Measurement of Gas^Engine Temperatures. 89 



the heat quantities being expressed in joules. Similarly, we 

 have 



X(0-0')=*^p (2) 



8' being the temperature shown on curve A. From these two 

 equations, eliminating 6: — 



so that A, can he determined. Having obtained X, the gas 

 temperature 6 is calculated from the wire temperature b} r the 

 aid of equation (2). The curve of gas temperature so obtained 

 is curve C. 



Before discussing the gas temperatures, it is advisable to 

 give an example of the calculations by which they are obtained. 

 Take, for instance, the points P, P'. Q, Q'. on the suction part 

 of the stroke. These four temperatures were obtained in 

 quick succession, so that the conditions would be substantially 

 the same for all. This is a necessary precaution because it is 

 difficult to keep the conditions absolutely constant over any 

 long period of time, and calculations based, as these are, upon 

 small differences of temperature are seriously vitiated by even 

 a small change in conditions. 



The following calculation is self-explanatory: — 



Temperature at P 84°- 7 C. 



P' 59°-6 



Q 40°-7 



Q' 22-°-l 



Mass of wire (by weighing) *053 gramme 



Specific heat of platinum *0324. 



Therefore capacity for heat of wire (k) is 



•0324 x -053x4-2 = -0072 joules per degree centigrade. 



Time interval from P to P' (90° of crank- angl e) =y§ second. 

 Rate of fall of temperature along PP' = 300° per second. 

 Rate of fall of temperature along QQ/ = 224° per second. 

 Rate of loss of heat of wire along PP' = 300° X '0072 = 2'16 

 dd" . [watts. 



[This is — k-rr in equation (1).] 



Rate of loss of heat of wire along QQ' = 224 X -0072 = P61 

 j(s! [watts. 



(This is -**). 



The mean resistance of the wire between P and P' is 



