup and archiving in favor of cloud-based data protection.
Getting started with customer-focused identity management
Learn why identity is a fundamental requirement to digital growth, and how without it there is no way to identify and engage customers in a meaningful way.
10 threats to successful enterprise endpoint backup
10 threats to a successful backup including issues with BYOD, slow backups and ineffective security.
Reg Reader Research: SaaS based Email and Office Productivity Tools
Read this Reg reader report which provides advice and guidance for SMBs towards the use of SaaS based email and Office productivity tools.
The Heartbleed Bug: how to protect your business with Symantec
What happens when the next Heartbleed (or worse) comes along, and what can you do to weather another chapter in an all-too-familiar string of debilitating attacks?