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Demonstration of Atomic Models. 399 



possible to secure a fairly uniform field over a considerable 

 area midway between the coils. Such arrangement (PI. II. 

 figs. 5 & G) allows of interesting experiments on a model atom 

 approximating more closely to modern ideas. A shallow 

 circular basin of mercury is placed on an adjustable stand 

 between the two coils. A number of steel pins with glass 

 heads serve as the elements (electrons) for the model. It 

 oue of these is placed with the glass head on the mercury 

 surface, it will float in a vertical position and tend to move 

 towards the centre ot the field, owing to the greater concen- 

 tration of lines of force. This force towards the centre may 

 be varied at will by adjusting the height of the mercury 

 surface, or by placing rods of soft iron along the axis of the 

 coils at adjustable distances above or below the mercury 

 surface. If a second pin be floated on the mercury surface, 

 it will repel the first with a force varying nearly as the 

 inverse square law when the distance apart is not too great. 

 A third pin may be added, when a triangular arrangment will 

 be formed. Successive pins give the familiar series of regular 

 polygons arranged in concentric rings. It is evident that 

 the great advantage of the A.C. field is to make the mag- 

 netical polarity of each of the pins very nearly equal, thus 

 giving rise to a remarkable symmetry in the arrangements 

 formed, as illustrated by figs. 7 (a) and 7(b) (PI. II.). 

 As before, the experiment can be carried out in such a way 

 that the various stable arrangements may be projected on a 

 screen. It is extremely simple to demonstrate the apparatus 

 at a moment's notice, the only precaution necessary being to 

 use clean mercury so as to allow a great mobility of the 

 floating pins on an uncontaminated surface. 



It is interesting to notice that rotation of the basin con- 

 taining mercury does not disturb any particular stable 

 arrangement, owing to the fact that the centrifugal force 

 is accurately balanced by the change of slope of the para- 

 boloidal mercury surface. 



The use of an A.C. field allows of the possibility of realizing 

 positive electrons and a central nucleus, the law of forces 

 between them being very nearly that of the inverse square 

 and at the same time very exactly that corresponding to 

 charges of ±e, ±2e, ±3^, etc. It is evident from fig. 5 

 (PI. II.), illustrating the model under consideration, that 

 electrons may be represented by lengths of soft-iron wire of 

 the same diameter arranged to move with both ends in the 

 same plane at distances not too far apart compared with their 

 length. In these circumstances we have repulsion according 

 to the inverse square law, the charge — e being represented 



