112 DESIGN IN NATURE 



explained, produces the striking, well-defined lines of force picture seen at Fig. 3 of Plate Iv. This picture resembles 

 in its general features that seen at Fig. 2, Plate Iv., and need not be further described. 



If a round bar-magnet be supported vertically on a horizontal sheet of white paper, over which iron-filings are 

 evenly spread, a fines of force picture in a horizontal plane is obtained, as seen at Fig. 4 of Plate Iv. 



This figure is very remarkable, as showing a radiating and concentric arrangement of the iron-filings such as 

 is frequently witnessed in crystals, plants, and animals, and which, as already explained, makes for symmetry 

 and strength. 



In the case of a very long, thin, bar-magnet the iron-fihngs attach themselves almost exclusively to its 

 extremities, and assume the form of nearly spherical tufts : the filings between the extremities, which constitute 

 its " indifferent zone," being scarcely at all influenced. The extremities or poles act, in a great measure, indepen- 

 dently. The field at either end of a long bar-maignet is said to be " unipolar." In the cross sections of such a 

 magnet the lines of force proceed radially. As many of the lines of force originate at the circumference of the end 

 of the magnet, the iron-filings aggregate and form a black ring in the middle ; the portions on either side of the 

 black ring being denuded of filings and appearing white, as shown at Fig. 4, Plate Iv. 



§ 24. The Compound Magnet. 



As already stated, the power of a magnet depends on the number of hues of force issuing from it ; the greater 

 the number of lines of force proceeding from a magnetically active plane, the greater their effect. The power of a 

 bar-magnet is increased by combining it with other bar-magnets, care being taken to arrange the ends of the bars 

 having the same land of magnetism together ; the lines of force all diverging from one end and converging again 

 at the other end. In this way a compound magnet is formed. 



There are various special magnets to which it is only necessary to allude in passing. Very thin magnets are 

 manufactured from clock-springs by cutting a long, thin strip of steel in the form of a very elongated rhombus and 

 magnetising the cut portion by stroking it with another magnet, so that the poles are at the acute comers. Such a 

 magnet forms the so-called magnetic needle so useful in navigation and in electric experiments generally. 



If a short, thick bar of steel be magnetised perpendicularly to its axis, " the transversely magnetised body so 

 obtained has polar regions on the two sides of its bounding surfaces." If the length of the bar be excessively small, 

 a plate with poles at opposite points of its circumference is produced. A steel ring can be magnetised to have 

 poles at opposite ends of a diameter. If a rod be strongly magnetic at both ends, and the length of the rod in the 

 direction of the axis sufficiently reduced, a plate which has a distribution of magnetism on each face is produced. 

 This gives rise to the " magnetic shell." In this case the fines of force proceeding from one face bend round, 

 embrace the rim of the plate, and reach the other face, where they terminate. A long bar-magnet may be regarded 

 as consisting of a succession of magnetic shells placed face to face, all the faces of one kind being turned in the 

 same direction. 



§ 25. The Horse-shoe Magnet. 



Seeing the effects produced by bar-magnets are most marked at the ends where the fines of force are most 

 numerous, stronger fields are secured by bending the bars into the form of horse-shoes, and so approximatiag 

 the ends. In the horse-shoe magnet the ends are filed to make their terminal faces fie in one plane ; the bar being 

 magnetised along its whole length. The power of the horse-shoe magnet is increased if the magnet is made of thin 

 steel plates separately magnetised. 



If such a magnet be supported vertically with its ends upwards and a lines of force picture formed on a sheet 

 of paper with iron-filings held immediately above it, a figure similar to that seen at Fig. 2, Plate Iv., will be obtained. 



The lines of force picture produced by the horse-shoe magnet is given at Fig. 2, Plate Iv. 



In the median plane of the magnet the fines of force proceeding from the ends n and s of the horse-shoe are 

 closely crowded together, while in the space between the limbs of the horse-shoe they pass by nearly the 

 straightest and shortest paths. Beyond this space the Unes are bent out into wider and wider arcs ; those pro- 

 ceeding from the outer portions of the ends extending to the more remote parts of the field. As the bend of the 

 horse-shoe is reached, the fines of force proceeding from the body of the magnet are diminished in number. Finally, 

 the Unes run parallel to the bend, where very little force is exerted upon them. The bend thus forms the 

 " indifferent zone " of the horse-shoe magnet. 



In the case of compound magnets, where the magnetic forces are strong, the lines of force can be traced not only 

 in a plane, but in three dimensions. The fines of force can also be fixed by stirring iron-fihngs into a warm solution 



