Reading a MachineTAS Summer 2006

Carol WenkEmail: 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 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 357oC)•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 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.

Harrison’s Temperature Compensation

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. 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: ApplicationsToasters: 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

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

References1. Howstuffworks.com/home-thermostat.htm2. virginia.edu/supplements/thermometers_and_thermostats.pdf; John Wiley & Sons, Section 6.6

Thermometers & Thermostats, 2001.3. En.wikipedia.org/wiki/bimetallic_strip; John_Harrison4. 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. www.madehow.com/Volume-7/Hair-Dryer.html; Aneroid-Barometer.html; Toaster.html7. www.madehow.com/Volume-4/Mercury.html8. www.bookrags.com/sciences/sciencehistory/thermostat-woi.html9. Trove Reference & Education: Science in Scotland:

findarticles.com/p/articles/mi_rlg4413/is_SLGG0003/ai_n1560712510. www.greatachievements.org/?id=376811. 199.249.170.176/europastar/watch_tech/nicolet6.jsp12. eo.ucar.edu/skymath/tmp2.html13. www.americanheritage.com/articles/magazine/it/2005/2/2005_2_6.shtml14. www.nmm.ac.uk/server/show/conWebDoc.355/setPaginate/No15. www.ce.berkeley.edu/Courses/E77/lecturenotes/1intro.pdf16. hyperphysics.phy-astr.gsu.edu/hbase/thermo/bimet.html17. www.jitterbuzz.com/indirn.html; waftrip.html18. www.madehow.com/Volume-6/Clothes-Iron.html19. www.columbia.edu/acis/history/census-process.jpg20. www.facstaff.bucknell.edu/mastascu/elessonshtml/Sensors/TempThermCpl.html21. www4.tsl.uu.se/~Atlas/DCS/DCSIL/therm.html