Other thermometers

Experiments can be carried out to show that liquids tend to expand at different rates over different temperature ranges. There are exceptinos like merury which expands quite uniformly for a good range of temperatures.

Mercury thermometers.

The mercury thermometer is a common type of thermometer in everyday use. The mercury in the thermometer expands when heated. The expansion pushes a thread of the liquid out of the bulb and up the capillary tube.
The narrow bore of te capillary tube makes the thermometer more sensitive. This is because a small expansion of the mercury in the bulb will cause a big change in the length of the mercury thread. The bulb is made of thin glass so that heat can be conducted quickly to the liquid. The round glass stem acts as a mgnifying lens enabling the temperature to be read easily.

Alcohol thermometers.

This is a cheaper liquid in glass thermometer. The fact that alcohol is safer than mercury is often an important consideration. It can also meeasure a lower temperature than a mercury thermometer wheich is an advantage in cold countries.
However, alcohol has severe disadvantages including its irregular expansion over different ranges of temperature, it slow reactino to temperature changes and its transparncy and tendency to stick to glass which make it difficult to see its meniscus. A dye is often added to the alcohol to make it more visible. Fortunately, in science laboratories, alcohol expands rather linearly over the temperature range encountered.

Clinical Thermometers

A clinical thermometer is a type of maximum thermometer specially designed for measuring the temperature of the human body. It is an ordinary mercury thermometer with two modifcaitions:

1. It shows a short range in temperature from about 35- 42 degrees celsius, centred around the normal body temperature which is about 37 degrees celius
2. It ihas a constriction in the capillary tube just above the bulb of the thermometer.


The bulb of the thermometer is held gently under the patient's tongue. When the temperature rises, the mercury expands, forcing its way through the constriction and into the capillary tube. On cooling, the mercury in the blub contracts but the constriction prevents the mercury from falling back. This enables the patient's temperature to be read at leisure. The mercury thread is shaken into the bulb before the thermometer is used again.