Mercury-in-Glass Thermometers

Mercury-in-Glass Thermometers

We limit ourselves to the study of mercury-in-glass thermometers. Construction: We take a glass tube of a uniform and fine bore called the capillary of the thermometer. The capillary tubing ends on a glass bulb. The bulb and the capillary tube are filled with mercury. After heating to drive out all the air, the capillary tube is scaled. As the liquid (mercury) cools and contracts, the space above it in the capillary tube becomes practically a vacuum. There are mainly three scales of temperature in which mercury is used as a thermometric substance. They are listed and described below:

(i) Celsius thermometer or Celsius scale of temperature

This scale the temperatures are measured with respect to one standard temperature called the “Triple

point” of water which is arbitrarily taken to be 273.16K (Note: Here K is pronounced as Kelvin). As an approximation, this temperature is taken as 273K. At this temperature all the three states of water (that is ice, water and water vapour) can coexist in equilibrium. This temperature is the zero of Celsius scale called 0°C. Here C stands for degree centigrade. This is the lower fixed point of the thermometer. The upper fixed point on this scale is the temperature of steam at one atmospheric pressure which is taken to be 100°C. The interval between these points is divided into 100 equal parts. Each part measures 1°C.

(ii) Kelvin Scale of Temperature

The lower fixed point of this thermometer is the triple point of water which 273K. The upper fixed point is the temperature of steam at one atmospheric pressure. This temperature is marked as 373K. The interval between these points is divided into 100 equal parts. Each part measures 1K (one Kelvin, not one degree Kelvin). An interval of 1°C is equal to the interval of 1K. The zero of this scale marked OK starts from —273°C. This is lowest temperature ever reached. This is the reason why this temperatures called absolute zero and the scale of temperature is also called absolute temperature scale or absolute scale of temperature.

(iii) Clinical Thermometer

This thermometer is used by doctors to find the temperature of a patient. It is also found nearly in every house and is used to know the temperature of an ailing member of the family. It is a modified form of an ordinary mercury thermometer. Since the normal temperature of a healthy person is about 37°C or 98.4°F, it has a short range of temperature from 35°C to 43°C or 95°F to 110°F. This thermometer has a constriction in its capillary tube just above the bulb. On putting the bulb of the thermometer into the patients mouth, the temperature rises. The mercury in the bulb expands and passes through the constriction into the capillary tube. On cooling the mercury in the bulb contracts, but the mercury in the tube does not fall back to the bulb because the constriction prevents it from doing so. Thus the temperature of the patient as shown by the thermometer can be read at leisure.