Temperature and its Measurement

What Is Temperature?
When your mother is cooking bread for you and you touch the bread, how does it feel? It feels hot. Again when you are eating an ice cream, how do you feel the ice cream? It feels cold. To describe how much hot or cold a thing is, scientists use the term "Temperature". A hot body is at high temperature and a cold body is at low temperature. Temperature of a body indicates how hot or cold the body is. We can guess the temperature of a body by touching it. But this is not an accurate and safe method for measuring temperature. In order to measure temperature accurately and safely at home or at school laboratory, we use an instrument called Thermometer

Thermometer is an instrument which measures the temperature accurately and safely.

Laboratory Thermometer 

Figure 6.1 shows a laboratory thermometer. A laboratory thermometer consists of a thick-walled glass tube having a narrow bore in it There is a glass bulb at lower end of the tube and its upper end is closed. The bulb is filled with alcohol or mercury. When bulb of the thermometer touches a hot body, the liquid in the bulb expands and rises up in the tube. In this way the thermometer shows a high temperature. But when the bulb of the thermometer touches a cold body the liquid contracts and comes down in the tube and the thermometer shows a low temperature

Temperature and its Measurement

Temperature Scales

The numbers written on the thermometer show the degree of hotness or coldness of a body. These numbers are called Temperature Scale. Now-a-days, two types of temperature scales are used. The most commonly used scale is the Celsius or Centigrade scale. The other scale is the Fahrenheit scale. We shall learn about these scales from the following activity.

Temperature and its Measurement

Clinical Thermometer

A clinical thermometer is used for measuring the temperature of the human body. Its glass tube is marked in centigrade scale from 35 to 42°C and in Fahrenheit scale from 95 to 110 °F (Figure 6.2).

A clinical thermometer has a small range. This is because the human body temperature cannot be below or above this range. A clinical thermometer has a small bending in the narrow bore just above the bulb. When we put the thermometer under the armpit of the patient, alcohol or mercury in the bulb rises up in the bore due to high temperature (Figure 6.3).

When we remove the thermometer from the armpit of the patient. liquid in the bulb contracts on cooling. The small bending in the bore prevents the fall of the liquid into the bulb. Therefore the liquid level does not change and we can get correct reading of body temperature. However, to use the thermometer again, we give it jerks to bring down the liquid back into the bulb.

Temperature and its Measurement

Take the following safety measures while using thermometers:

 1. Keep the thermometer upright while measuring the temperature.

2. Mercury is harmful to health, therefore, in case of breakage of a thermometer, do not touch it with hands.

3. Do not touch the bulb of a clinical thermometer with fingers. It should be cleaned with spirit and water before use on another patient.

4. After using a clinical thermometer, it should be cleaned, disinfected and placed in an appropriate container for storage.

5. Do not keep a clinical thermometer below 0°C and above 50°C.

6. Do not put the clinical thermometer into hot water. It may burst and injure the user or the person nearby