It has two heat-sensitive thermocouples or thermistors. Typical electro-mechanical sensors used in mechatronics are strain gauges, thermocouples, and pressure transducers. Power generation using multiple thermocouples, as in a thermopile, is common. Some pH meters provide built-in temperature-coefficient correction, with temperature thermocouples in the electrode probes. Metal thermocouples have low thermal-to-electrical efficiency. These thermocouples are well suited for measuring extremely high temperatures. The key feature is the existence of two thermocouples connected to a voltmeter. Typically a large number of thermocouples are connected in series to generate a higher voltage. Constantan is also used for electrical resistance heating and thermocouples. The system generated electricity by circulating NaK around lead tellurium thermocouples. It consisted of a central fuel capsule surrounded by concentric rings of thermocouples. Types B, R, and S thermocouples use platinum or a platinum/rhodium alloy for each conductor. A combination of bismuth and tellurium is most commonly used for the thermocouples. Usually a number of thermocouples connected in series called thermopiles are used. It is composed of several thermocouples connected usually in series or, less commonly, in parallel.