Structure of Clinical Thermometer

Tina’s pet dog Harry was not feeling well.

Tina took Harry to a veterinary hospital.

There the vet took an instrument and checked Harry’s body with it and said that Harry's temperature was normal. It was just a food issue and gave some tablets.

Tina is a 7th grader, she got curious as to how that instrument worked and asked the doctor, "What is that thing you checked Harry's temperature with? How does that work?"

The doctor said it's called a "thermometer" or more accurately "a clinical thermometer" and he started to explain about it.

Clinical Thermometer

A clinical thermometer is generally used in hospitals to measure human as well as animal’s body temperature.

The bulb at the bottom has mercury in it. There is kink just above the bulb.

There is a capillary tube just above the kink, into which the mercury can move.

Mercury has a property of expanding and depending on the heat, it will expand. The more the heat, the more the expansion. The thermometer is calibrated accordingly to show us the temperature.

There are a few different scales of temperature based on the scientist who proposed it.

Let us now study the scales of temperature.

There are three types of scales on a clinical thermometer.

In clinical thermometer, the range of temperature is from 35℃ to 42℃ as it is the normal range of human temperature.

In the Fahrenheit scale, the temperature of the thermometer ranges from 94℉ to 108℉.

The normal temperature of the human body in Fahrenheit is 98.6℉ and in Celsius scale, it is 37°C

How to use a clinical thermometer safely.

Before using, the thermometer should be cleaned with hot water or spirit and shaken so that all the mercury is at the bottom.

We have to make sure that the level of mercury is below 37℃ or 98℉, the normal human body temperature, before using.

It is then kept under the tongue for adults or in the armpits for babies to get the readings on the thermometer.

The level of the mercury increases slowly and stops at a reading, that reading gives the temperature of the body.

A clinical thermometer should be used with care as it breaks easily. If broken, the mercury may leak. It is not useable again.

Since mercury is a toxic element, it can cause mercury poisoning.

To avoid this, nowadays, digital thermometers are used which overcome these drawbacks.

We also use the more advanced infrared thermometers which are even better than the digital thermometer.

Now, let's revise what we studied so far.


A clinical thermometer is generally used in hospitals to measure human as well as animal’s body temperature.

It has a bulb with mercury, a kink to avoid rapid fall of mercury which gives us time to record the measurement. The mercury rises into the capillary tube. The tube is calibrated.

Before using a thermometer, we need to bring the mercury level to the normal temperature by shaking it. We also need to clean it with spirit or hot water.

The End