612 THURAS: DENSITY MEASUREMENTS 



The changes in density corresponding to these changes in tem- 

 perature are negligible. 



To determine the rapidity and reliability of the method, a 

 series of tests were made on the same sample of water, no single 

 test taking longer than ten minutes. The following results 

 were obtained using a new sample in the test tube for each 

 measurement : 



Sample No. Equil. temp. 



1 28.40 



2 28.38 



3 28.39 



4 28.38 



5 28.40 



6 28.39 



Mean 28.39 



This maximum variation of 0?01 C. corresponds to less than 

 three in the sixth decimal place of density. 



The formation of bubbles on the bobbin is troublesome when 

 a high degree of accuracy is required. It is fortunate, however, 

 that when bubbles are forming on the bobbin it is impossible to 

 obtain an equilibrium temperature. This difficulty may be 

 overcome either by lifting the bobbin out of the liquid just 

 before a reading is taken, or by driving off the air in the liquid 

 by heating the sample to 50° or 60° before making a measurement. 

 The latter method was found the more satisfactory. It elimi- 

 nated the difficulty completely, but, of course, care must be taken 

 to avoid evaporation. 



Conclusion. A simple apparatus is described by which the 

 density of sea water can be measured on board ship with speed 

 and precision. With carefully calibrated bobbins a density 

 measurement of a liquid of known temperature coefficient can 

 be made in less than ten minutes to an accuracy of more than 

 two in the fifth decimal place. The particular advantage of the 

 method lies in the facts that (1) by changing the temperature of 

 the liquid its density can easily and quickly be brought exactly 

 to the density of the bobbin, and (2) at equilibrium temperature 

 the sensitivity of the method is unaffected by the .motion of the 

 vessel, the liquid and bobbin having the same density. 



