MEASUREMENT OF TEMPERATURE 



but useful as insulators. Many heat 'insulators' do not however rely 

 entirely on the thermal conductivity of their material, but on cellular air 

 spaces and a restriction of convection. When fluid is heated locally it 

 expands in volume; the attendant decrease in density causes it to tend to 

 rise in the surrounding cooler material. Circulation of thermal origin set 

 up in this way is called convection: such convection speeds the heating of 

 a body of fluid appreciably, for clearly it takes place faster than the heat 

 energy can be transmitted through the fluid by conduction. Gases, such as 

 air, are extremely poor conductors of heat, but this property can be utiHzed 

 for insulation purposes only when the air is sealed in small cells to prevent 

 convection — hence the great value of expanded plastic foams, cork, slag 

 wool, etc. Conversely, while convection may assist in the heating of a fluid 

 environment it can only take place where temperature differences exist, 

 thus indicating a lack of temperature uniformity. Thermal gradients set 

 up by convection can amount to several degrees Centigrade change. Heat 

 can also be transferred in the form of electromagnetic radiation — typically 

 the infra-red. The important point to realize is that many materials, 

 including air, transmit heat in this form without being appreciably heated 

 themselves ; hence if a visibly red heater is placed in a box of air, an appreci- 

 able portion of the heat emission is absorbed directly by the walls of the box 

 without heating the air. On the other hand, Hght (as well as mechanical 

 energy) entering an environment is normally converted into heat, so that 

 radiation may prove a source of unwanted heat in a controlled enclosure. 



MEASUREMENT OF TEMPERATURE 



Temperature is measured by a thermometer, which to most people is a 

 mercury-in-glass thermometer. The universal use of this machine and its 

 'convenience' is accepted. The outline of limitations which may lead to 

 incorrect information from the mercury thermometer, and which may make 

 it necessary to use one of the admittedly less convenient electrical methods, 

 is given below. 



Limitations of the mercury-in-glass thermometer 



(a) Contact; while most biological work will involve the measurement of 

 the temperature of fluids, anyone who has tried to measure the temperature 

 of a solid will know the inconvenience of the device. 



(b) Thermal capacity; for any reasonable volume change of mercury 

 the bulb must be large — hence the thermal capacity is correspondingly 

 large; thus equilibration will be slow, and according to the size and nature 

 of the body may lead to an appreciable heat exchange and an alteration 

 of the body's temperature. This is particularly true of a body such as air. 

 The most sensitive type of thermometer— the Beckmann — is capable of 

 readings more accurate than 0-0 1°C but it measures only temperature 

 differences; reference calibration is very difficult. The mercury thermometer 

 is useless for following rapid temperature changes. 



(c) Hysteresis; the glass envelope of a thermometer has appreciable 

 hysteresis if subjected to a rapid temperature cycle; this can materially 

 affect measurements of accuracy greater than 0-l°C: permanent distortion 

 is unlikely. 



385 



