BOMB CALORIMETER 25 



white enamel, and contains within it, but insulated from it by 

 a thin ebonite stand, 3. the bright nickel-plated water holder. 



Besides this it is necessary to have a stirring device, a ther- 

 mometer to read to 1/100 C., and a means whereby oxygen at 

 15-20 atmospheres pressure can be put into the bomb. 



Calibration. 



Certain values have got to be determined before the apparatus 

 can be employed. 



1. Calorie Value of Match. 



In order to convert energy from the potential to the free state, 

 we have already seen that some free energy must be added 

 the material must be ignited. Various forms of match are em- 

 ployed. Some workers prefer to suspend a dried cotton thread 

 of known weight from a platinum wire connecting D and R. 

 The thread dips into the crucible T, and touches or is embedded 

 in the material to be burned. On completion of an electric 

 circuit through Pi and P2 the platinum wire glows and sets off 

 the cellulose match, which in turn causes the foodstuff to ignite. 

 Others prefer to weigh out a piece of iron wire, 5-6 cms. long 

 and 0-1 mm. thick, and put it in place of both the platinum 

 wire and the cotton thread. In any case, the amount of heat 

 evolved in the ignition process has to be determined carefully, 

 and deducted from the heat evolved, in a complete estimation 

 or in the determination of the water equivalent. 



2. Water Equivalent. 



The apparatus itself vessels, thermometer, stirrer is heated 

 along with the water it contains. Its water equivalent, i.e. 

 the quantity of water which has the same heat capacity as the 

 apparatus, must be determined and added to the quantity of 

 water actually employed in the experiment. Several methods 

 exist for this determination. The most exact, and at the same 

 time the most convenient, is to burn in the calorimeter a weighed 

 quantity of a substance whose calorie value is known with 

 absolute certainty, and ascertain the resultant change in the 

 temperature of the water. The water equivalent is : 



(Total heat generated (calculated) \ iri ... ,. 



\ - (Quantity of water in 

 Observed increase in temperature of calorimeter water/ apparatus (in gins ) ) 



3. Calibration of Thermometer. 



The thermometer has to be calibrated, and a correction 

 applied for this. 



