| 摘要 |
<p>A tennis court line calling system including an electrically conductive tennis ball (20) and an electrical sensing circuit (32) having sets of exposed, spaced apart conductors (34) bounding selected areas of the tennis court and extending along the top of the net to sense touchdown of the electrically conductive ball. An adjustable resistor (76) makes the circuit insensitive to water on the court. The electrically conductive tennis ball has a cover (24) of woven fabric (30, 30a) in which the yarn used for weaving the fabric in at least one direction is made by twisting together a blend of electrically conductive and electrically non-conductive filament fibers (48, 50, 48a, 50a). The electrically non-conductive fibers may predominate, and the woof yarn (46) may be thicker than the warp yarn (44) which may be so woven with the woof yarn that the latter occupies the major part of the ball's surface.</p> |
