56 Testing Milk and Its Products. 



removed until the machine stops; it should be tight 

 fitting, since test bottles sometimes break while the ma- 

 chine is running at full speed, and every possible pre- 

 caution should be taken to protect the operator from 

 any danger from spilled acid or broken glass. 



66. Speed required for the complete separation of 

 the fat. There is a definite relation between the diame- 

 ter of the Babcock testers' and the speed required for a 

 perfect separation of the fat. In the preliminary work 

 with the Babcock test the inventor found that with the 

 machine used, the wheel of which had a diameter of 

 eighteen inches, it was necessary to turn the crank so 

 as to give the test bottles seven or eight hundred revo- 

 lutions per minute, in order to obtain a maximum sepa- 

 ration of fat; later work has shown that this speed is 

 ample. Taking the higher figure as a standard, the cen- 

 trifugal force to which the contents of the test bottles 

 are subjected when supported on an eighteen-inch wheel 

 and turned 800 revolutions per minute, can be calcu- 

 lated as follows: 



The centrifugal force, F, acting on the bottles is expressed by 

 the formula 



F = ^'''l. . . . (1) 



32.2r ■ ■ ■ • ^-^J 



in which w = the weight of the bottle with contents, in pounds ; 

 V ^ the velocity, in feet per second, and r := the radius of the 

 wheel in feet. 



When the wheel is turned 800 times a minute, a bottle sup- 

 ported on its rim will travel StttX "J'„" = 2x3.1415Xi\X|!;!] = 62.83 

 feet per second. The weight of a bottle, with milk and acid, is 

 about 3 ounces, or Jj of a pound. Substituting these values 

 for V and w, gives 



