Beading Resistance-Thermometers, 721 



advantage ot this combination lies in the fact that the thermo- 

 electric power is large and almost constant over a good range 

 of temperature, the lines of the iron and German-silver in 

 the thermoelectric diagram being very nearly parallel : thus 

 the voltages given are very nearly proportional to the tem- 

 perature-differences. Fortunately the Nickel line is also very 

 nearly parallel to that for Iron, and is lower than that for 

 German-silver, making the combination Iron-Nickel con- 

 siderably more powerful than that of Iron-German silver. 

 vSince the introduction of Constantau (or Eureka, a copper- 

 nickel alloy), it has been largely used, particularly in 

 Germany, in conjunction with iron, as a sensitive thermo- 

 junction. The thermoelectric power is likewise very nearly 

 constant and it is considerablv greater than either of the 

 others already mentioned, being about 50 microvolts per 

 degree C. The Iron-Xickel combination, however, has for 

 some purposes the advantage over the other two, in that its 

 resistance is much lower for the same size of wires. In all 

 these combinations, the voltage obtained is only approximately 

 proportional to the temperature difference. To make the 

 proportionality much closer I would suggest the use of 

 composite junctions, first employed by Tait in mapping out 

 the thermoelectric diagram 



\n rig. 7 let ABC and ADC be wires of two different 

 metals joined in parallel, and let MP and NP be their 

 respective lines on the thermoelectric diagram. Then the 

 line for tie- composite wire AC will pass through P (the 

 Neutral Point of ABC and ADC) in some direction such as 

 QP, and. by altering the ratio of the resistances of ABC and 

 ADC, the direction of QP can be altered continuously so as 

 to lie anywhere desired between MP and NP. 



