SPECIFIC HEAT OF AMMONIA. 345 



rt, dR^ ^^^ ^ R^-^ ^^^ _ ^^^ _ k ^^^^ _^^_ ^^^ ^^^ _ ^^^ ^^^^ 



Jh at h — to 2 



Now the observed temperature change is {R2 — -Ri) where Ro and Ri 

 have been obtained graphically as explained above, and the cooling 

 correction is given by (16). Calling 8R the corrected value of the 

 change of resistance corresponding to the change of temperature in the 

 calorimeter 



57? = R,- R,- f^'^ dt = 



J I (it 



R'2 -Ri+l {2Rm -Ri- Ro) (to - t,) - ^ — ^ {t, - /i) (17) 

 2 ti — to 



where the value of k/2 is given by (13). 



Thus 8R is given in terms of four points, (Roto), (Riti), (-R2/2), and 

 (Rats), and in. terms of Rm- The four points are obtained by plotting 

 the initial and final lines, and Rm is obtained b}' averaging the observed 

 values of R during the heating period. 



To convert 8R into degrees C, use is made of a formula given by 

 Dickinson and Mueller. ^'^ Starting with the Callendar-Griffiths 

 equations for the resistance of platinum as a function of the tempera- 

 ture, they obtain. 



100 

 86 = ^"'"° ~ ^"° 8R (18) 



^ ^ 100 100^ '"^ 



where Rm°, Ro°, and 5c, are the constants of the Callendar-Griffiths 

 equation for the observing thermometer, and dm is the mean tempera- 

 ture, d -^ 89/2. dm need be known only approximately, and in this 

 work was taken to be the temperature corresponding to Rm- 



5. Callendar-Griffiths Equation for Resistance of Platinum 

 as a Function of the Temperature.^* 



6-dp = 8c(~ - 1)-^ (19) 



"^ uoo y 100 



0p = 100 ^~^°° (20) 



illOO — ito 



13 Bull. Bur. of Standards, 9, (1913), p. 483. 



14 Reference 8, p. 201. 



