CHAP. IL] ELECTRICITY AND MAGNETISM. 105 



Lesson XXV. See also Sir George Airy's Treatise on 

 Magnetism. 



125. Unit Strength of Pole. We found in Cou- 

 lomb's torsion-balance a convenient means of comparing 

 the strengths of poles of different magnets ; for the force 

 which a pole exerts is proportional to the strength of the 

 pole. The Second Law of Magnetic Force (see Art. 

 1 1 6) stated that the force exerted between two poles 

 was proportional to the product of their strengths, and 

 was inversely proportional to the square of the distance 

 between them. It is possible to choose such a strength 

 of pole that this proportionality shall become numerically 

 an equality. In order that this may be so, we must 

 adopt the following as our unit of strength of a pole, or 

 unit magnetic pole : A Unit Magnetic Pole is one of such 

 a strength that^ when placed at a distance of one centi- 

 metre from a similar pole of equal strength it repels it 

 with a force of one dyne. If we adopt this definition we 

 may express the second law of magnetic force in the 

 following equation : 



where f is the force (in dynes), m and m' the strengths 

 of the two poles, and d the distance between them (in 

 . centimetres). This subject is resumed in Lesson XXV., 

 Art. 310, on the Theory of Magnetic Potential. 



126. Theory of Magnetic Curves. We saw (Art. 

 1 08) that magnetic figures are produced by iron-filings 

 setting themselves in certain directions in the field of 

 force around a magnet. We can now apply the law of 

 inverse squares to aid us in determining the direction 

 in which a filing will set itself at any point in the field. 

 Let N S (Fig. 59) be a long thin magnet, and P any 

 point in the field due to its magnetism. If the N.- 

 seeking pole of a small magnet be put at P, it will be 

 attracted by S and repelled by N ; the directions of these 

 two forces will be along the lines P S and P N. The 



