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Reading a Machine TAS Summer 2006 Carol Wenk Email: cwenk@westfieldnjk12

Reading a Machine TAS Summer 2006 Carol Wenk Email: cwenk@westfieldnjk12.org. 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

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Reading a Machine TAS Summer 2006 Carol Wenk Email: cwenk@westfieldnjk12

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  1. Reading a MachineTAS Summer 2006Carol WenkEmail: cwenk@westfieldnjk12.org

  2. 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.

  3. Thermostat: How does it work? • Temperaturelever 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.

  4. Mercury Use • Hg barometer: Toricelli 1643 • Hg thermometer: Fahrenheit 1714 • Hollerith tabulating machine & sorter 1890 Advantages • Liquid metal (-39 to 357oC) • Conducts electricity • Uniform thermal expansion • Visible

  5. 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.

  6. 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

  7. Bimetallic Switch: History • Invented by English carpenter & clockmaker John Harrison in 18th century (1739-1749). • 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.

  8. Harrison’s Temperature Compensation Gridiron Pendulum H1 H2 H3

  9. 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.

  10. Bimetallic Switch: Applications Toasters: • 1st 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: • 1st 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

  11. 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): 1st idea by Leibniz in 1700; Lucien Vidie in 1843. • Hair dryer safety cut-off switch (1st dryers 1925; safety 1970s) • Circuit breakers: strip bends to open circuit; 1904

  12. References • Howstuffworks.com/home-thermostat.htm • virginia.edu/supplements/thermometers_and_thermostats.pdf; John Wiley & Sons, Section 6.6 Thermometers & Thermostats, 2001. • En.wikipedia.org/wiki/bimetallic_strip; John_Harrison • William J.H. Andrewes, The Quest for Longitude (Cambridge, MA: Collection of Scientific Historical Instruments, Harvard University, 1996). • Dava Sobel & William J.H. Andrewes, The Illustrated Longitude (New York: Walker and Co., 1995). • www.madehow.com/Volume-7/Hair-Dryer.html; Aneroid-Barometer.html; Toaster.html • www.madehow.com/Volume-4/Mercury.html • www.bookrags.com/sciences/sciencehistory/thermostat-woi.html • Trove Reference & Education: Science in Scotland:findarticles.com/p/articles/mi_rlg4413/is_SLGG0003/ai_n15607125 • www.greatachievements.org/?id=3768 • 199.249.170.176/europastar/watch_tech/nicolet6.jsp • eo.ucar.edu/skymath/tmp2.html • www.americanheritage.com/articles/magazine/it/2005/2/2005_2_6.shtml • www.nmm.ac.uk/server/show/conWebDoc.355/setPaginate/No • www.ce.berkeley.edu/Courses/E77/lecturenotes/1intro.pdf • hyperphysics.phy-astr.gsu.edu/hbase/thermo/bimet.html • www.jitterbuzz.com/indirn.html; waftrip.html • www.madehow.com/Volume-6/Clothes-Iron.html • www.columbia.edu/acis/history/census-process.jpg • www.facstaff.bucknell.edu/mastascu/elessonshtml/Sensors/TempThermCpl.html • www4.tsl.uu.se/~Atlas/DCS/DCSIL/therm.html

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