DAIRYING 25 



of the force which acts on milk while it is in a centrifugal separ- 

 ator. The contrast between the action of the force of gravity on 

 a body and the centrifugal force exerted on a body revolving 

 about a center may be illustrated by comparing the weight, or 

 the gravity of a body with the pull on a string to which the same 

 body is attached, when whirled around one's head at a high speed. 

 If a weight is hung on a spring balance and swung around in a 

 circle at various speeds, it will be noticed that the pull of the 

 weight on the balance will increase if the speed increases. A dif- 

 ference will also be noticed between the pull exerted by a small 

 weight as compared with a larger one where both are revolved 

 at the same speed. This explains the action of centrifugal force 

 on milk ; the cream is separated from the skim milk because the 

 fat is lighter than the serum when milk is revolved at a high 

 speed. 



264. If the weight of a certain volume of milk serum is 1.04 

 Ibs., then that of an equal volume of butter fat is .90 lb.* The 

 difference between the weights of these two volumes is .14 lb., 

 and the effect which centrifugal force may have to separate the 

 fat from the serum can be calculated by determining the pull 

 which i-s exerted on a weight of .14 lb. when revolved at any 

 given speed in a circle of a given diameter. If the weight .14 lb. 

 is hung from one end of a spring balance one foot long, and this 

 is revolved about the other end as a center at the speed of 100 

 revolutions per minute, the indicator on the balance will show 

 that the weight is pulling .48 lb. Increasing the speed to 200 

 revolutions makes the pull 1.92 Ibs. At a speed of 1000 it is 

 48 Ibs., and at 5000 revolutions per minute the pull of this .14 

 pound weight on the balance is 1200 Ibs. This shows the differ- 

 ence between gravity and centrifugal force to be as .14 is to 1200 

 when milk is revolved at a speed of 5000 revolutions per minute 

 in a circle having a radius of one foot. 



Calculation of Centrifugal Force 



The centrifugal force in pounds exerted on any body may be 

 calculated by multiplying the weight of the body in pounds by the 



* These numbers are taken because they represent the spec- 

 ific gravity of milk serum and of butter fat respectively. 



