THE SEPARATION OF THE CRYSTALS 



419 



meter of the basket increases so also must the peripheral speed, if it is desired 

 to maintain the centrifugal force constant. 



When the Weston centrifugal first came into use it was designed for 

 1,440 r.p.m. with reference to a 3O-inch machine, which for a number 

 of years was the only size built. But within a few years the makers 

 reduced this speed to 1,200 r.p.m. and the centrifugal force corresponding 

 to this diameter and to this speed remains generally a standard at present. 

 The table below gives the r.p.m. in other sizes required to give an equal 

 peripheral speed, and an equal centrifugal force. 



EQUIVALENT SPEEDS REFERRED TO 1,200 R.P.M. AND 30-INCH MACHINE. 



Diameter, 

 inches. 

 30 

 36 

 40 

 42 

 48 

 54 



EQUAL PERIPHERAL 



SPEED. 



Revs, per min. 

 1,200 



1,000 



900 



857 

 750 

 667 



EQUAL CENTRIFUGAL 



FORCE. 



Revs, per min. 



1,200 



1,095 



1,039 



1,013 



948 



894 



It also follows that with the larger-sized machines, run at equal centri- 

 fugal force, the stress in the shell of the basket is greater, necessitating either 

 a greater section or the use of materials of higher tensile strength. 



The relation between speed of rotation and water left in the material 

 has not, the writer believes, been worked out in detail. Reasoning by 

 analogy a law similar to that found by the writer as holding between pressure 

 and quantity of juice extracted on crushing cane (cf. Chapter XI) would 

 probably result. If such be the case great increases in the speed would be 

 accompanied by but small decreases in the quantity of water left in the dried 

 material. The following data on drying yarn made in 1878 have been given 

 to the writer by Mr. A. R. Robertson, of the firm of Watson, Laidlaw & Co., 

 Ltd., and, though incomplete, bear out the ideas put forward above. 



