thermometer Any property of a substance that varies uniformly with temperature may be utilized as a basis of thermometer. Commonly used thermometers are liquid in glass tube, the platinum resistance thermometer, thermocouple thermometers * and constant volume gas thermometers.

Liquid in glass tube thermometers: Celsius devised a scale based on freezing point and boiling point of water at 1 atmosphere. The positions of a liquid (mercury or alcohol) in a scaled glass capillary tube at these two temperatures are marked, and distance between them is divided into equal intervals. Major limitation of the liquid thermometer is due to non uniformity of thermal expansion at different ranges. Another limitation is that the range of the thermometer is small, which is between the freezing point and boiling point of the liquid.

The constant volume gas thermometer: The thermometer is shown in fig. t4. A gas confined to a bulb is kept in a liquid bath whose temperature is to be measured. The change in the temperature of the gas alters its pressure. The volume of the gas is held constant by adjusting the reservoir level. The difference of height of mercury levels in tubes A and B gives the pressure of the gas. The pressure at the triple point * of water, p_t is determined. Pressure, p at any other temperature is related linearly to p_t, for an ideal gas,

p= P_t [T /273.16] (t4)

Temperatre, T can be found using this relation. Helium gas is nearest to an ideal gas, and can be used down to 1 K. Although it is cumbersome, it is accurate. It is used for calibration of other thermometers.

Platinum resistance thermometer: The property of variable electrical resistance with temperature in conductors is utilized in resistance thermometers Pure platinum wire forms one arm of Wheatstone bridge circuit. The balancing arm contains a variable resistance and potentiometer wire (see fig. t5), for accurate bridge balancing.

Since P=Q

or S + l₁r = R + l₂r

where r is the resistance per unit length of the potentiometer wire. After the platinum coil attains new temperature its resistance changes to R¢ and the balance shifts by x in the potentiometer.

\ S + ( l₁ + x) r = R¢ + (l₂ - x) r

The resistance variation with temperature occurs according to the approximate relation,

R = R₀ ( 1 + aT + bT² ) (t5)

where R is the temperature T, and R₀ is the temperature at 0^oC. a and b are constants. The calibration curve is obtained by taking observations at three different points which enables unknown temperature to be measured by the instrument.