THERMO-ELECTRIC QUALITY UNDER PRESSURE. 



287 



higher temperatures, at all pressures, the Peltier heat becomes posi- 

 tive, passes through a maximiuu, and decreases again. The Thom- 

 son heat, on the other hand, is initially positive for low temperatures, 

 but becomes negative at higher temperatures. 



Tin was one of the metals measured by Wagner.^ He made only 

 one measurement, at 300 kg. and 100°, and found for the effect 

 —0.95 X 10~^- volts per degree per kg. The measurements above 



0'' 



20" 40" 60" 

 Temperature 



80" 1 00° 20" 



Tin 



40" 60° 

 .Temperature 



80' 



100" 



Figure .3. Tin. On the left, the heat absorbed by unit quantity of electri- 

 city on flowing from uncompressed metal to metal compressed to the pressure 

 indicated on the curves, as a function of temperature. On the right, the 

 excess of Thomson heat in metal compressed to the pressure indicated on the 

 curves over uncompressed metal, as a function of temperature. 



make the effect positive at all pressures for a temperature range of 

 100°, and would indicate, if interpolation is justifiable, + 0.43 X 10"'^ 

 volts per degree per kg. Wagner's one reading allowed him to form 

 no idea of the complicated state of affairs for this metal. There can 

 be no comparison between his results and mine, because it will be 

 shown later in the case of Al and Fe, which also show reversals in 

 sign like Sn, that small variations in the purity of the metal, or the 



