Bi metallic Mechanical Thermometer and Applications

By using the bi metallic mechanical thermometer we can convert the change of temperature in to useful mechanical displacement. We can see the two strips and they are made of two different metals. When they are heated the metals expand at different portions. Regularly they use copper and steel , in some cases they prefer brass and steel. Throughout the length the strip joints are joined with the help of welding, riveting or brazing.

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The dissimilar expansion forces the flat strip to twist one way if heated. Below the initial temperature is cooled in the opposite direction. The outer side of the curve is the metal which is the greater coefficient of thermal expansion, and when the strip is heated the inner inside of the tube is cooled. The sideways movement of the strip is greater than the low lengthways enlargement in both of the two metals. These types of results are used in the electrical devices and mechanical devices. In the flat from in some applications the bimetal strip is used. In some cases it is covered with a coil for firmness. The greater measurement of the coiled version gives improved sensitivity.

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We can see the applications of the bimetallic strips in the clocks, thermostats, thermometers, electrical devices and heat engine.


In clocks the mechanism is very sensitive to the change in temperature. It leads to the errors in time observance. In some mechanisms bimetallic strip is used to compensate the errors. In bimetallic construction the common method used for the circular rim is balance wheel. With increasing the temperature, the spring adjusting the balance becomes weaker. To keep the periodic oscillations the balance becomes smaller in the diameter.


By using the thermostats we can operate different types of temperatures by regulating the cooling and heating. In this process one end of the bimetallic strip is fixed and the other end is attached to electrical power source.  Moving the strip helps to carry the electrical contact. By using the regulating lever or knob another contact of the thermostats is positioned. The regulated temperature position is known as set point.

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Now thermostat with bimetal coil:

In some thermostats they use mercury switch and it is connected to both the electrical leads. The angle is adjusted to control the set point of the thermostat. The application usage depends on the contact, if it is an open contact at the higher temperatures or it is a closed contact.

The power must be controlled directly with the help of the electrical control. It is also controlled indirectly with the help of the switch through a relay or fuel oil or supply of natural gas through an electrically operated valve. In natural gas heaters the control may be delivered with a thermocouple and the thermocouple is heated by the pilot light.


A dial thermometer with direct indicating uses a bimetallic strip covered with a coil. To housing of the device on one end of the coil is fixed and the other end acts as a needle. It is also known as recording thermometer. In Breguets thermometer it consists of a tri – metallic helix.

Heat engines:

To drive heat engines uncomplicated toys are constructed to explain the principles of usage.

Electrical devices:

Bimetal strips are used in small circuit breakers. They are used to protect the circuits from the excess of current. A bimetal strip is heated with the help of the coil of wire. The wire coils bends and controls the linkages that open a spring operated contact. It disturbs the circuit and it can rest when the bimetal strip is cooled down.

In time delay relays fluorescent lamp starter and for lamp flasher , the bimetal strips are used. In some devices the current flows directly into the bimetal strip and the heat is insufficient. Thus, It operates the strip directly.

Breguets thermometer

Breguets thermometer is also known as spiral thermometer. It is one of the types of the thermometer. It uses the expansion of metals lower than heat to produce a measurement more sensitive and with higher range. The cases are used for both the air thermometer and mercury thermometer. The working principle of the bimetallic strip consists of very lean strip of platinum soldered to a similar strip of silver between a slip of gold soldered. The strips are bent in to a helix. The upper edge of the helix is joined to a metallic support. The lower edge is connected to an index and projects over an advance circle. If the temperature is rises then we can see the expansion of the silver. In platinum we can see twice the expansion, gold exits as well. Moving the index the temperature’s rise or fall will cause an analogous twist in the spiral. Sudden starts are used to stop the slip of gold.

Coulomb blockade thermometer

Coulomb blockade thermometer is abbreviated as CB. It is used to measure the primary temperature. The temperature must range from 10mk to 0.2 k. It must be having the capability of tolerating high magnetic fields. For measuring and the calibration of the temperature low noise preamplifiers are used and the data is saved in the files .Without using any preamplifier and graphical software we can measure the temperature.

Advantages of Coulomb Blockade Thermometer:

  • External calibration is not required in the primary thermometer.
  • Secondary mode is used for the fast measurement of temperature and zero bias resistance.
  • It is insensitive the magnetic field.
  • Using lock in amplifier simple measurements are done.

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