Calorimetric Absolute Measurements. 191 



pile in Wheatstone's scheme was taken by a sensitive galva- 

 nometer; that of the resistance to be measured in Wheat- 

 stone's plan was occupied by the pile whose resistance and 

 electromotive force were to be measured, the single tangent- 

 compass (R = 165*7 nrillim.), and the other wire resistances 

 which were included in the resistance w x . The resistances of 

 the measuring-wire and the galvanometer branch had been 

 accurately ascertained. With the bridge open and the rest of 

 the circuit closed the galvanometer and the tangent-compass 

 indicated certain deflections. The point of connexion of the 

 bridge-wire with the measuring-wire was now so chosen that 

 the deflection of the sensitive galvanometer remained invari- 

 able whether the bridge was open or for an instant closed. As 

 soon as this point was found, according to known rules, first, 

 the resistance n\ (in S. M. U.), could be determined, which the 

 pile employed, the tangent-compass, and the wires belonging 

 to them possessed at that determinate current-intensity i x 

 which had been indicated by the tangent-compass with the 

 bridge open; secondly, the electromotive force exhibited by 

 the pile when traversed by the current i t could be calculated 

 in relative measure (founded on the absolute electromagnetic 

 current-unit and Siemens 's unit of resistance). 



After this the absolute value of the same electromotive force 

 was determined, by means of the amount of heat which it ge- 

 nerated in its circuit by a current i, maintained constant (which 

 was always approximately =z'i), during the time z. For this 

 purpose the pile, the tangent-compass, and the wires which 

 were also comprised in the resistance iv x were combined with 

 the platinum resistance w in the calorimeter to form a circuit, 

 through which the constant current i then passed during the 

 time z. The quantity of heat Q, which this current would 

 have called forth during this time in the calorimeter if the 

 platinum resistance had possessed, not the alternating tempe- 

 ratures of the calorimeter, but the constant temperature t a of 

 the environment, is, according to equation (2) in section III., 



•9 



— % w z 



Q=Mc a [t-t +B(t-t a )z] = ^- 



This heat was calculated from M, c a , t, t, t , t a , and z by the 

 previously indicated process. 



Immediately after the conclusion of the calorimetrical mea- 

 surement, the resistance iu 1 and the electromotive force e were 

 measured a second time in relative measure according to the 

 above-described procedure, in order to control any variation 

 in the two quantities that might have taken place during the 



