

TERRESTRIAL MAGNETISM. 



[Sect. IL 



ApP* 



niagnet, ?'. e,, the magnet of 3-00 inches In lengtli (a single fibre of silk 

 is generally strong enongh to support it); affix the deflecting rod, and 

 the telescope and scale ; level the base circle very correctly, suspend the 

 3-inch magnet, and raise it until it is in the same horizontal plane -with 

 the deflecting nijgnet when resting on the supports. 



2. Place the 3-67-inch (or deflecting) magnet on the graduated support 

 to the ^est of the suspended magnet, with its centre at the distance de- 

 termined upon, and "with the north end towards the East. Turn the 

 telescope in azimuth until the central division of the scale attached to 

 the telescope is on the vertical wire ; write down the readings of the 

 %^erniers, and the height of the thermometer placed near the deflocting 

 m agnet. 



3. Reverse the deflecting magnet, placing the south end where the 

 tiorth was, and bringing its centre precisely to the same distance from 

 the suspended magnet as before. Turn the telescope until the central 

 division of the scale is again on the vertical wire, and write down the 

 readings^ together with the height of the thermometer. 



4. Eepoat these observations by reversing the deflecting magnet again 

 and again^ until, judging by the agreement of the several results, suffi- 

 cient accuracy appears to be obtained. Two or three repetitions will 

 generally be found to suffice. 



5. Place the deflecting magnet at the same distance on the opposite 

 support, and repeat the > 



series of observations, commencing as before 

 with the north end of the magnet towards the East. Take a mean of 

 the readin|i,o of the circle in the 1st and Srd positions of the deflecting 



magnet, and another mean of the readings in the 2nd and 4th positions ; 

 half the difR^^rence of these means will be the angle u corresponding to 

 the distance r. The distance r should be such as to produce a sufuciently 

 large deflection iu the suspended magnet. At Woolwich, where the 

 value of the horizontal component of the terrestrial force is 3' 7? the de- 

 flecting magnets of 3*67 inches m length, when recently magnetized, will 

 usually produce a deflection of about 10", when the distance from centre 

 to centre of the magnets is 1-3 foot But with a constant distance the 

 deflection would inert: je in localities where the horizontal component is 

 less, and diminish where it is greater: the deflection would also be 

 diminished if the deflecting magnet lost any portion of its original charge. 

 It is not desirable that the deflection should be less than 6^. 



When the diitanee required to produce a sufficient deflection is less 

 than ii\v times Vi^^ length of the deflecting magnet, the amount of the 

 deflection is liable to be influenced by the distribution of the magnetism 

 in the m_agncts employed, which does not follow the same law in all 

 magnets. In such cases it is necessary to employ two or more distances 



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