412 M. Regnauh on the Meamremetit of Temperatures 



with whatever care the bearing and the pivot have been worked, 

 it is impossible to give to the needle a sufficiently great mo- 

 bility to cause it to obey the feeble variations in the intensity 

 of the current. It is necessary to give the instrument a few 

 shakings to overcome the inertia of the needle ; and the direc- 

 tion in which it stops, after having removed it from its position 

 of equilibrium, varies very perceptibly, although the current 

 always preserves the same intensity. Thus, in a perfectly 

 constructed compass with an agate bearing, on which I have 

 experimented, the uncertainty amounted to ^°, which conse- 

 quently gives an uncertainty of 10'^ per cent. 



Much greater mobility is given to the needle by suspending 

 it to a cocoon thread ; but then other inconveniences arise, 

 which occasion similar uncertainty. The accurate centring 

 of the needle becomes difficult; it may perceptibly vary during 

 the course of the experiments ; the extreme mobility of the 

 needle causes it to oscillate constantly around its position of 

 equilibrium ; it is difficult to alter the direction of the current, 

 so that it coincide with that of the magnetic axis of the needle; 

 and if the temperature which it is desired to measure is not 

 absolutely stationary for a pretty long time, it becomes almost 

 impossible to make an observation at the suitable moment. 

 In all cases the measurement of the deviations presents great 

 uncertainty, unless we have much time at our disposal to ad- 

 just the apparatus. 



If the instrument is not to be employed to measure very 

 high temperatures, but is merely to be used between 0° and 

 400° Cent., it may be arranged so as to obtain for a difference 

 of temperature of 100° of the two solderings a deflexion greater 

 than 5°. However, this is not always easy when it is desired 

 to use but a single couple, and not to have recourse to a pile 

 composed of several elements ; when, moreover, this couple 

 cannot be formed of those metals which produce the most 

 powerful currents, as bismuth and antimony, on account of 

 their great fusibility. Greater deflexions, it is true, are ob- 

 tained by increasing the number of the convolutions of the 

 wire which act upon the needle; but this increase has a limit, 

 because the wires should possess great conductibility for ther- 

 mo-electric currents, and consequently present a considerable 

 diameter. 



Any degree of sensibility may be obtained by substituting 

 for the simple needle a system of two partially compensated 

 needles; but then we meet with the same incoveniences as the 

 galvanometer presents, especially those dependent on the 

 magnetic alteration of the system. I have made numerous 

 experiments with compasses arranged in this manner, but the 



