CHAPTER VII. 

 CONDUCTIVITY. 



The Passage of Heat from one Body to Another Conductivity Differs enormously 

 in different Substances General Remarks on Conductivity in the Three States 

 Definition of Conductivity Diffusivity Emissivity Measurements of Con- 

 ductivity Pe"clet's Method Bar Methods of Despretz, Forbes, Neumann, and 

 Angstrom Gray's Method Berget's Experiment on Mercury Experiments of 

 Wiedemann and Franz Kundt's Experiments Senarmont's Experiments on 

 Crystals Lees's Experiments Lundquist Weber Conductivity of Gases- 

 Experiments of Stefan, Kundt, and Warburg. 



Transference of Heat by Conduction. 

 The Passage of Heat from one Body to Another. There are 



two modes in which heat is transferred from one portion of matter to 

 another conduction and radiation. In conduction, the matter receiving 

 the heat is in contact with the matter from which it receives it, and the 

 temperature falls continuously along the course by which the heat is 

 flowing. If, for example, I put one end of a poker in the fire and hold 

 the other end, the heat is conducted along the poker from the warmer 

 to the colder portions, the heat passing down the slope of tempera- 

 ture, warming the iron as it travels, so that all the intervening portions 

 of the poker are intermediate in temperature between that of the fire 

 and that of my hand. 



In radiation, the matter receiving the heat is not in contact with 

 the matter from which it receives it. If I warm my hands before a fire, 

 I do so by radiation, the heat received by my hands passing through the 

 intervening air without warming it. In fact, in the case of radiation, 

 any matter through which the radiation passes may be colder or hotter 

 than either or both of the bodies between which it is passing. We 

 cannot, therefore, suppose that the energy passes from one to the other 

 as heat, but that, on leaving the sender, it is converted from heat- 

 energy into another form which we term radiant energy, to be recon- 

 verted into heat on reaching the receiver. 



Conductivity. We have already noted as the chief characteristic 

 of heat conduction, that the heat always travels from hotter to colder 

 matter. The greater the slope that is, the difference of temperature 

 between neighbouring points a given distance apart the greater the 

 amount of heat conducted. If a thermometer at the temperature of the 

 room be placed in a vessel of hot water, it rises much more rapidly at 

 first, when the temperature slope between the water and the mercury is 

 great. The rate of rise gradually slackens till, ultimately, when the 

 mercury and the water are at the same temperature, there is no further 



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