Working Principles of Thermometers

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Bimetallic Strip Thermometers

Measuring temperature accurately can be done with bulb thermometers, but controlling temperature is more difficult. On the other hand, bimetallic strip thermometers, which are made of metal, are useful in controlling temperature.

Bimetallic strip thermometers work based on the principle that different metals expand at different rates when they are heated. By bonding two different metals together, a simple electric controller that can withstand high temperatures can be made. Such controllers are commonly used in ovens. The following is the general layout:

The bimetallic strip is made up of two metals, which is why it is called so. In this diagram, the green metal is chosen to expand faster than the blue metal if the device is used in an oven. In a refrigerator, the opposite setup is used, so that as the temperature rises, the blue metal expands faster than the green metal. This causes the strip to bend upward, making contact, so that current can flow. By adjusting the size of the gap between the strip and the contact, temperature can be controlled.

Long bimetallic strips are often coiled into spirals. This is the typical layout of a backyard dial thermometer. By coiling a very long strip, it becomes much more sensitive to small temperature changes. In a furnace thermostat, the same technique is used, and a mercury switch is attached to the coil. The switch turns the furnace on and off.


1. What is a thermometer?

A thermometer is an instrument used to measure temperature. It typically consists of a glass or plastic tube containing a liquid, such as mercury or alcohol, that expands or contracts as it is heated or cooled. The temperature is read by observing the level of the liquid in the tube against a scale marked on the side.

2. How does a mercury thermometer work?

A mercury thermometer uses the expansion and contraction of mercury to measure temperature. Mercury is a liquid metal that expands when heated and contracts when cooled. The mercury is contained in a glass tube with a narrow bore and a bulb at one end. As the temperature changes, the mercury expands or contracts, causing it to move up or down the tube. The temperature is read by observing the level of the mercury against a scale marked on the side of the tube.

3. What is a digital thermometer?

A digital thermometer is an electronic device that measures temperature using a sensor and displays the result on a digital screen. The sensor can be a thermistor, which changes resistance with temperature, or a thermocouple, which generates a voltage proportional to temperature. The temperature is displayed on a digital screen in degrees Celsius or Fahrenheit.

4. How accurate are thermometers?

The accuracy of a thermometer depends on the type of thermometer and the quality of its construction. Mercury thermometers are typically accurate to within 1 degree Celsius or Fahrenheit. Digital thermometers can be more accurate, with some models claiming an accuracy of 0.1 degrees Celsius or Fahrenheit. However, the accuracy can be affected by factors such as the calibration of the thermometer and the environment in which it is used.

5. How do you calibrate a thermometer?

To calibrate a thermometer, you need to compare it to a known standard. For example, you can place a mercury thermometer in a mixture of ice and water, which should give a reading of 0 degrees Celsius or 32 degrees Fahrenheit. If the thermometer reads higher or lower than this, you can adjust it by moving the scale or the bulb. Digital thermometers can often be calibrated by pressing a button or following a calibration procedure in the user manual.

6. What are some common uses of thermometers?

Thermometers are used in many different applications, from measuring body temperature to monitoring the temperature of industrial processes. Some common uses of thermometers include checking the temperature of food, testing the temperature of water in swimming pools, and measuring the temperature of the air in a room. In medical settings, thermometers are used to measure body temperature, which can be an indication of illness or infection.

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