56 Testing Milk and Its Products. 



the machine; test bottles sometimes break while the 

 machine 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 diameter 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 revolutions per 

 minute, in order to affect a maximum separation of fat; 

 later work has shown that this speed is ample. Taking 

 therefore this as a standard, the centrifugal force to which 

 the contents of the test bottles are subjected when sup- 

 ported on an eighteen inch wheel and turned 800 revolu- 

 tions per minute, can be calculated as follows: 



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

 the formula 



w. v 2 



F =~32F (I) 



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 2?r rx 8 6 =2X3.1415 X & X 8 e =62.83 

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

 very near 3 ounces, or j 3 e of a pound. Substitutiug these values 

 for v and w, gives 



=30.65 lb, 



The bottles are therefore, under the conditions given, sub- 

 jected to a pressure of about 30.65 pounds. IB order to calculate 



