Sect. II.] 



TERRESTRIAL MAGNETISM. 



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Absolute Measurement of the Maanetic Force. 



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3. No satiBfactory method has yet been brouglit into 

 practice for the direct absolute measurement in one opera- 

 tion of the vjhole magnetic force of tlie Earth (called the 

 ^' total force'') at any particular point of its surface. But 

 that portion of the force whicli acts in a direction parallel 

 to the surface of the Earth (called the " horizontal com- 

 ponent") may be measured with considerable accuracy by 

 a process, of which the following brief description may suf- 

 fice to give a general idea. If a magnet be suspended 

 horizontally by a few fibres of silk, and made to vibrate in 

 the horizontal plane on either side of its position of rest, 

 the square of the number of vibrations in a given time is 

 a measure of the horizontal component of the magnetic 

 force of the Earth, but is also dependent on the individual 



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properties of the magnet employed. Inese properties 

 influence the time of vibration in two respects : first, by 

 the gr^r-^ter or less magnetic force which the magnet itself 

 possesses ; and, secondly, by the effect which the form and 

 weight of the magnet produce on the time of vibration. 

 The latter effect may be eliminated wlien the moment of 



maemet 



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ither be 



calculated by known rules, or may be ascertained experi- 

 mentally by vibrating the ma 



net 1^ in its usual state, and 



2^ with its momont of inertia increased by a known amount. 

 The influence of the magnetic force possessed by the mag- 

 net may also be eliminated by ascertaining its. magnetic 

 moment. This is accomplished by using it to deflect a 

 second magnet similarly suspended in another apparatus. 

 The deflecting magnet is placed at one or more exactly 



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