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



Section 2. 



The magnetic elements which form the essential feature 

 of the apparatus to be described consist of a number of steel 

 spheres or small soft-iron rods magnetized in a strong 

 alternating field. 



One such model is shown diagrammatically in PL II. fig. 1, 

 while fig. 2 shows the actual apparatus. The coil A consists 

 of 340 turns of number 12 B. k S. copper wire (2 mm. diam.); 

 inside radius of winding 8"8 cm., outside radius 13'5 cm., 

 width of coil 3" 9 cm. Such a coil has a resistance of 

 approximately 1*3 ohms and self-inductance of about 32 

 millihenries. It may be connected directly to a 110-volt 

 60-cycle A.O. circuit without overheating. In such cir- 

 cumstances it draws a current of about 9 amperes. It is 

 approximately of such dimensions as to give a maximum 

 field strength at the centre of the coil. 



Placed over the opening of the coil is a large watch-glass 

 B whose radius of curvature is approximately 25 cm. If 

 available, an accurately ground concave glass mirror may 

 be used to advantage. If, now, a supply of steel ball-bearings 

 about 3 mm. in diameter is available, these may be placed 

 on the concave surface B, where they will experience an 

 attraction towards the lowest point approximately pro- 

 portional to the distance. When the maximum current is 

 passed through the magnetizing coil, the steel spheres will 

 become A.C. magnetic doublets of very uniform magnetic 

 moments. It will be noticed that the magnetic axis will 

 always be very accurately along the direction of the mag- 

 netic field, independently of the rolling motion of the balls. 

 Furthermore, if the spheres are of fairly uniform quality and 

 the field strength sufficiently great, the instantaneous mag- 

 netic moments of these doublets will be equal in magnitude 

 and phase. In these circumstances the steel spheres will 

 repel each other with a force varying as the inverse fourth 

 power of the distance, the constant of proportionality being 

 accurately the same for all the spheres. With the attraction 

 to the centre varying as the distance, it maybe expected that 

 the magnetic elements will form remarkably symmetrical 

 stable groupings. One such grouping is illustrated in 

 PI. II. fig. 3 (a). 



It is obvious that by a very simple arrangement of lenses 

 and mirrors this model atom may be projected on a screen. 

 The concave surface B may, if desired, be mounted so as to 

 allow of rotation, thus increasing the interest of the " atomic " 

 arrangements. This experiment is extremely convenient for 

 lecture-room purposes, as it requires no preparation and is 



