A thermometer is an instrument used to measure the temperature. It can measure the temperature of a body or even of air or atmosphere. It helps in regulating chemical reactions by controlling temperatures of the solutions. It also used to measure the melting points of different solids and boiling points of liquids.
There are several types of thermometers. The main types are: 1. Liquid-in-glass thermometers. 2. Bimetallic strip thermometers. 3. Electrical thermometers. 4. Gas thermometers.
The descriptions of each type of thermometers are below:
i. Liquid-in-glass thermometers:
The most common liquid-in-glass thermometers make use of mercury or alcohol as thermometric liquid. These thermometers work on the principle that liquids expand on heating. A thermometer of this type made up of a glass tube with a narrow bore through it. At the bottom of the glass tube, a small bulb has blown which works as the reservoir of the liquid. The glass tube has filled with mercury or alcohol. It then puts in a hot bath. Some amount of the. The liquid will expel from the tube. The thermometer's range has decided by the temperature of the bath. Finally, its upper end is sealed. The sealed glass tube now puts on ice to mark the lower fixed point. Then it put in another hot bath to mark the upper fixed point, which indicates the maximum temperature for which the thermometer has been constructed. The distance between the lower fixed point and upper fixed point divided into equal parts. When we wish to measure the temperature, the thermometer puts in contact with that body. When it comes into the contact, the liquid expands and stops when the temperature of the bulb becomes equal to the temperature of the body whose temperature has measured. The temperature then reads from the upper point of the liquid.
Mercury as a thermometric liquid, has certain advantages. It is a good conductor of heat. It does not stick to the walls of the glass. It is bright and easily visible. Its freezing point and boiling point have sufficient range of temperature and hence can be used to make thermometers of wide range freezing point-39°C and boiling point 357°C. On the other hand, although alcohol doesn't have so many advantages it can also be used as a thermometric liquid. For a given temperature, it expands more than mercury. While using it in thermometers it is usually dyed red or blue.
The ordinary thermometers used in chemical laboratories are usually mercury thermometers. Clinical thermometers also contain mercury. Meteorologists use 'maximum' and 'minimum' thermometers to record the highest and lowest temperatures of the day. It contains both mercury and alcohol
ii. Bimetallic strip thermometers:
A bimetallic strip thermometer consists of a strip of two different metal's having different co-efficient of expansion. The two metals are usually brass and Invar. Brass is an alloy of copper and zinc, while Invar is an alloy of iron and nickel. The two strips are joined together. When the temperature changes, the two metals expand and contract at different rates because of different co-efficient of expansion. This causes the strip to bend. The strip is attached to a pointer, which indicates the temperature.
Bimetallic strip thermometers are used in refrigerators for temperature control. They are also used in thermometer. A the thermometer records a graph of temperature. Instead of a pointer, a pen is attached to the bimetallic strip, which records the temperature on a moving chart. The chart of the temperature is called a thermo gram.
iii. Electrical thermometers:
There are two common forms of electrical thermometers: 1. resistance thermometer and 2. thermocouple thermometers.
A resistance thermometer works on the principle that the resistances of metals increase with the rise in temperature. A resistance thermometer consists of a sealed tube containing tightly-coiled platinum wire. As the temperature increases, the resistance of the platinum wire also increases. By measuring the resistance, the temperature is determined.
A thermocouple thermometer consists of wires of two different metals. One of the junctions of the wires is kept in ice and the other with the source whose temperature is to be measured. Thermocouple changes heat into electricity and produces a voltage known as thermo e.m.f.. The voltage varies according to temperature. The voltage is applied to a galvanometer needle and the galvanometer needle shows the current which is calibrated in terms of temperature.
iv. Gas thermometers:
In these thermometers, gases like air, nitrogen, hydrogen, and helium are used as thermometric substances. These thermometers are of two types: 1. constant volume thermometers and 2. constant pressure thermometers. They work on the principle that gases expand on heating. By recording the expansion of gas, the temperature is measured.
Now the question arises, what are the different scales of temperature during use?
There are four scales of temperature 1. Celsius scale 2. Fahrenheit scale 3. Reaumur scale and 4. Absolute scale of temperature. The Celsius or Centigrade scale has 0° as the freezing point of water and 100° as the boiling point of it. Fahrenheit scale has 32° as the freezing point of water and 212° the boiling point. Reaumur scale has the 0° as the freezing point of water and 80° the boiling point. Absolute scale has 273°as the freezing point of water and 373° as the boiling point. -273° Celsius is the lowest temperature which has not yet been achieved. This is also called absolute zero.