ON METHODS OF DETERMINING THE DRYNESS OF STEAM. 403 

 Or taking the specific heat of water constant : — 



l; -- 



_ 1116-<i-0-19<3 + 0-48i!2 

 1116-0-71<i 



No weighing is required, and temperatures only have to be observed. 

 The apparatus must be in operation for twenty to thirty minutes before 

 observations are commenced. Then observations may be begun and con- 

 tinued for as long a period as is desired. It is not a mere small sample 

 of steam, therefore, which is observed, but a considerable quantity taken 

 from the steam pipe at a uniform rate. The observations are as simple as 

 possible, and the apparatus is used with the greatest facility. Presently 

 some tests will be given tending to show that the instrument is very 

 trustworthy. 



Suppose the steam is initially quite dry. Then x=l in the formula 

 above. 



A,+Li=:a;3 + L3 + 0-48(«2-«3) 

 «2=0-604i!i + 0-395<3 



From the small excess of pressure over atmospheric pressure to is about 

 214°. Then 



<2=0-604i!,+84-5. 



This may be termed the normal temperature in the chamber B for dry 

 steam. In proportion as the temperature falls below this the steam is 

 initially moist. If it contains initially more than a certain amount of 

 moisture, the temperature in the chamber falls to 214° or below it, and 

 no calculation of the dryness is possible. 



If i.2=220° and <3=214° are taken as the limiting values for which the 

 instrument can safely be used, there being then 6° of superheating in B, 

 then 



_ 1181-<i 

 ^•"'^ 1116-0-71<i 

 and /, V 0-29iJ,-65 



1116-0-71«, 



give the minimum initial dryness or maximum amount of initial moisture 

 in steam for which the apparatus is suitable. 



J _, Greatest initial 



1 moisture per cent. 



250° 0-80 



300 2-44 



350 4-21 



400 6-13 



YII. Combined Sejmrating and Wire-drawing Method. — To extend 

 the usefulness of the wire-drawing method Mr. Barrus added a separator 

 (C, fig. 5). The steam first passes through the separator, leaving most of 

 its moisture, and the remainder is measured by a wire-drawing calorimeter. 

 When thus arranged the use of the instrument is much more complicated, 

 as the amount of steam flowing through and the amount of moisture 



n n 2 



