MEGASS AS FUEL. 



the specific heats of carbon dioxide, nitrogen, and water vapour being '22, *24, 

 and '48 respectively. If twice the quantity of air necessary for combustion 

 was used, the combustion temperature will be 



939 x -22 + 4-788 X '24 + '683 X '22 + '771 X '48 

 since there are now 4-788 Ibs. nitrogen and -683 Ib. oxygen in the flue gases. 



The more air admitted to the furnaces the lower is the temperature, and 

 hence observations of the temperature of the furnace give a means of overseeing 

 the carefulness of the firing. 



If the temperatures in the furnace, i.e., the combustion temperature, and 

 the temperatures of the waste gases be known, then the efficiency of the 

 furnace can be at once obtained from the expression, 



j< _ ^ 

 Efficiency == - , 



where T and t are the excess temperatures of combustion and of the waste 

 gases over and above the temperature of the outside air ; thus, if the com- 

 bustion temperature be 1884 P., the waste gas temperature be 584 P., and 

 that of the outside air be 84 F., then the efficiency is 



1800-500 



1800 



perfect work being represented by unity. This calculation does not allow for 

 heat losses due to radiation and to unburnt fuel, or to air leaks between the 

 combustion chamber and the fuel, but will give strictly comparative results from 

 day to day. The immediately preceding sections show how from the flue gas 

 analysis and the megass analysis, first the excess air and then the combustion 

 temperature can be calculated. What is required, however, in actual work is 

 a continuous automatic record to which the mill superintendent can refer at 

 his leisure, and immediately below attention is called to the very beautiful 

 instruments which have been devised in the last decade for the accurate 

 determination of high temperatures, and which are now to be obtained from 

 dealers, 



1. Electric Resistance Pyrometers. The resistance of a conductor to the 

 passage of an electric current is a function of the temperature prevailing in 

 the conductor ; this relation has been very carefully worked out for a number 

 of conductors, and especially so in the case of platinum ; hence if a current of 

 fixed voltage or E.M.F. flow through a platinum wire, observations of the 

 current produced give means of obtaining the resistance of the conductor by 

 means of Ohm's law, current X resistance = voltage, whence from previously 

 ascertained experimental observations the temperature of the conductor and 

 (in the special instance considered) of the gases in the combustion chamber of 

 the furnace are obtained. These instruments are constructed so that readings 

 are obtained aurally by means of a telephone attachment, or visually by means 

 of the deflection of a galvanometer needle. 



423 



