Why is the bulb of the clinical thermometer long and cylindrical?

Question

My text book says that this bulb is a special feature of the clinical thermometer which differentiates it from the normal mercury thermometer found in weather stations. A question paper asks this question and I am unable to find it. Best of luck and look out for the ten points. :-)

Answer 1

A liquid-in-glass thermometer uses a column of liquid, usually mercury or alcohol, that is contained in long glass tube. The length of the liquid column changes with temperature. A scale is marked on the glass tube. In a common mercury thermometer, the scale is normally given 100 divisions . Zero on the scale corresponds to the melting point of ice and the 100 corresponds to the boiling point of water. This liquid-in -glass thermometer is therefore capable of measuring temperatures in the range zero to 100 degrees celsius.

The clinical thermometer is just a special type liquid-in-glass thermometer. However its use is for clinical purposes eg measuring body temperature. That is why it is called a clinical thermometer. The clinical thermometer uses mercury, but it is different from the common mercury thermometer in a number of ways. It is designed to read the range of body temperatures so its scale has a range of only a few degrees, usually from 35 to 42 degrees Celsius. Another difference is that it has a small bulb and a much narrower mercury column. This makes it very sensitive to small changes in temperature. The clinical thermometer also has a kink in the tube above the bulb. When the thermometer is removed from the body, the mercury starts to contract. The kink in the tube causes the mercury thread to break away from the mercury in the bulb. A column of mercury is left above the kink and this enables the temperature to be read accurately even when the thermometer is no longer in contact with the body. Another difference is that clinical thermometer has a bulge in the glass. It is more triangular in shape than the common mercy thermometer. The bulge acts as a lens, making it easier to see the narrow thread against the scale. I hope this helps.



Dictionary
clinical thermometer
n.
A thermometer used to measure body temperature, especially a small glass thermometer designed with a narrowing above the bulb so that the mercury column stays in position when the instrument is removed from the body.

Medical
clinical thermometer
n.
A thermometer having a graduated glass tube with a bulb containing a liquid, typically mercury or colored alcohol, that expands and rises in the tube as the temperature increases.

Answer 2

Bulb Thermometer

Answer 3

Cylindrical shape has high surface area and if the bulb is also made long, it is able to hold the volume of the thermometric liquid inside the thermometer when it rises very high due to heat.