WEIGHT AND MATERIAL. 



connection not shown in the diagram. At 

 the end of each arm is a swivel ring into which 

 a test-tube, containing the substances to be 

 separated by weight or its substitute, can be 

 dropped. To avoid shaking, equal weights 

 should be placed opposite to each other. 



When the apparatus is spun horizontally, 

 centrifugal force causes the test-tubes to swing 

 outwards from the position A. When they 

 swing as far as B, so as to make an angle of 

 forty-five degrees, they are half-way between 

 the vertical and the horizontal position. The 

 vertical force of gravity is just balanced by 

 the horizontal force of centrifugal action. One 

 ounce of material inside each tube, in addi- 

 tion to its one ounce of vertical weight, due 

 to gravity, has acquired one ounce of horizontal 

 weight due to centrifugal action. 



Now the centrifugal force increases in pro- 

 portion to the square of the velocity, so that 

 when the machine is spinning so as to carry 

 round the test-tubes twice as quickly, their 

 contents have four times the centrifugal force, 

 or four ounces of horizontal weight, urging 

 them away from the centre. And when the 

 machine is carrying the tubes four times as 

 swiftly, each ounce of material in them, besides 

 its own ounce of vertical weight, has one pound 

 of horizontal weight. Thus a very small original 

 difference between the weights of two of the 

 things in a test-tube becomes a very consider- 



H 97 



