CENTRIFUGES 



81 



In general, the rate at which a particle settles depends upon the rela- 

 tive centrifugal force, the size and shape of the particle, the viscosity of 

 the liquid in which it is suspended, and the difference between the 

 density of the particle and the den- 

 sity of the liquid. Let us call the 

 rate of sedimentation s cm/sec, and 

 assume a spherical particle of 

 radius r and density S, suspended 

 in a liquid of density p and vis- 

 cosity r). Then 



^~ \.\25ri 



where N is revolutions per second 

 and R is the radius of rotation as 

 before. This is a rather complex 

 interrelationship of factors, but if 

 we know all but one, we can cal- 

 culate the last. In addition, if we 

 measure all the factors, s, r, 8, etc., 



Fig. 7-2, Sedimentation in the angle 

 head. Particles need not move the full 

 length of the tube against the viscosity 



we can check on the validity of the of the medium. (Courtesy Ivan Sorvall, 

 assumption that the particles are Inc.) 

 spherical. If particles are rods in- 

 stead of spheres, their sedimentation behavior will not obey this equation. 

 Even the viscosity of protoplasm has been determined by measuring the 

 rate at which particles sediment through it. 



Types of centrifuges 



Such a variety of centrifuges is available that it is impossible to de- 

 scribe them all. Probably the easiest way to gain information about the 

 types and sizes is to read catalogue descriptions and advertisements. Even 

 any attempt to classify the various types is completely arbitrary, but 

 for the sake of discussion they can be divided into cHnical centrifuges, 

 "high-speed" centrifuges, and preparative and analytical ultracentrifuges. 

 In common usage any centrifuge which turns faster than about 20,000 

 rpm is now called an ultracentrifuge. 



The clinical centrifuge is a small, portable model, easily used on a 

 bench-top. The capacity is usually not more than about 200 ml, and 



