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



And of course operation of the equipment must be smooth enough to 

 ensure good optical definition in the photographs. 



Convective disturbances may be caused by temperature gradients 

 in an ultracentrifuge whenever these gradients have any component 

 that is not radially directed so as to produce a positive density gradient 

 away from the axis of rotation. These disturbances are magnified in 

 proportion to the centrifugal force and mth a given temperature 

 gradient are most pronounced with low concentrations of highly dif- 

 fusible or polydisperse material in large cells (76) . These are exactly 

 the conditions that must be attained for precise measurements on 

 particles of low molecular weight, and hence it is of considerable im- 

 portance to select, especially for such studies, an ultracentrifuge that 

 approaches the convection-free ideal as nearly as possible. It should 

 be noted that a sharp boundary does not indicate an absence of con- 

 vection; its sharpness may in fact be due partly to one or several 

 circulating currents above or below the boundary, affecting its appar- 

 ent rate as well as its shape. While viscous drag against the walls of 

 the cell tends to slow down such convection, only sufficient density 

 gradient within the fluid (such as exists within a sharp boundary) can 

 prevent it. Convective disturbances can also be caused by an im- 

 proper shape or misalignment of the cell (7) . 



In the oil-driven ultracentrifuges of Svedberg (1) the convection 

 is so pronounced during acceleration of the rotor that no boundary is 

 formed until conditions become uniform at full operating speed. 

 This is apparently due to a rapid and continual rise in rotor tempera- 

 ture because of heating associated with friction from the bearings 

 (which are integral with the rotor and support its full weight), the 

 impinging oil (against turbines), and the enveloping hydrogen. To 

 minimize convection at operating speed, constant and balanced (with 

 respect to the two ends of the rotor) conditions must be maintained. 

 With the vacuum centrifuge convection is minimized by reducing air 

 resistance (pressures of a micron or less) and thermal contact with 

 the bearings as much as possible. In properly designed vacuum ultra- 

 centrifuges, no appreciable convection occurs during acceleration. 

 It is therefore important that such machines be capable of very rapid 

 acceleration to full speed in order that a boundary be sedimented 

 away from the meniscus before its concentration distribution becomes 

 unduly affected by "reflection" from the meniscus. This is especially 

 important when the starting position of the boundary is used in the 

 determination of sedimentation rate. It is of interest in this connec- 



