﻿Demonstration of Atomic Models. 391 



always certain to give results which never fail to delight an 

 audience. 



An interesting variant of this experiment is to make use of 

 the arrangement of two coils described in Section 5 (figs. 

 5 & 6). A surface of clean mercury is placed midway between 

 the two coils. A number of steel balls floating on this surface 

 will repel each other as already described, and will all tend 

 towards the centre, owing to the greater intensity of field. 

 The remarkably regular arrangement taken up under these 

 conditions is shown in fig. 3 (6). The damping is so slight 

 that the system may be set into oscillation in various ways 

 by means of external magnets, giving a good illustration of 

 internal vibrations in the atom. It would, moreover, be 

 possible with no very great expenditure of labour to deter- 

 mine the frequency of various modes and compare the results 

 with theoretical calculations. 



Section 3. 



The same apparatus may also be used to illustrate the 

 motion of the molecules of gas or the Brownian movements. 

 For this purpose an elongated piece of iron is employed, 

 e. g. a short cylinder of iron or steel wire about 1 cm. in 

 length by 1 mm. in diameter. In the alternating field of the 

 coil such a magnet experiences a very strong torque, which 

 vanishes when the axis lies along the direction ot the 

 resultant A.O. field. If such a magnet is placed in a flat 

 cylindrical glass vessel occupying the centre of the coil, ai.d 

 the field suddenly applied, violent movements of the little 

 iron rod will be observed. The instantaneous moments set 

 up by the field will be sufficient to make the rod leave the 

 surface on which it is resting and describe a trajectory 

 under the combined effect of gravity and the magnetic field. 

 At the termination of the flight, it will again strike the glass 

 plate and will then receive an additional impulse made up of 

 the magnetic torque and the elastic reaction at contact with 

 the glass. This will start it on a new trajectory, and the 

 process will be continued indefinitely until the rod makes 

 contact with the plate at the termination of its flight in such 

 a way that the instantaneous torque is zero. Then it stops 

 dead with the axis pointing along the direction of the field. 

 This is an event which happens very rarely. Several such 

 rods enclosed within a glass vessel will keep in constant 

 motion in a manner resembling the motion of molecules in a 

 rarefied <:as. An interesting variant of this experiment is to 

 insert .-hort steel wires along the diameters of small pith balls 

 which hop around, describing flights in the glass vessel as if 



