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Reading a Machine TAS Summer 2006 Carol Wenk

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1 Reading a Machine TAS Summer 2006 Carol Wenk

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?  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.

4 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

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 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.

8 Harrison’s Temperature Compensation Gridiron Pendulum H1H2H3

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:  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

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): 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

12 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


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