﻿398 Prof. L. V. King on a Lecture-Room 



they were animated with life. As before, the glass vessel 

 and its contents may be projected on a screen, the resulting 

 effect being illustrative of molecular movements. 



Section 4. — Experiments on Electrodynamic Repulsion. 



Owing to the distribution of the magnetic field around the 

 coil employed in cbis experiment, the same apparatus is well 

 suited to the demonstration of electrodynamic repulsion. For 

 this purpose several plates of aluminium or copper should be 

 cut with a radius approximately equal to the outer radius 

 of the coil. Such a disk may be anchored by three strings 

 fastened at equidistant points of the circumference so as 

 to allow it to move vertically, with its centre over the 

 axis of the coil, which is laid in a horizontal position. 

 On applying A.C. circuit, the plate will float three or 

 four centimetres above the coil. By placing a light iron 

 rod (3 cm. x 1 mm.^ on the plate, the direction of the 

 A.C. field is easily demonstrated, as shown in PI. II. fig. 4. 

 It will be noticed that over an annular region bounded by 

 the outer edge of the plate and a circle of half its radius, the 

 lines of force are inclined at approximately 45° to the vertical. 

 It is the reaction of the horizontal component of the A.C. 

 field with the induced current due to the vertical component 

 which causes the repulsion referred to. To demonstrate this, 

 a circular plate may be cut up into several concentric rings 

 and laid on a sheet of glass. When current is applied it is 

 only the outer rings which are repelled, the force on the 

 inner rings gradually becoming less, until that on the central 

 disk in a practically uniform field perpendicular to its plane 

 is practically nil. 



Iron filings poured on a glass plate laid horizontally over 

 the coil assume an interesting laminar distribution, which 

 again may be projected on a screen. The iron filings tend 

 to arrange themselves in a series of vertical planes about 

 1 cm. high arranged radially. It is easily seen that this 

 arrangement is due to the fact that under the influence of the 

 alternating field, each of the radial planes represents a series 

 of vertical A.C. magnets which repel each other. Their 

 height is limited by the vertical stability of the plates under 

 the combined effect of gravity and of the alternating field. 



Section 5. — Experimental Model of the Rutherford Atom. 



By using two coils of the dimensions already described, 

 arranged with their planes horizontal at a distance apart 

 equal to the mean radius (Helmholtz arrangement), it is 



