Presentation on theme: "Reading a Machine TAS Summer 2006 Carol Wenk"— Presentation transcript:
Reading a Machine TAS Summer 2006 Carol Wenk
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 ( ). Developed precise marine clocks (chronometer) to calculate longitude. Bimetallic strip compensated 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 Temperature Compensation Gridiron Pendulum H1H2H3
Bimetallic Switch: Applications Bimetallic strip thermometers: Johann Lambert & David Rittenhouse in 1767 in Philadelphia; James Crichton in 1803 in Scotland. Meat & candy thermometers. Uses a helical bimetallic strip to turn needle, or uses gears like in a watch. Thermostats: Cornelius Drebbel (Dutch) in 1660 to control temperature of an egg incubator. U-shaped tube filled with Hg, raised a rod that controlled the furnace’s flue.
Bimetallic Switch: Applications Toasters: 1 st electric (no controls): Frank Shailor of GE in 1909 Toastmaster pop-up by Charles Strite in 1919 Late 1920s use bimetallic strips Sunbeam in 1940s improved bimetal (based on bread, not heating element) Irons: 1 st electric cord: Earl Richardson in 1903 in CA Temperature control with Ag thermostat: Joseph Myers of Silex in 1927 Bimetallic strips: American Beauty in 1943 & Unique thermometer in 1942
Bimetallic Switch: Applications Automatic Coffeepot: Russell Hobbs in 1952; bimetallic strip stops device when coffee is perked. Waffle irons: Manning-Bowman Twin-O-Matic with thermostat & thermometer using bimetallic strips in 1939 by Alan M. Young. Aneroid barometer (non-mercury): 1 st idea by Leibniz in 1700; Lucien Vidie in Hair dryer safety cut-off switch (1 st dryers 1925; safety 1970s) Circuit breakers: strip bends to open circuit; 1904
References 1. Howstuffworks.com/home-thermostat.htm 2. virginia.edu/supplements/thermometers_and_thermostats.pdf; John Wiley & Sons, Section 6.6 Thermometers & Thermostats, En.wikipedia.org/wiki/bimetallic_strip; John_Harrison 4. William J.H. Andrewes, The Quest for Longitude (Cambridge, MA: Collection of Scientific Historical Instruments, Harvard University, 1996). 5. Dava Sobel & William J.H. Andrewes, The Illustrated Longitude (New York: Walker and Co., 1995). 6. Aneroid-Barometer.html; Toaster.html Trove Reference & Education: Science in Scotland: findarticles.com/p/articles/mi_rlg4413/is_SLGG0003/ai_n /europastar/watch_tech/nicolet6.jsp 12. eo.ucar.edu/skymath/tmp2.html hyperphysics.phy-astr.gsu.edu/hbase/thermo/bimet.html 17. waftrip.html www4.tsl.uu.se/~Atlas/DCS/DCSIL/therm.html