Bimetallic strip
The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature.
The invention of the bimetallic strip is generally credited to John Harrison, an eighteenth-century clockmaker who made it for his third marine chronometer (H3) of 1759 to compensate for temperature-induced changes in the balance spring.
[1] Harrison's invention is recognized in the memorial to him in Westminster Abbey, England.
The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature.
The sideways displacement of the strip is much larger than the small lengthways expansion in either of the two metals.
according the formula derived by French physicist Yvon Villarceau in 1863 in his research for improving the precision of clocks:[2] where
[3][4] The earliest surviving bimetallic strip was made by the eighteenth-century clockmaker John Harrison who is generally credited with its invention.
He made it for his third marine chronometer (H3) of 1759 to compensate for temperature-induced changes in the balance spring.
[5] It should not be confused with the bimetallic mechanism for correcting for thermal expansion in his gridiron pendulum.
His earliest examples had two individual metal strips joined by rivets but he also invented the later technique of directly fusing molten brass onto a steel substrate.
Harrison's invention is recognized in the memorial to him in Westminster Abbey, England.
The metals involved in a bimetallic strip can vary in composition so long as their thermal expansion coefficients differ.
Copper, steel, brass, iron, and nickel are commonly used metals in bimetallic strips.
The most common method is to use a bimetallic construction for the circular rim of the balance wheel.
As the spring controlling the balance becomes weaker with the increasing temperature, the balance becomes smaller in diameter to decrease the momentum of inertia and keep the period of oscillation (and hence timekeeping) constant.
Nowadays this system is not used anymore since the appearance of low temperature coefficient alloys like nivarox, parachrom and many others depending on each brand.
In the regulation of heating and cooling, thermostats that operate over a wide range of temperatures are used.
In adjustable thermostats another contact is positioned with a regulating knob or lever.
The angle of the entire mechanism is adjustable to control the set point of the thermostat.
In some natural gas heaters the power may be provided with a thermocouple that is heated by a pilot light (a small, continuously burning, flame).
In devices without pilot lights for ignition (as in most modern gas clothes dryers and some natural gas heaters and decorative fireplaces) the power for the contacts is provided by reduced household electrical power that operates a relay controlling an electronic ignitor, either a resistance heater or an electrically powered spark generating device.
Breguet's thermometer consists of a tri-metallic helix in order to have a more accurate result.
Moreover, the bimetallic strips cannot produce strength in its moves, the reason why is that in order to achieve reasonables bendings (movements) both metallic strips have to be thin to make the difference between the expansion noticeable.
A coil of wire is used to heat a bimetal strip, which bends and operates a linkage that unlatches a spring-operated contact.
Bimetal strips are also used in time-delay relays, gas oven safety valves, thermal flashers for older turn signal lamps, and fluorescent lamp starters.
