82 CENTRIFUGES 



relative centrifugal forces up to about 2000 X g can be attained. A variety 

 of swinging bucket and conical or angle heads can be used. Also, several 

 special rotors are available, such as a horizontal rotor for spinning the 

 small tubes used in blood analysis. 



The larger "high-speed" centrifuges exist in the greatest variety. Most 

 modern units rotate at speeds up to about 20,000 rpm, developing cen- 

 trifugal fields up to about 50,000 X g. As air friction generates consider- 

 able heat, most of these centrifuges are refrigerated. Because friction on 

 the bearings is reduced to a minimum, the rotor would spin freely for 

 some time after the motor is shut off if it were not for the magnetic 

 brakes. The Servall KSA system provides for continuous flow of liquid 

 through the instrument while the rotor is turning at high speed. Sedi- 

 ment is collected in centrifuge tubes, and the clarified liquid flows into 

 a collecting ring and then into a separate container. 



The Spinco Preparative Ultracentrifuge develops centrifugal fields up 

 to about 150,000 X g. Refrigeration is provided. Air friction becomes 

 so important at speeds up to 50,000 rpm that the rotor of this instrument 

 turns in an evacuated chamber. The liquid to be centrifuged is placed in 

 sealed tubes inside a sealed rotor. Several different rotors are available, 

 one of which has a capacity of about 600 ml— so fairly large amounts of 

 material can be handled. 



Analytical ultracentrifuges usually spin very small samples of material, 

 primarily for the purpose of determining sedimentation rate. The rotor 

 must turn at very high speeds. Thus, compressed air bearings, magnetic 

 bearings, or other novel bearings are used. Sedimentation in the sample 

 tube can be viewed or photographed through a special illumination and 

 optical system. Flashes of light are synchronized with the rotation, so in 

 effect the rotation is stopped. The rotor, of course, operates in a vacuum 

 under refrigeration. Some commercial models are available, but some 

 of the analytical ultracentrifuges are built in the laboratories where they 

 are used. 



Research on centrifugal principles still continues. Pea-sized rotors have 

 been subjected to centrifugal forces exceeding five million X g, and much 

 smaller rotors have been turned at 1.5 million revolutions per second, 

 providing a force of over a billion X g. Under these conditions the rotor 

 tears itself apart and explodes. An interesting illustrated article by Beams 

 describes some of this work.^ 



1 Jesse W. Beams, "Llltrahigh-speed rotation," Scientific American 204: 4 (April, 

 1961), pp. 134-147. 



