2 HEAT. 



These, then, are our two fundamental ideas: that bodies are comparable 

 as to their hotness or coldness, and that in general, on becoming hotter 

 they receive something which we call heat the amount of heat received 

 depending both on the quantity and nature of the matter and on the 

 degree to which it becomes hotter. 



Our first aim must be to render our conceptions more definite, by 

 obtaining some numerical scale to express how hot a body is. 



We shall be occupied, in the earlier chapters of the book, with a 

 description of the mode in which such a scale is obtained, and an account 

 of its use in the investigation of the change of dimensions of bodies when 

 heated. Afterwards we shall show how numerical expressions may be 

 obtained for quantities of heat, and how these quantities may be deter- 

 mined by experiment. 



Temperature and Thermal Equilibrium. The number which 

 expresses on some definite scale how hot a body is, is termed its 

 temperature. 



Any instrument, such as the ordinary mercury and glass instrument, 

 used to obtain the temperature of a body is termed a thermometer. 



We know from common observation that bodies in contact with each 

 other, and not subjected to changes of external conditions, after a time 

 get neither hotter nor colder i.e. heat does not pass from one to another. 

 They are then said to be in thermal equilibrium with each other. 



For example, in a room not exposed to draughts or to sunlight, and 

 containing no fire, the objects lying on a table will after a time all be in 

 thermal equilibrium with the table and with each other. A glass of 

 water on the table will neither be heated nor cooled by putting into it 

 an iron rod which has also been lying 011 the table. The sensation 

 received by an observer touching the various objects may, however, be 

 very different. The iron will feel colder than the table on which it has 

 been placed, and the water in the glass will feel warmer than the iron 

 and colder than the table. But if, instead of the finger, we use a 

 thermometer, we find that it will register the same whether it is laid on 

 the table, placed against the iron, or put in the water the difference of 

 sensation not being due to difference of temperature but to different 

 rates of communication of heat from the hand to the surfaces touched. 



The thermometer, then, shows us that bodies in thermal equilibrium 

 with each other are at the same temperature, and as the thermometer is 

 also one of the bodies it is also at the same temperature as the substance 

 in which it is placed. 



Conversely it is true that bodies at the same temperature are in 

 thermal equilibrium with each other. 



This may be proved by direct experiment ; by pouring, for instance, 

 a quantity of mercury into a vessel containing water at the same 

 temperature, and noting that the temperature remains constant. 



Thermometers. In order to obtain a definite scale of temperature, 

 we make use of the fact that, in general, bodies expand on being heated. 

 Various simple experiments show this. For instance, if a metal rod 

 (Fig. 1) be fixed at the end V in a vice and if the end A presses down 

 on a small roller r on a flat plate, then a pointer P attached to the 

 roller will move over a scale S when the rod is heated, say by a gas 

 flame. 



