EGGS OF ARBACIA PUNCTULATA 451 



of this construction over the nonvacuum type is that it obviates the 

 undesirable effects of air friction and the need for great precision in 

 the dynamical balance of the rotor. Furthermore, it has the capacity 

 for handling much greater volumes of material. On the other hand, 

 it is considerably more complicated mechanically. 



Electrically Powered, Magnetically Supported Vacuum-Type 

 Centrifuge. In order to overcome the irregularities occasioned by 

 variations in gas pressure in the preceding types of centrifuge, 

 various methods have been employed to regulate the pressure, but 

 the most successful among these have been rather complicated. 

 Hence Skarstrom and Beams (1940) devised an electric centrifuge 

 in which the rotor was supported magnetically and allowed to spin 

 in a vacuum. For constancy of speed and ease of operation this 

 instrument appears to be superior to gas-driven vacuum types. 

 Refinements of the apparatus have been appearing continuously 

 from Beams' laboratory at the University of Virginia. In more 

 recent models the centrifuge is suspended from a vertical iron rod 

 supported by the field of a solenoid. An automatic regulatory mech- 

 anism is employed whereby the current in the solenoid is controlled 

 by the height of the rotor, decreasing as the rotor rises and increas- 

 ing as it falls. In this fashion the height of the rotor is kept constant 

 and free from any mechanical contact. Consequently, when the rotor 

 is spun in a vacuum by a rotating magnetic field, extremely high 

 speeds can be obtained. With this technique MacHattie (1941) 

 spun a V32 in. steel ball at 6,600,000 R.P.M., and when the driving 

 field was cut off at 6,000,000 R.P.M., it lost only about 1% of its 

 speed in an hour. 



