76 E. G. P I C K E L S 



tation in a centrifugal field could be measured quantitatively. How- 

 ever, it has become common practice to associate the term "ultracen- 

 trifuge" with any type of high speed centrifuge rather than with the 

 original connotation of quantitative measurement. 



In 1927 Svedberg and Lysholm (IS) substituted an oil drive for the 

 electrical drive. In the present model (1,13,90) used for sedimenta- 

 tion velocity measurements a rotor of special steel is driven by oil 

 under pressure about a horizontal axis and in an atmosphere of hy- 

 drogen gas at a pressure of about 20 mm. The hydrogen gas is 

 necessary for conducting away from the rotor the comparatively large 

 amount of heat developed through friction in the mechanical bearings. 

 Fortunately, from the standpoint of avoiding temperature gradients 

 (which would produce convection currents) across the cell in an axial 

 direction, the rotor is symmetrical about a plane perpendicular to the 

 axis, having a similar bearing and turbine assembly on either side. 

 The temperature of the rotor is measured by a radiation thermo- 

 couple placed about 0.25 mm. from the rotor near the cell hole posi- 

 tion. Speed is determined by a stroboscope or a frequency meter. 

 With certain special experiments small rotors with a cell height of 

 8 mm. and an average radial distance of 3.25 cm. have been operated 

 at speeds up to 140,000 r.p.m. However, it has been found that for 

 general purposes the most practical rotor is one capable of operating 

 at speeds up to about 65,000 r.p.m. with a fluid column approximately 

 17 mm. long at about 65 mm. from the axis of the rotation. Fluid 

 column thicknesses up to about 12 mm. are used according to the 

 concentration and nature of the solute. Cell distortion, leakage, 

 and cell failure become serious problems as centrifugal forces consid- 

 erably above 300,000 times gravity are employed. Since the rotor 

 shaft and the bearings are comparatively rigid in the oil-driven ultra- 

 centrifuge, dynamic balance is very critical, although a damping de- 

 vice (1) used in the later models has reduced vibration somewhat. 



For use as an equilibrium centrifuge Svedberg and Sjogren (14) 

 in 1929 improved the early low speed electrical centrifuge by sub- 

 stituting a direct motor drive for the earlier gear drive. The drive 

 rotates about a vertical axis and is of a self-balancing type used for 

 spinning viscose thread in the artificial silk industry. It is fed with 

 three phase a.c. current of variable frequency from a special generator 

 and has a short-circuited squirrel cage rotor moving in ball bearings. 

 The 15 cm. rotor spins in an atmosphere of hydrogen and has an ac- 

 curate temperature control. The transparent cell and counterbal- 



