MEASUREMENT AND CONTROL OF TEMPERATURE 



in the temperature of the thermometer as such, but of the object in contact 

 with the thermometer, a thermometer must equiUbrate to the temperature 

 of its surroundings rapidly — which usually means it must be a good con- 

 ductor of heat — and in that process it must not change the heat content of 

 its surrounds appreciably or the resultant temperature measured will not 

 be that prevailing before the thermometer was introduced. 



Temperature scales 



It is known that water in contact with melting ice, under stated conditions, 

 has a constant and repeatedly obtainable temperature. The thermal exchange 

 of equilibrating a thermometer with an ice-water mixture involves only 

 a change in the amount of ice — not of the temperature. Hence this is used 

 for reference purposes as the 'lower fixed point' of temperature. Similarly, 

 a 'higher fixed point' is obtained from water boiling at atmospheric pressure. 

 On the Centigrade scale, these are the 0° and 100° points; on the Fahrenheit 

 scale they are called 32° and 212°. Once these two points have been calibrated 

 on the instrument whose temperature-dependent property is being used as 

 a thermometer, the intervening change of property is divided into equally 

 spaced intervals: on the Centigrade scale into 100 parts, on the Fahrenheit 

 into 180. No account is taken of whether the addition of equal quantities 

 of heat produces successively similar changes of property — for example, 

 the mark 50°C on the mercury-in-glass thermometer is half way between 

 the and 100° marks, on the stem; 50°C on the platinum resistance ther- 

 mometer is half way in resistance between that at and 100°C. However 

 unless the laws governing the change of volume, and of resistance, with 

 successively added quantities of heat are identical, 50°C will not be the same 

 temperature on both scales — and in fact, readings taken in the same medium 

 around 50°C do vary between the two, by almost one degree. Whenever 

 specifying temperature to an accuracy greater than 0-5°C it is necessary to 

 state the measuring element used, and this is especially important if the 

 measurement was made on an instrument other than a mercury thermometer, 

 or one not calibrated against a mercury thermometer. 



Heat quantity 



The thermal capacity of a body is the quantity of heat needed to raise 

 its temperature through one degree. For unit mass of material the figure 

 is called the specific heat of the material : thus the product of specific heat 

 and mass gives the thermal capacity. Specific heats vary widely — that of 

 water is highest, while metals are commonly low. With a heterogeneous 

 collection of objects, such as might constitute a biological environment, 

 either in water, or more particularly air, it must be appreciated that in 

 raising the temperature of the whole collection the added heat energy has 

 to be distributed very unevenly according to the specific heats of the objects. 



Conduction and insulation; convection and radiation 



The thermal conductivity of a material gives the rate at which heat flows 

 through it for a unit temperature gradient across it. Generally speaking, 

 good thermal conductors are also good electrical conductors, while elec- 

 trical insulators are also poor heat conductors, and thus difficult to heat, 



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