How do
you weigh a dust mite? Or determine the force required to pull
a molecule apart? Such tasks require a device that measures nanonewtons—forces
1 billion times smaller than the force required to hold an apple
against Earth’s gravity. Nanonewton forces are estimated
with atomic force microscopes and instruments that measure the
properties of ultrathin coatings like those used on computer
hard drives or turbine blades. But the accuracy of such estimates
is unknown because they haven’t been calibrated with force
standards based on the kilogram, the internationally accepted
unit of mass.
Luckily, there is hope
on the horizon. In a paper presented June 4 at the annual conference
of the Society of Experimental Mechanics
in Charlotte, N.C., National Institute of Standards and Technology
(NIST) engineers describe a prototype instrument that reliably measures
forces as small as tens of nanonewtons and simultaneously ties those
measurements to forces a thousand times larger based on the kilogram.
The device works by connecting a well-calibrated spring-loaded scale
with a set of electrodes that generates an electrostatic force. The
instrument balances the downward force produced by a one-milligram
mass artifact, by keeping the distance between the electrodes constant
but varying the amount of voltage between them. The result is a force
determination accurate to a few parts in 10,000 that is measured
with voltage, electrical capacitance and distance (the location of
the electrodes as measured in wavelengths of laser light). “It
is much easier to make small changes in voltages, than to make small
mass standards,” explains NIST mechanical engineer Jon Pratt.
The NIST researchers hope
to extend the instrument’s resolution
to tens of piconewtons (trillionths of a newton).
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Porter
Manufacturing Metrology Division
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