410 M. Regnault on the Measurement of Temperatures 



The employment of thermo-electric couples for measuring 

 temperatures would, under many circumstances, present such 

 great advantages over the ordinary processes, especially when 

 it is desired to determine temperatures within limited spaces, 

 that I have frequently devoted attention to this subject ; but 

 I must confess, that, notwithstanding the very numerous and 

 varied experiments which I have made, my researches have 

 met with little success, and I have not been able to obtain a 

 comparable instrument whose indications might at all times 

 inspire confidence. There is such instability in the molecular 

 conditions which determine the thermo-electric currents, that 

 we are never certain of obtaining a current of constant inten- 

 sity when the two solderings are raised at repeated intervals 

 to the same temperatures ; the variations are especially con- 

 siderable when, during the interval, the apparatus has been 

 heated to very different temperatures. 



The instrument which is used for determining the inten- 

 sity of currents is far from presenting the perfection which is 

 necessary in accurate investigations, especially if these inten- 

 sities vary considerably, as happens when thermo-electric cur- 

 rents are employed for the measurement of temperatures. 



The intensity of electric currents is measured, either by the 

 deflexions which they produce upon a fieely suspended mag- 

 netic needle, or by the chemical decompositions which they 

 effect. The second method, which is of great importance for 

 the measurement of powerful currents, is inapplicable in the 

 case of thermo-electric currents, which are always very feeble, 

 and present too little after resistance to overcome the least 

 obstacles introduced into the circuit. 



The only instruments which have hitherto been employed 

 for measuring thermo-electric currents, are based, therefore, 

 on the deviations which these currents impress on the mag- 

 netic needle; they are the galvanometer and the sine com- 

 pass. 



The galvanometers with two partially compensated mag- 

 netic needles are most suited for measuring very weak cur- 

 rents; they appear, consequently, to be adapted principally 

 for thermo-electric currents. Unfortunately', the deviations 

 of the needles are not proportional to the intensity of the cur- 

 rents except between very restricted limits; and for somewhat 

 considerable deflexions, it is necessary to construct a table in 

 which the intensities corresponding to the observed devia- 

 tions are found. The direct construction of this table would 

 not be a matter of any moment if the same table were of ser- 

 vice lor any time; but experience has shown, that in a system 

 of two partially compensated needles, the magnetic intensity 



