268 



Table 298. 

 THERMOELECTRIC POWER. 



The thermoelectric power of a circuit of two metals is the electromotive force produced by one 

 degree C. difference of temperature between the junctions. The thermoelectric power varies with 

 the temperature, thus : thermoelectric power = Q = dE /dt^=A + Bt, where A is the thermoelec- 

 tric power at o° C, B is a. constant, and / is the mean temperature of the junctions. The neutral 

 point is the temperature at which dE ldt = o, and its value is — A jB. When a current is caused 

 to flow in a circuit of two metals originally at a uniform temperature, heat is liberated at one of 

 the junctions and absorbed at the other. The rate of production or liberation of heat at each 

 junction, or Peltier effect, is given in calories per second, by multiplying the current by the co- 

 efficient of the Peltier effect. This coefficient in calories per coulomb = QT/J, in which Q is in 

 volts, T^is the absolute temperature of the junction, and ^ = 4.19. Heat is also liberated or ab- 

 sorbed in each of the metals as the current flows through portions of varying temperature. The 

 rate of production or liberation of heat in each metal, or the Thomson effect, is given in calories 

 per second by multiplying the current by the coefficient of the Thomson effect. This coefficient, 

 in calories per coulomb, ^^ BT6 /y, in which B is in volts per degree C, 7" is the mean absolute 

 temperature of the junctions, and Q is the difference of temperature of the junctions. [BT] is Sir 

 W. Thomson's " Specific Heat of electricity." The algebraic signs are so chosen in the following 

 table that when A is positive, the current flows in the metal considered from the cold junction to 

 the hot. When B is positive, Q increases (algebraically) with the temperature. The values of 

 A, B, and thermoelectric power, in the following table are with respect to lead as the other metal 

 of the thermoelectric circuit. The thermoelectric power of a couple composed of two metals, i 

 and 2, is given by subtracting the value for 2 from that for i ; when this difference is positive, the 

 current flows from the cold junction to the hot in i. In the following table, A is given in micro- 

 volts, B in microvolts per degree C, and the neutral point in degrees C. 



The table has been compiled from the results of Becquerel, Matthiessen and Tait; in reducing 

 the results, the electromotive force of the Grove and Daniell cells has been taken as 1.95 and 

 1.07 volts. The value for constantin was reduced from results given in Landolt-Bornstein's 

 tables. The thermoelectric powers of antimony and bismuth alloys are given by Becquerel in the 

 reference given below. 



Substance. 



Aluminum 



Antimony, comm'l pressed wire 



" axial 



" equatorial . . . . 



" ordinary . . . . 

 Argentan 



Arsenic 



Bismuth, comm'l pressed wire . 



" pure " " 



" crystal, axial .... 



" " equatorial . . 



" commercial . . . . 

 Cadmium 



" fused 



Cobalt 



Constantin 



Copper 



" commercial . . . . 



'* galvanoplastic .... 

 Gold 



Iron 



" pianoforte wire .... 

 " commercial 



Lead 



Magnesium 



Mercury 



Nickel 



" (—18° to 175°) . . . . 



" (25o°-30o°) 



" (above 340°) 



A 

 Microvolts. 



0.76 



11.94 



2.63 



■1-34 



—2.80 

 —17-15 



21.8 

 83-57 

 3-04 



B 



Microvolts. 



1.0039 



0.0506 



1.0424 



1.0094 



— O.OIOI 



0.0482 



0.0000 



0.0094 



0.0506 



-0.2384 



0.0506 



Thermoelectric power 



at mean temp, of 

 junctions (microvolts). 



20° C. 



0.68 



—6.0 



—22.6 



-26.4 



—17.0 



12.95 



13-56 

 97.0 

 89.0 

 65.0 



45-0 



22. 

 —1-52 



O.IO 



-3-8 

 — 1.2 



—3-0 

 — 16.2 



— 17-S 



0.00 

 — 2.03 

 0.413 



50° C. 



0.56 



14.47 

 12.7 



39-9 



—4-75 

 —2-45 



+ 19-3 

 — 1.81 



—3-30 



—14.74 



— 12.10 

 — 9.10 

 0.00 

 —1-75 



3-30 

 15-50 

 24-33 



Neutral 

 point 

 A 



195 



-236 



—62 



—143 



[- 



[- 



277] 

 356 



236 

 ■431] 



Author- 

 ity. 



T 



M 



B 

 T 

 B 

 M 



T 



M 



M 

 B 



T 

 M 

 B 



Smithsonian Tables. 



