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
 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 357oC)
•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 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: 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
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
References
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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_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