文档介绍：Bimetallic Switch: How does it work? ? Metals expand when heated ? A strip of 2 different metals that are connected (coil is more sensitive) ? Metals will have different rates of expansion ? If metal on inside of coil expands more when heated, coil will unwind It’ s a thermometer! Converts temperature change to mechanical movement. Thermostat: How does it work? ? Temperature lever attached to center of bimetallic coil . ? Mercury tilt switch attached to end of coil. When mercury contacts wires, a circuit is closed. (Need to mount level) Bimetallic strip can be a mechanical automatic control device. Similar purpose to flyball governor. Mercury Use ? Hg barometer: Toricelli 1643 ? Hg thermometer: Fahrenheit 1714 ? Hollerith tabulating machine & sorter 1890 ? Liquid metal (-39 to 357 o C) ? Conducts electricity ? Uniform thermal expansion ? Visible Advantages Thermostat: How does it work? ? Heat anticipator (resistor): Based on setting, current travels through resistor. Generates heat, which warms coil. Shuts off heat before room heat actually reaches thermostat. Variations: Digital Thermostats ? Thermistor: Electrical resistance changes with temperature. Converts resistance to temperature. ? Programmable thermostats Thermocouples ? 2 wires of different metals are joined ? Measures change in voltage Bimetallic Switch: History ? Invented by English carpenter & clockmaker John Harrison in 18 th century (1739-1749). ? Developed precise marine clocks (chronometer) to calculate longitude. Bimetallic pensated for temperature changes in the balance spring. ? When receiving Copley Gold Medal, referred to invention as Harrison ’s “ new metalline thermometer ”. ? Memorial in Westminster Abbey, England in 2006. Harrison ’ s pensation Gridiron Pendulum H1 H2 H3 Bimetallic Switch: Applications ? Bimetallic strip thermometers : Johann Lambert & David Rittenhouse in 1767 in Philadelphia; James Crichton in 1803 in Scotland. ? Meat & candy thermometers . U