Crystalline element 1, of Rochelle salt, has the shape of a plate
fixed at one end. This element is subject to bending by the
action of stylus a as it traces over surface d whose roughness is
to be determined. Ow ing to the piezoelectric effect, electric
charges are developed on the metal electrodes of the crystal at
the faces of the crystalline plate. These charges are strictly
proportional to the deformation, i.e. to the deflections of the
free end of the plate. These, in tu rn, are equal to the vertical
displacements of stylus a. At a definite value of the electric
capacitance between the electrodes of the crystal, the electrodes
set up a potential difference proportional "to the charge. As
stylus a moves continuously along surface d being inspected, the
potential difference over the electrodes varies continuously,
in proportion to the displacements of stylus a, i.e. to the o rd inates
of the profile of surface d. These variations in potential
difference can be am plified by am plifier 2 and measured by
suitable electrical instrument 3.
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