Mass artefacts employed in this prototype are stainlesssteel wires ranging in mass from 0.5 mg to 500 mg, for anominal deadweight range of 5 µN–5 mN. The force cellis an instrumented indentation transducer obtained through aspecial arrangement with Hysitron, Inc. The capacitance ofthis transducer is a function of mechanical force; we use anAndeen–Hagerling 2500-A capacitance bridge (AH bridge) tomeasure its value. The transducer has a loading button with avee-groove for holding the wire weights. At the very centreof the loading button, there is a recessed mica flat to provide asmooth, flat location for applying the indenter forces. Clearly,the various wire weights can produce torques that would notresult from a direct point loading at the centre of the transducer.However, we deliberately applied loads with large offsets (asmuch as 5 mm) to assess this dependence and found it to benegligible. Furthermore, care was taken to ensure that all wireweights were centred on the loading button.To calibrate the transducer force sensitivity, a givendeadweight is set on the loading button 10 times in succession.After the tenth force repetition, the deadweight loader isindexed to a new deadweight and the process is repeated for anew force level. The data are fitted to a third-order polynomialgiven byC =A0+A1F+A2F2+A3F3,(4)where Cis the change in capacitance of the device, Fis theapplied force and A0through A3are polynomial coefficients.The capacitance change is calculated byCi,j =12((Ci−Cj)+(Ci−Cj+1)), (5)where Ciis the capacitance value in the loaded condition, Cjisthe capacitance value in the unloaded condition and Ci,j isthe average change in capacitance due to a force betweensuccessive unloaded conditions,

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