Feb 25 2008
When chemists want to measure the bonding forces in molecules or other most minuscule forces very accurately, they have to calibrate their measuring instruments (for example the cantilevers, i.e. the measuring tips, of scanning force microscopes). And if it is a matter of comparing the attained results with other results, one must refer to a common basis.
In the case of scanning force microscopes, the nominal values for bending stiffnesses deviate distinctly from the actual values. With the current devices, calibrations of cantilevers are accurate to > 5%. For forces in the nano- and piconewton range one therefore requires more accurate realisations and stable transfer standards.
In order to offer this in future, the Physikalisch-Technische Bundesanstalt (PTB) has set up the protoype of a nanonewton force-measuring device. First measurements show that the measuring principle functions well: The very small force (of approx. 50 pN) of a laser beam on the pendulum, the "heart" of the apparatus, is measured with a voltage (acting as counterforce), and this with a measuring uncertainty of 5 % to 10 %.
First measurements have shown that the measuring device is sufficiently protected against vibrations (so-called "seismic noise"). A large-scale device, which is to be set up next year, is envisaged to bring still further improvements here. Furthermore, other changes are also needed to be able to actually measure on cantilevers (as transfer standards).