THERMAL CONDUCTIVITY. 19 



the thickness of the wall a unit of length, and it is cus- 

 tomary nowadays to employ generally the centimeter, the 

 gramme, and the second as our units of length, mass, and 

 time. Consequently, let us make our wall one centimeter 

 thick, and then in place of " d " we should have to write 

 " one." Let us take the difference of temperature as equal 

 to 1, and then instead of 6 l 0<> we have to write 1. Let 

 us take the area of our wall through which the flux of heat 

 is measured as equal to one centimeter squared. The unit 

 of area A, then, we replace again by one. If we measure 

 the quantity of heat which passes through in a minute, 

 then g^ will pass through in a second, and let us take one 

 second as the unit of time instead of T. Then what we 

 measure is the flux of heat through a piece of the substance 

 one centimeter thick over an area one centimeter square, 

 with a difference of temperature of 1 and during one second 

 of time, that is, the conductivity is the quantity of heat 

 which flows across from unit of surface of a body whose 

 thickness is one unit of length and the difference of tem- 

 perature of whose sides is a unit of temperature during 

 unit of time. That is Fourier's definition of conductivity. 

 Now, there are very great practical differences in deter- 

 mining the value of this quantity K, but there is one 

 condition which was pointed out by Fourier, and which has 

 been almost invariably used for the measurement of the 

 conductivity of different bodies. This is what Fourier 

 called the permanent state. If I take a rod of metal, such 

 as this, and heat it by any means at one end and let the 

 other end gradually cool by radiation to the air and by 

 convection, then this end will always be the hottest, but a 

 certain amount of increase of temperature will be propagated 

 along the bar, and finally, after some hours it may be, the 

 rod will acquire a permanent condition of temperature, 

 provided the temperature of the heated end remains constant 

 and the temperature of the air remains constant. Then 

 we shall have a perfectly gradual decrease in the tempera- 

 ture from the hot end to the temperature of the air at the 

 other end, and a steady flow of heat is passing through the 

 bar in order to maintain this constant condition of tempera- 

 ture. Under these circumstances, when a body is in such 

 a permanent condition, it is possible to measure the flux of 

 heat. Fourier gives a number of formulae which were 



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