A thermometer is an instrument used for measuring the temperature of our body or atmosphere. The first thermometer was produced by the Italian scientist, Galileo Galilei. Thermometers help in regulating chemical reactions by controlling temperatures of the solutions. They are used to measure the melting points of different solids, and boiling points of liquids.
The main types of thermometers are: I. Liquid-in-glass thermometers. II. Bimetallic strip thermometers. III. Electrical thermometers. IV. Gas thermometers.
Liquid-in-glass thermometers: The most common liquid-in-glass thermometer makes use of mercury or alcohol as thermometric liquid. The thermometer is made up of a glass tube with a narrow bore through it. At the bottom of the glass tube, a small bulb is blown, in which the liquid mercury or alcohol is kept. It is then put in a hot bath, as a result of which some of the liquid is expelled. The thermometer’s range is decided by the temperature of the bath. Finally its upper end is sealed.
The sealed glass tube is now put in ice to mark the lower fixed point. This indicates the minimum temperature for the thermometer. Then it is put in another hot bath to ascertain the maximum temperature. The distance between the lower fixed point and the upper fixed point is divided into equal parts. When we wish to measure our body temperature, the thermometer is put into contact with the body. The thermometric liquid expands and stops when the temperature of the bulb becomes equal to the temperature of the body. The temperature is then read from the upper point of the liquid in the capillary.
Clinical thermometers also contain mercury. Meteorologists use ‘maximum’ and ‘minimum’ thermometers to record the highest and lowest temperatures of the day. They contain both mercury and alcohol.
Bimetallic strip thermometers: A bimetallic strip thermometer consists of a strip of two different metals having different co-efficients of expansion. This means that different metals expand unequally at the same temperature. The two metals used 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. 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 thermographs. A thermograph 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 which is known as a thermogram.
Electrical thermometers: These are of two types: Resistance thermometers and Thermocouple thermometers.
A resistance thermometer works on the principle that the resistance of metals increase with the rise in temperature. The 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 ends of the wires is kept in ice, and the other with the source whose temperature is to be measured. The thermometer 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 which then shows the current.
Gas thermometers: These are the most accurate thermometers, which also have the widest range (from -273°C to about 1500°C). In these thermometers, gases like Nitrogen, Hydrogen, and Helium etc. are used. By recording the expansion of gas on heating, the temperature is measured. These thermometers are of two types (a) constant volume thermometers and (b) constant pressure thermometers.
There are four scales of temperature (I) Celsius scale (II) Fahrenheit scale (III) Reaumur scale and (IV) 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 0° as the freezing point of water and 80° as 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 be achieved. This is also called absolute zero.