59$ MAGNETIC FIELD. 



Dr. Lloyd* acted on a declination compass by one end of a 

 vertical soft iron bar, which acquires an induced magnetism pro- 

 portional to the component Z. Observation gives thus a quantity 

 proportional to the ratio of the components Z and H, and con- 

 sequently to the tangent of the inclination. 



This method was used by Lament, f It has the advantage of 

 giving all the elements of the earth's magnetism by a single in- 

 strument. 



In Lament's theodolite a bar of soft iron, or, better, two sym- 

 metrical bars, are arranged vertically in a fixed mounting in such 

 a manner that the top of one and the bottom of the other are 

 in the plane of the movable magnet. The action of the induced 

 magnetisation on the movable magnet may be represented by CZ, 

 and if the deflecting bars are in a plane perpendicular to the 

 magnet, the mean deflection a gives 



GZ 



sma = - = Ctan I, 

 H 



the coefficient C being determined by comparison with a dip needle. 

 A series of reversals render it possible to eliminate the defects of 

 symmetry of the bars and their residual magnetism. 



The properties of iron are, however, modified by so .many 

 causes, physical and mechanical, that we can never be certain of 

 the bar being always in the same condition. Experiment shows, 

 in fact, that the coefficient C is not invariable, and that we obtain 

 thus only an approximate value for the inclination. 



1169. INDUCTION METHOD. In Weber's J induction inclino- 

 meter, the ratio of the components Z and H is determined by 

 discharges induced in a frame which turns through 180 about a 

 vertical from a plane perpendicular to the meridian and about* a 

 horizontal parallel to the meridian starting from a horizontal plane 

 (529). 



With this arrangement it is necessary to measure the swings of 

 the ballistic galvanometer, which corresponds to two induced dis- 

 charges, and the relative error of the inclination of the same .order 

 as that of the two readings. 



* LLOYD. Account of the Magnetic Observatory in Dublin. 1842. 

 f LAMONT. Handbuch des Erdmagnetisimis, p. 212. 1849. 

 % W. WEBER. Pogg. Ann., Vol. XLIII., p. 293. 1886. 



