BOMB CALORIMETER 



507 



3. The calorimeter vessel is a cylindrical copi)or can heavily nickel plated 

 and capaltle of containing the bomb and about 2,000-2,500 c.c. of water. 

 On the floor of the can is a pad of cork or fibre, ois which the bomb rests. 



Besides this it is necessary to have a stirring device, a thermometer to read 

 to 1/100° C, and a means whereby oxygen at 15-20 atmospheres pressvire 

 can be put into the bomf). 



Calibration. — Certain values have 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 employed. 

 Some workers prefer to suspend a dried cotton thread of known weight from 

 platinum wire connecting D and 7?. The thread 

 (lips 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 oft' the cellulose match, which in 

 turn causes the foodstuff to ignite. Others prefer 

 to weigh out a piece of iron wire, 5-6 cm. 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 incorporated 

 in the correction called 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 calori- 

 meter 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. 



If we burn a certain quantity of naphthalene (9,668 calories) or of cane 

 sugar (3,988 calories per gram), which would evolve Q gram cals.. the actual 

 rise of temperature shown by the thermometer is t° C, then Q = {m -\- ii)t 

 where m = water equivalent of the apparatus and ju. = weight of water 

 (in grams) in the apparatus. Transposing, we have 



Fig. 102. — Section tlirough 

 a Kroeker bomb (see text). 



m 



Q 



( 



That is, the water equivalent is : 



Total heat generated (calculated) 

 Observed increase in temperature of calorimeter water 



(Quantity of water in 



apparatus (in gms.) ] 



(3) Calibration of Thermometer. — The thermometer has to be calibrated, 

 and a correction applied for this. 



(4) Cooling Constant. — Another correction to be made in the final calcula- 



