PHYSICAL AND CHEMICAL EXAMINATION OF MILK 



163 



the wheel. The smaller the diameter, the greater should be the 

 number of revolutions per minute. The following figures give the 



Fig. 56. Electric centrifuge for the Babcock test. (A. H. Thomas Co.) 



required number of revolutions per minute for centrifuges of differ- 

 ent diameter (Farrington and Woll) : 



Diameter of wheel. Number of revolutions required. 

 10 inches 1074 



24 



909 



848 



759 

 724 



When a hand centrifuge is used the speed can be judged by 

 counting the number of revolutions made by slowly turning the 

 handle and then turning the handle as many times per minute as 

 required. 



The water which is used for filling the bottles so as to bring the 

 fat column into the neck should be hot and soft. Hard water may 

 contain carbonates and the acid then liberates carbon dioxid gas. 

 Bubbles of this gas may then disturb the uniformity of the fat 

 column and cause inaccurate reading. Hard water is made suit- 

 able by boiling 



The percentage of fat should be read at about 120 to 140 F. 

 (50-60 C.). If the temperature should be higher than this, read- 

 ing must be deferred until the right temperature has been reached. 



