Class 7 Science Heat by CBSE Lectures

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Temperature and Its Measurement
Transfer of Heat
Seasons and Clothes
Temperature is a measure of the degree of hotness of a body. The SI unit of temperature is kelvin (K). But temperature is also measured in celcius (°C) or fahrenheit (°F) scales. The instrument used to measure temperature is called a thermometer.

Thermometer is a device which is used to measure temperature.

Thermometer is made of a long narrow glass tube; with a bulb at one end. The narrow tube appears as a continuous silver line; because it is filled with mercury. Mercury is a metal which is in liquid state at room temperature and it readily expands or contracts at the slightest change in temperature. Hence, mercury is used in thermometer.

Types of thermometer:

Laboratory Thermometer: Laboratory thermometer is used to measure the temperature. The scale of temperature is graduated generally from –10°C to 110°C over the glass tube. Each division of temperature scale is further divided into 10 parts to read fraction of temperature.

Clinical Thermometer

These thermometers are used to measure the temperature of the human body, at home, clinics and hospitals. All clinical thermometers have a kink that prevents the mercury from flowing back into the bulb when the thermometer is taken out of the patien’s mouth, so that the temperature can be noted conveniently. There are temperature scales on either side of the mercury thread, one in Celsius scale and the other in Fahrenheit scale. Since the Fahrenheit scale is more sensitive than the Celsius scale, body temperature is measured in °F only. A clinical thermometer indicates temperatures from a minimum of 35 °C or 94 °F to a maximum of 42 °C or 108 °F.

Precautions to be Followed While Using a Clinical Thermometer

The thermometer should be washed before and after use, preferably with an antiseptic solution.
Jerk the thermometer a few times to bring the level of the mercury down.
Before use, the mercury level should be below 35 °C or 94 °F.
Do not hold the thermometer by its bulb.
Keep the mercury level along your line of sight and then take the reading.
Handle the thermometer with care. If it hits against some hard object, it may break.
Do not place the thermometer in a hot flame or in the hot sun.

Laboratory Thermometers

Lab thermometers are used to measure the temperature in school and other laboratories for scientific research. They are also used in the industry as they can measure temperatures higher than what clinical thermometers can record. The stem and the bulb of a lab thermometer are longer when compared to that of a clinical thermometer and there is no kink in the lab thermometer. A laboratory thermometer has only the Celsius scale ranging from −10 °C to 110 °C.

Precautions to be Followed While Using a Laboratory Thermometer

Do not tilt the thermometer while measuring the temperature. Place it upright.
Note the reading only when the bulb has been surrounded by the substance from all sides.

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