An improved means for measuring recesses in an electrically conductive surface

917,893. Determining physical qualities of materials. SUN PRINTERS Ltd. Sept. 13, 1961 [June 13, 1960], No. 20683/60. Class 37. The integrated characteristic of the recesses in an area of an electrically conductive surface such as the intaglio cells of a gravure printing cylinder, is measured by measuring the capacitance between a test unit or so-called " probe," and the area to be tested. The test unit or probe shown in Fig. 1 comprises a bottom casing 3 closed at the top and bottom by flexible dielectric material 17, 1 and filled-with mercury. 2. When the casing is associated with the conductive surface 5, the mercury 2 and the surface 5 form the plates of a capacitor, which plates are separated by the dielectric material 1. The mercury not only serves as the plate of a capacitor but also provides the necessary pressure to hold the dielectric 1 in firm contact with the surface to be tested. In a modification (Fig. 3, not shown) mercury is dispensed with, the dielectric 1 being provided with a metallized portion to act as a capacitor plate and being held in contact with the area under test by air pressure. As described, the capacitance is measured by an A.C. bridge circuit including a transistor oscillator and a transistor amplifier.