A zero-point detecting method of this invention is performed prior to testing the electrical characteristics of a wafer by bringing an object to be tested on a stage and probes of a probe card into contact with each other. When used with network computer systems or communications systems, securing a contact imager is difficult. The additives are in electrically conductive contact with one another, in order to form the electrically conductive layer. The solid lubricant pieces are embedded in a plurality of depressions of the inner annular wall surface by coating and grinding and each has a radial inside annular surface in a curve conforming to that of the inner annular wall surface. Next, during normal system operation, the system periodically performs a selected pilot transmission by transmitting pilot signals through a selected subset of the available transmit antennas. The rigid layer includes a plurality of turned edges.