In: Physics
How likely is it that the disagreement between your experimental and accepted value for Plank’s constant is due to uncertainty in the measurement of the potential or the wavelength? You can obtain specifications for resolution of the sensors at the Vernier web site.
The measurement of the Planck constant, h, is entering a new
phase. The CODATA 2010 recommended value is 6.626 069 57 × 10(-34)
J s, but it has been a long road, and the trip is not over yet.
Since its discovery as a fundamental physical constant to explain
various effects in quantum theory, h has become especially
important in defining standards for electrical measurements and
soon, for mass determination. Measuring h in the International
System of Units (SI) started as experimental attempts merely to
prove its existence. Many decades passed while newer experiments
measured physical effects that were the influence of h combined
with other physical constants: elementary charge, e, and the
Avogadro constant, N(A). As experimental techniques improved, the
precision of the value of h expanded. When the Josephson and
quantum Hall theories led to new electronic devices, and a hundred
year old experiment, the absolute ampere, was altered into a watt
balance, h not only became vital in definitions for the volt and
ohm units, but suddenly it could be measured directly and even more
accurately. Finally, as measurement uncertainties now approach a
few parts in 10(8) from the watt balance experiments and Avogadro
determinations, its importance has been linked to a proposed
redefinition of a kilogram unit of mass.The path to higher accuracy
in measuring the value of h was not always an example of continuous
progress. Since new measurements periodically led to changes in its
accepted value and the corresponding SI units, it is helpful to see
why there were bumps in the road and where the different branch
lines of research joined in the effort. Recalling the bumps along
this road will hopefully avoid their repetition in the upcoming SI
redefinition debates. This paper begins with a brief history of the
methods to measure a combination of fundamental constants, thus
indirectly obtaining the Planck constant. The historical path is
followed in the section describing how the improved techniques and
discoveries in quantum mechanics steadily reduced the uncertainty
of h. The central part of this review describes the technical
details of the watt balance technique, which is a combination of
the mechanical and electronic measurements that now determine h as
a direct result, i.e. not requiring measured values of additional
fundamental constants. The first technical section describes the
basics and some of the common details of many watt balance designs.
Next is a review of the ongoing advances at the (currently) seven
national metrology institutions where these experiments are
pursued. A final summary of the recent h determinations of the last
two decades shows how history keeps repeating itself; there is
again a question of whether there is a shift in the newest results,
albeit at uncertainties that are many orders of magnitude less than
the original experiments. The conclusion is that there is room for
further development to resolve these differences and find new ideas
for a watt balance system with a more universal application. Since
the next generation of watt balance experiments are expected to
become kilogram realization standards, the historical record
suggests that there is yet a need for proof that Planck constant
results are finally reproducible at an acceptable
uncertainty.