Thermometer: Clinical & Laboratory Thermometer

Temperature and Thermometer

Temperature: The temperature of a body is basically a measurement of the degree of hotness or coldness. The molecules of a gas or liquid or a solid are in constant motion. This motion of molecules constitutes the kinetic energy of each molecule. As a matter of fact, the temperature is nothing but the average kinetic energy of all the molecules present in the body. Greater is the kinetic energy, higher is the temperature. In addition, the temperature is one of the seven standard physical quantities which cannot be derived from any other quantity. There are three main scales known as Celsius (0C), Fahrenheit (0F) and Kelvin (K) on which the temperature is to be measured.

Thermometer: A thermometer is a tool by which temperature or the temperature difference of an object is measured. Thermo means hot; hence thermometer is a device that measures the degree of hotness or coldness.

Note: Coldness is nothing but the absence of hotness. 

Santorio, who was an Italian physician, introduced the first thermometer ever. However, the standardization of the thermometer took place later. Advancement has been made not only in the design but also in the materials used in thermometer which eventually enhances the sensitivity of the device. There are a plethora of uses of the thermometer in the field of medical science, meteorology, monitoring thermodynamic or chemical or various physical processes, food technology, engineering applications and many more. In today’s time, numerous thermometers, for instance, thermistor, thermocouple, passive infrared thermometers, gas thermometers, Manometric thermometers have been discovered. But, there are chiefly two thermometers : 

1.Clinical thermometer

2.Laboratory Thermometer

Clinical Thermometer

Clinical thermometer is also known as a medical or doctor’s thermometer. The first clinical thermometer was invented by Thomas Clifford Allbutt in 1866. As the name suggests, Clinical thermometers are used for clinical purposes, mainly to measure the human body temperature by the doctors. The early clinical thermometers used to be large in size and took a longer time to measure the temperature. Over the centuries, clinical thermometers have been developed in such a way that now they are not only available in smaller sizes but also with higher sensitivity to measure the temperature quickly. During the 20th century, clinical thermometers with a glass tube filled with mercury were predominant. Nowadays, digital thermometers, which are portable and easy to use, have become popular and ubiquitous. The small range of the temperature 35oC- 42oC (or 95 oF-108 oF) could be read using a clinical thermometer. Generally, both the scales Celsius and Fahrenheit are present on the thermometer.

Design of the clinical thermometer: Here we discuss a clinical thermometer which comprises of a glass tube with the scale inscribed on it as shown in Figure 1. One of the tube ends is filled with liquid mercury which is of red colour. An extremely thin capillary tube runs through the middle of the glass tube. There is a small kink capillary thread near the neck of the bulb.

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Figure 1: Clinical thermometer with a mercury bulb and a capillary tube with a kink in it.

Working of the clinical thermometer: This thermometer must be kept clean and sterilized before taking the temperature of the patient. The thermometer is placed inside the mouth of the patient to measure the body temperature. As the temperature of mercury in the glass bulb rises up due to the heat of the body, mercury expands and it starts to flow through the thin capillary thread at a certain level. The new level of mercury indicates the body temperature. The reading of the temperature does not change even after removing the thermometer from the mouth because the kink in the capillary does not allow the mercury to flow back. Therefore, the new level of mercury does not change. To bring the mercury back in the bulb, the thermometer needs to be jerk or swing sharply a few times. This would allow the mercury back to the glass bulb.

Advantages of mercury: Mercury is the most commonly used material in different types of thermometers for various reasons. First, mercury is a metal which is a good conductor of heat and expands instantly as it starts to heat. Secondly, it remains in liquid form in a wide range of temperatures from -39 oC and 357 oC. Thirdly, it is non-adhesive to glass.

Disadvantage: Mercury is poisonous in nature. In case, the glass bulb breaks while taking the temperature, mercury can leak into the mouth. That is why electronic digital thermometers and scanner thermometers have gradually replaced the mercury thermometers.

How to read a Thermometer?

First, clean and sterilize the thermometer.

Swing or jerk the thermometer so that the mercury is brought back to the glass bulb. 

Note the temperature difference between two bigger marks on the scale of the thermometer. Let’s say, the difference is of 1 degree. If there are 10 divisions between the two consecutive bigger marks, then one smaller division will be equal to 0.1 oC.

The thermometer is to be placed under the tongue inside the mouth of the patient to obtain the temperature of the body.

When the reading is taken, the thermometer needs to be held vertical to the eyes and the eyes must be at level with the top of the mercury in the capillary thread.

Laboratory Thermometer


Laboratory thermometers cannot be used for clinical purposes because such thermometers read temperatures ranging from -10 oC to 110 oC, which is very high as compared to the human body temperature range. As the name suggests, these thermometers are used in the laboratory to check the boiling and the freezing points of any liquid/solvent or the temperature of any other object other than the human body. The basic laboratory thermometer is similar to the clinical thermometer except that the laboratory thermometer does not have any kink. Due to the absence of the kink, these laboratory thermometers are used in a continuous process and need to be monitored all the time to get accurate results. 

Moreover, the size of the laboratory thermometer is larger as it measures a wide range of temperatures. Also, these thermometers have a high resolution.