A gain-control method and device that enable high-speed gain switching of cascaded programmable gain amplifiers without a high-resolution AD converter is provided. A ferroelectric single crystal consisting essentially of potassium tantalate and the alkali metal substituting from about 1% to 3% of potassium or a group metal substituting in part for tantalum, having cubic form of perovskite crystalline structure, and essentially free of impurities and defects is also disclosed. The sensor is provided with a travel sensor which is disposed parallel to the seat fastening screw between the two fastening areas or to these rigidly connected parts and which measures the distance of these fastening areas parallel to the seat fastening screw. A gap is defined between each block portion and its corresponding sidewall, and each gap receives a resilient member.