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Appendix.] 



TERRESTRIAL MAGNETISM 



33 



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ArPENDIX No. 1. 



Description and Use of the Unifiiar Magnetomkxeh 



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The Unifilar Magnetometer consists of two parts—an apparatus for 

 deflection, and an apparatus for vibration : these correspond with the 

 two parts of the process by which the absolute horizontal force is de- 

 termined; the experiments of deflection consist in observing the angular 

 deflection of a suspended magnet produced by the iniiuence of a second 

 magnet, which is placed on a support at one or more known distances 

 from the suspended magnet, and in a line drawn from its centre per- 

 pendicular to its direction : the experiments of vibration consist in 



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suspending th^^ magnet which was used -as the deflecting magnet in 

 experiments of deflection^ and observing its time ^^f vibration. By the 

 first part of the process (or the experiments of deflection) we obtain the 

 ratio of the magnetic moment of the deflecting magnet to the Earth's 

 horizontal magnetic force at the place of observation ; the latter being 

 to the former as 1 to the sine of the angle of deflection multiplied by half 

 the cube of the distance employed ; — or if m denote the magnetic moment 

 of the needle, X the Earth s horizontal force, r the distance apart of the 

 centres of the magnets, and u the angle of deflection, the expression 



m 

 X 



J r^ sin u. 



By the second part of the process (or the experiments of vibration) the 

 product of the same two qualities is obtained : being the quotient of a 

 constant, which we may call t^ K (see p. 3S), divided by the square of 

 the time of vibration: or if T be the time of vibration. 



m X 



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«K 



T* 



m 



The values of m X and ~ being known^ those of m and X may be 



X 



obtained separately: for if we call m X :^ a and 



m 

 X 



jS, m (the 



magnetic moment) = V ^ /^' ^"*^ ^ (the horizontal component of the 



Earth's magnetic force) 



DefuJcion Ea-perments, 



1. Place the tripod base and copper box upon the stand, and screw 

 on the torsion tube; form a thread capable of carrying the smaller 



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