UK AT. 



337 



ture 





HEAT. U 



EFFECTS OF HAT EXI'ANHION MOORS OF MEASURING IT 

 MKNT - CHANUE OF BTATE. 



WE have now to notice tho principal effects which heat pro- 

 duces on different bodies submitt. .1 t . in influence. Take a rod, 

 A i Fitf . 5), of brass or copper, about half an inch in diameter, 

 iiinl cut a gauge of metal of the shape shown at B, ao that tho 

 rod may just fit length way* between the end* of the gauge, and 

 also fit tightly in the hole, 

 <. If now tho rod be 

 dipped in boiling water, 

 or held over a source of 

 heat so that it* tempera- 

 may be raised, we 

 lin'l Unit it will no 

 nter the gauge nor 

 pass through the hole. It 

 is clear, then, that the di- 

 mensions of tho rod have 

 l>wn increased by its ele- 

 vation of temperature, and 

 we thus learn that one of 

 the effects of heat is to 

 produce expansion. 



If we take a flask (Fig. 6) 

 with a long narrow neck, 

 and fill it with water BO 

 that tho liquid may stand 

 a little way in the nock, we 

 shall find, on exposing tho 

 bulb to the flame of a lamp, 

 that tho column of liquid 

 will rise. Tho experiment 

 may easily be shown in a 

 large room, by placing the 

 flask in the course of tho 

 rays from the electric or 

 some other powerful lamp, 

 and placing a lens so that 

 an enlarged image of the 

 tube may bo thrown on a 

 screen. It will then be 

 seen that on tho applica- 

 tion of the source of heat 

 to the flask, tho column of 

 liquid falls slightly before 

 it begins to rise. This at 

 first seems strange, but it 

 arises from the fact that 

 the heat reaches the gloss 

 first, and expands that be- 

 fore it has had time to 

 expand the water. The 

 flask accordingly becomes 

 slightly larger, and hence 

 the column of water sinks 

 in it. The expansion of 

 metals is so great that in 

 large engineering works 

 as, for instance, long iron 

 bridges allowance has to 

 be made for it, as otherwise 

 the structure would be dis- 

 torted and weakened. It is 

 very important, therefore, 



to ascertain the exact amount of expansion which different sub- 

 stances undergo when their temperature is raised. The simplest 

 means of doing this is to take a rod of the metal, and having 

 placed it so that one end presses against an adjusting screw and 

 the other against the short end of a lever, heat it by means of a 

 spirit-lamp. The longer limb of the lever then serves as an index, 

 and shows the amount of elongation. Sufficient accuracy cannot, 

 however, be obtained in this way, as the exact temperature of the 

 bar cannot be determined. The method devised by Lavoisier 

 and Laplace, and represented in Fig. 7, is therefore frequently 

 adopted. A metal trough is placed over a furnace between four 

 stone supports, and tho bar to be tested is placed in this. A 



126 N.E. 



p.-l of glaM i* placed between the rapport* at one end of the 

 trough, *o that the bar may prou against it. On the top of the 

 r two is a rod turning in bearing*, and carrying at on* end 

 ; a teleaoope. Fixed to thw rod i* another at right angle* to it, 

 which pre**e* againit the other end of the bar under erainina- 

 An accurately-divided scale i* placed on the wall of the 

 room opposite to the telescope, which ha* crow wire* placed in 

 it, HO a* to mark the centre of iU field of view. It will easily be 

 I Men that when the rod elongate* it will torn the axle which car- 

 ries the teleaoope, ao that 

 by looking through the lat- 

 ter we shall be able to read 

 off on the scale the amount 

 of deviation, and by an 

 ea*y calculation learn the 

 exact increase in length. 

 The viraal ray here aerveo 

 a* a long index hand, and 

 enable* n* to take oar 

 measures accurately. 



When an experiment i* 

 to be made, the bar i* 

 placed in position, and the 

 trough filled with melting 

 ice. In a little time it will 

 have attained the tempera- 

 ture of 32, and an ob- 

 servation i* then made 

 through the teleaoope so 

 as to determine the degree 

 of the scale to which it 

 points. The ice is now 

 removed, and the trough 

 filled with mercury or oil, 

 and raised to the required 

 temperature. When it has 

 been stationary at this 

 point for a short time, as 

 shown by thermometer* 

 placed in the trough, a se- 

 cond observation i* taken, 

 and in this way the expan- 

 sion is ascertained. This 

 fraction is usually known 

 as the co-efficient of linear 

 expansion, and in most 

 tables it is given for the 

 expansion between 32 C and 

 212, or the freezing and 

 boiling points of water. 

 The following table show* 

 tho extent of this increase 

 for a few common sub- 

 stances : 



It must be remembered that this table merely indicate* the 

 linear increase that is, the increase in one direction. Most rub- 

 stances, however, expand equally in each direction, and then the 

 cubical expansion may be taken at three times the above 

 fractions. 



The enlargement of bodies by heat is easily accounted for 

 by the dynamical theory, for, when the particles vibrate more 

 widety, they naturally endeavour to get further apart, ao as to 

 have more space. We may regard the particles of any body a* 

 being held together by two opposing forces cohesion, which 

 toniln to draw them more closely together, and heat, which tends 

 to drive them further apart. If the heat be increased the body 



