When you touch tea, you find that it’s hot. Similarly, when you touch ice, you find that it’s cold. But, how do you find out how hot or cold a substance is? The physical quantity that tells us how hot or cold a substance is, is known as Temperature. Temperature can be defined as-
‘The degree of hotness or coolness of an object is known as its temperature’.
So, how do we find out the specific temperature of a substance? We can measure the temperature of an object by using a device known as a Thermometer.
The first ever thermometer was invented by the prolific inventor, Galileo, way back in 1593 AD. Galileo used air in his thermometer to measure temperature, because air expands on heating and contracts on cooling. However, this thermometer was found to be very slow in measuring temperatures, and could not be used in complex experiments. Additionally, air pressure interfered with the measurements, leading to problems in accuracy.
The first modern thermometer, the mercury thermometer with a standardized scale, was invented by a German instrument maker, Daniel Gabriel Fahrenheit, in 1714. Before that, he also invented the alcohol thermometer in 1709. Both materials are still in use in clinical and laboratory thermometers even today.
Celsius and Fahrenheit are the two temperature scales commonly used around the world. In India, we use the Celsius scale to measure the temperature of the atmosphere, but use the Fahrenheit scale to measure body temperature.
The fixed temperatures marked on the stem of a thermometer by the common consent of scientists are called standard points or fixed points. Two fixed points are chosen in order to mark the thermometer, one for the lowest temperature and one for the highest temperature. All the markings in between these two points are known as degrees.
In the Celsius scale, the melting point of ice at atmospheric pressure is taken as the lower standard point. Its value is taken as 0
The boiling point of pure water at atmospheric pressure is taken as the upper standard point. Its value is taken as 100 .
The thermometer is divided into 100 equal markings or degrees between the two standard points.
Mercury as a thermometric liquid-
The liquid used in thermometers is known as thermometric liquids. Mercury makes a very good thermometric liquid, due to the following reasons-
- Mercury expands with a very small amount of heat. Thus, it can measure even small changes in temperature.
- It has a high boiling point of 357. Thus, it can record temperatures up to 357 .
- Mercury has a very low freezing point of – 39. It can thus measure temperatures as low as – 39 .
- Mercury is a non-sticky material, and does not stick to the sides of the glass tube of the thermometer. It can therefore give accurate temperature readings.
- Mercury is a shining and opaque material. It can thus be seen clearly even in the form of a fine thread through the glass of the thermometer.
In special situations, alcohol thermometers are used to measure temperatures instead of mercury ones. Alcohol has a freezing point lower than – 100 . Thus it can be used to measure ultra-low temperatures, such as in regions like the Arctic and Antarctic polar ice caps.