CHAP, ii.] ELECTRICITY AND MAGNETISM. 99 



ducing this twist corresponding to 12 x 35 (or 420) of 

 torsion ; and to these the actual torsion of 12 must be 

 added, making a total of 2880 + 420 + 12 = 3312. 

 The result then of halving the distance between the 

 magnet poles was to increase the force fourfold, for 

 3312 is very nearly four times 864. Had the distance 

 between the poles been reduced to one-third the force 

 would have been nine times as great. 



120. Method of Oscillations. 1 If a magnet sus- 

 pended by a fine thread, or poised upon a point, be 

 pushed aside from its position of rest, it will vibrate 

 backwards and forwards, performing oscillations which, 

 although they gradually decrease in amplitude, are 

 executed in very nearly equal times. In fact, they follow 

 a law similar to that of the oscillations executed by a pen- 

 dulum swinging under the influence of gravity. The law 

 of pendular vibrations is, that the square of the number 

 of oscillations executed in a given time is proportional to 

 the force. Hence we can measure magnetic forces by 

 counting the oscillations made in a minute by a magnet. 

 It must be remembered, however, that the actual number 

 of oscillations made by any given magnet will depend 

 on the weight, length, and form of the magnet, as well 

 as upon the strength of its poles, and of the " field " 

 in which it may be placed. 



121. We can use this method to compare the intensity 

 of the force of the earth's magnetism 2 at any place with 

 that at any other place on the earth's surface, by oscil- 

 lating a magnet at one place and then taking it to the 

 other place and oscillating it there. If, at the first, it 

 makes a oscillations in one minute, and at the second, b 

 oscillations a minute, then the magnetic forces at the 



1 It is possible, also, to measure electrical forces by a "method of oscil- 

 lations;" a small charged ball at the end of a horizontally-suspended arm 

 being caused to oscillate under the attracting force of a charged conductor 

 near it, whose " force" at that distance is proportional to the square of the 

 number of oscillations in a given time. 



z Or, more strictly, of its horizontal component. 



