ELEMENTARY LESSONS ON [CHAP. n. 





opposite the " centre of gravity " of this triangle ; or, in 

 other words, the " pole," which is the centre of the result- 

 ant forces, is not at the end of the magnet. In thin 

 bars of magnetised steel it is at about tV of the magnet's 

 length from the end. 



123. Method of Deflections. There are a number 

 of ways in which the deflection of a magnet by another 

 magnet may be made use of to measure magnetic forces. 1 

 We cannot here give more than a glance at first principles. 

 When two equal and opposite forces act on the ends of 

 a rigid bar they simply tend to turn it round. Such a 



pair of forces form what 

 is called a " couple," and 

 the effective power or 

 " moment " of the couple 

 is obtained by multiplying 

 one of the two forces by 

 the perpendicular distance 

 between the directions of 

 the forces. Such a couple 

 tends to produce a motion 

 of rotation, but not a 

 motion of translation. 

 Now, a magnetic needle 

 placed in a magnetic field 

 across the lines of force, 

 experiences a " couple," 

 tending to rotate it round 

 Flg< 58< into the magnetic meridian, 



for the N. - seeking pole is urged northwards, and 

 the S. -seeking pole is urged southwards, with an equal 

 and opposite force. The force acting on each pole 

 is the product of the strength of the pole and the 

 intensity of the " field," that is to say, of the horizontal 

 component of the force of the earth's magnetism at the 



1 The student desirous of mastering these methods of measuring magnetic 

 forces should consult Sir G. Airy's Treatise on Magnetism. 



