of Thermo-electric Phenomena. 527 



of the button on the cord at any instant (this is Ohm's law ; 

 see § 9 equation (1) and § 10); hence the work done in one 

 excursion from right to left is 



-y. 



' + a Tj- 



W= \ rvdoc=-^r ra 2 V k. 



The work done in unit time is therefore 



^=i,.A=i,.V3. (1) 



and this is the rate * at which a button loses its energy when 

 left to itself without assistance from others, or the rate of cool- 

 ing of a substance placed in an enclosure at absolute zero. 

 Hence, assuming that the absolute temperature 6 is connected 

 with the maximum velocity V and the atomic weight m by 

 the relation 



±mY 2 = s6, 



the rate of cooling is given as 



0= ^- r0, (2) 



dm v y 



which would be simply proportional to 6 (Newton's law), ex- 

 cept that the resistance r itself varies with the temperature. 



§ 23. The above conclusion that the radiating power of a 

 body is directly proportional to its specific electrical resistance 

 and inversely as its atomic weight, is not "negatived by the expe- 

 riments of Leslie, and of Provostaye and Desains,as far as they 

 go ; for Leslie's order of the metals in radiating-power agrees 

 nearly with their inverse order in conductivity. Moreover, if 



one finds the value of the expression — ^ for the few metals more 



accurately experimented on by Provostaye and Desains, one 

 gets a number whose constancy is indeed rough but quite as 

 good as could be expected. The only metals apparently whose 

 radiating-powers were ascertained by them were platinum, 

 copper, gold, and silver; and of these gold was only tried as 

 gold-leaf, which is scarcely a fair test, because radiation is 

 more than a surface action. The following Table contains in 

 the first column a mean value of the radiating-power, taken 



* If, instead of assuming a simple harmonic motion, one takes any mo- 



2 

 m haying average velocity - V, the rate of do 



7T 



;r~V 2 , which is only slightly different from (1). 



2 

 tion having average velocity - V, the rate of doing work will be nearly 



4 n 



