118 RADIO-ACTIVITY 



A model may make this clearer. The outer particles may be 

 represented by a number of exactly similar sewing-needles, 

 magnetised simultaneously in a solenoid. They are floated 

 vertically in a small trough (Fig. 21), by having, say, their N 

 poles inserted each to the same depth in exactly similarly pieces 

 of cork. The place of the positive core is taken by a bar magnet 

 set vertically, N pole upwards, below the trough. It will be 

 noticed that the needles arrange themselves in two rings. If ten 



FIG. 21. A " Model " Atom. (From Crowther's Molecular Physics.) 



needles are floated, seven will be in the outer ring and three in 

 the inner. 



It has been found that the greatest number of magnets which 

 we can have in an empty ring is five. If a sixth is added, two rings 

 are formed with five needles on the outside and one in the middle. 

 The number which must be placed in the middle rapidly increases 

 with the number in the outer ring. The removal of one needle 

 from the outer ring may cause a complete rearrangement of the 

 needles, i.e. a new series of concentric circles may be formed 

 differing from the first series in the number of the component 

 needles in each ring. 



As we have mentioned, the electrons forming an atom are 

 supposed to be in constant rapid and irregular motion. If for 



