III. CENTRIFUGATION 75 



parable time by reversing and regulating the current through the armature. 

 Accurate speed control at any of thirty different values isobtained by matching 

 a greatly reduced speed from the drive against a selected speed controlled by 

 a constant speed, synchronous motor. Any difference motivates a differen- 

 tial gear, which in turn actuates the electrical controls that regulate the power 

 supply to the drive motor. The speed is also indicated by an electrical ta- 

 chometer. 



To permit viewing and photographic registration of the sedimen- 

 tation according to methods discussed later, light from a mercury 

 arc is directed upward through the revolving cell and through windows 

 (which serve as collimating lenses also) in the vacuum chamber 

 against a 45° mirror near the top of the instrument and thence hori- 

 zontally through appropriate lenses to the photographic plate at the 

 right end of the machine. By means of a half-reflecting mirror, some 

 light is diverted to a viewing screen so that the sedimentation pat- 

 tern may be viewed at any time, even while photographs are being 

 taken. The photography is automatic, there being appropriate ad- 

 justments for preselecting both exposure time and the interval be- 

 tween photographs. Exposure time is usually of the order of fifteen 

 seconds. 



A smaller (half size), relatively simple air-driven vacuum type 

 ultracentrifuge patterned after that described by Beams and Pickels 

 {17) has been commercially produced {91); its resolving power, how- 

 ever, is rather low {1, p. 47). 



Since the description of the first air-driven centrifuge of the vac- 

 uum type was published in 1935 {19a), several improved designs for 

 driving mechanisms have been described in the literature. De- 

 scriptions of or reference to most of these can be found in the publi- 

 cations of Beams {8) and Pickels {2,4). Of particular interest are the 

 electrically driven (high frequency induction type), magnetically 

 supported drive of Skarstrom and Beams {10) and the air-driven, air- 

 supported "turret" type drive of Pickels {11), which has been in 

 successful use at a number of research institutions for several years. 



2. Svedberg Ultracentrifuges 



The original Svedberg ultracentrifuge was a small "optical cen- 

 trifuge" developed by Svedberg and Nichols {9) at the University of 

 Wisconsin in 1923. Later Svedberg and Rinde {20) used the machine 

 to study size distribution among gold sols and proposed the name 

 "ultracentrifuge" denoting an instrument by means of which sedimen- 



