Reading a Machine TAS Summer 2006 Carol Wenk

Reading a Machine TAS Summer 2006 Carol Wenk Email: cwenk@westfieldnjk 12. 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 · 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 Advantages • Liquid metal (-39 to 357 o. C) • Conducts electricity • Uniform thermal expansion • Visible

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 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 H 1 H 2 H 3

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 1920 s use bimetallic strips · Sunbeam in 1940 s 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 1843. · Hair dryer safety cut-off switch (1 st dryers 1925; safety 1970 s) · Circuit breakers: strip bends to open circuit; 1904

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 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_rlg 4413/is_SLGG 0003/ai_n 15607125 www. greatachievements. org/? id=3768 199. 249. 170. 176/europastar/watch_tech/nicolet 6. jsp eo. ucar. edu/skymath/tmp 2. html www. americanheritage. com/articles/magazine/it/2005/2/2005_2_6. shtml www. nmm. ac. uk/server/show/con. Web. Doc. 355/set. Paginate/No www. ce. berkeley. edu/Courses/E 77/lecturenotes/1 intro. 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/Temp. Therm. Cpl. html www 4. tsl. uu. se/~Atlas/DCSIL/therm. html