Thermometers

Watch the mercury rising!:

A thermometer is an instrument that is used to measure temperature. The most commonly used thermometer is the mercury-in-glass type, which consists of a uniform-diameter glass tube that opens into a mercury-filled bulb at one end.

This is sealed to preserve a partial vacuum in the tube. If the temperature increases, the mercury expands and rises in the capillary (very thin tube or vessel). The temperature may then be read on an adjacent (nearby) scale.

Mercury is widely used for measuring ordinary temperatures; alcohol, ether and other liquids are also used for this purpose. The thermometer was invented by Galileo Galili.

The modern alcohol and mercury thermometers were invented by the German physicist Gabriel Fahrenheit, who also proposed the first widely adopted temperature scale, named after him, in which 32 degrees Fahrenheit is the freezing point of water, and 212 degrees Fahrenheit is its boiling point at standard atmospheric pressure.

Various temperature scales have been proposed since his time; in the Centigrade or Celsius scale, devised by the Swedish astronomer Anders Celsius and used in most parts of the world. The freezing point is 0 degrees Celsius (Centigrade) and the boiling point is 100 degrees Celsius (Centigrade).

Types of thermometers

A wide variety of devices is employed as thermometers. The primary requirement is that one easily measured property, such as the length of the mercury column, should change noticeably and predictably with changes in temperature.

The variation of that property should also remain linear (in line) with variations in temperature. In other words, a unit change in temperature should lead to a unit change in the property to be measured at all points of the scale.

The optical pyrometer is used to measure temperatures of solid objects at temperatures above 700 degrees C (about 1300 degrees F), where most other thermometers would melt.

At such high temperatures, solid objects radiate sufficient energy in the visual range to permit optical measurement by exploiting the so-called glow colour phenomenon (happening).

The colour at which hot objects glow changes from dull red through yellow to nearly white at about 1300 degrees C (about 2400 degrees F). The pyrometer contains a light bulb-type of filament controlled by a rheostat (dimmer switch) that is calibrated (measured) so that the colours at which the filament glows correspond to specific temperatures.

The temperature of a glowing object can be measured by viewing the object through the pyrometer and adjusting the rheostat until the filament blends into the image of the object. At this point, the temperatures of the filament and the object are equal and can be read from the calibrated rheostat.

Special-purpose thermometers

Thermometers may also be designed to register the maximum or minimum temperature attained.

A mercury-in-glass clinical thermometer, for example, is a maximum-reading instrument in which a trap in the capillary tube between the bulb and the bottom of the capillary permits the mercury to expand with increasing temperature, but prevents it from flowing back unless it is forced back by vigorous shaking.

Maximum temperatures reached during the operation of tools and machines may be estimated by special paint patches that change colour when certain temperatures are reached.

Accuracy of measurement

The accurate measurement of temperature depends on the establishment of thermal equilibrium (state of balance) between the thermometric device and its surroundings; that is, when at equilibrium, no heat is exchanged between the thermometer and the material it touches or material in its surrounding area.

A clinical thermometer, therefore, must be inserted long enough (more than one minute) to reach near equilibrium with the human body to yield an accurate reading. It should also be inserted deep enough, and have sufficient contact with the body, to indicate temperature accurately.

Any thermometer indicates only its own temperature, which may not agree with the actual temperature of the object to be measured. In measuring the air temperature outside a building, for example, if one thermometer is placed in the shade and one in the Sun, only a few centimetres away, the readings on the two instruments may be quite different, although the air temperature is the same.

The thermometer in the shade may lose heat by radiation to cold building walls. Its reading, therefore, will be slightly below the true air temperature. On the other hand, the thermometer placed in the Sun will absorb the Sun's radiant heat.

As a result, the indicated temperature may be significantly above the true air temperature. To avoid such errors, accurate temperature determinations require the shielding of the thermometer from hot and cold sources or from which heat might be transferred by radiation, conduction or convection (explained in earlier artiles on this page).

Make your own thermometer

What you need

* A plastic bottle weighing about 16 ounces

* Warm water

* Food colouring

* A clear plastic straw

* Clay

* A bowl of ice

* A marker pen

Method

1. Fill the bottle to the very top with warm water and colour it with a few drops of food colouring.

2. Insert the straw into the bottle, upto about 1/3 of its length. Use clay to seal the bottle so that the straw sticks straight up. When you do this, you will see some water go up the straw.

3. Mark the water level in the straw so that you know where it was when you started.

4. Stick the bottle in the bowl of ice and watch what happens to the level of water in the straw.

Compiled by Janani Amarasekara