CHAPTER 



7 



Centrifuges 



A centrifuge is an instrument designed to separate materials of different 

 density from each other by virtue of a centrifugal force. Since the cen- 

 trifugal force is similar in its effects to gravity, most things that can be 

 separated in a centrifuge u^ould eventually settle because of gravity, but 

 a very long time might be required. The centrifuge allows us to hasten 

 this effect by applying a larger force. 



In laboratory centrifugation, at least one of the components to be 

 separated is a liquid. The other might be solid particles, another liquid, 

 or, rarely, even bubbles of gas. The separation of parts of cells, as de- 

 scribed in the previous chapter, is one common use of the centrifuge. 

 So many other kinds of mixtures must be separated in the laboratory 

 routine, that a centrifuge is used almost daily. In addition to its use 

 in preparing materials, the centrifuge is a valuable analytical tool. 



The usual centrifuge consists of a rotor or head, driven by a motor. 

 Several types of rotors are available. Some hold only a few very small 

 tubes or vials of material, while others hold bottles with a total capacity 

 of a liter or more. As the rotor turns, the liquid and its suspended ma- 

 terial are subjected to the centrifugal force. The various rotors in ordi- 

 nary use fall into two general classes: those in which tubes of liquid 

 are held firmly at some fixed angle (like 35°), and those in which tubes 

 or bottles are placed in metal buckets which swing out to a horizontal 

 position as the rotor turns. Each type has certain advantages. 



Centrifugal force 



Any rotating body is subject to a constant acceleration inward, toward 

 the center of the circle. A weight whirled on a string must be pulled 



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