128 



AKSEL S. STEEN. TERRESTRIAL MAGNETISM. [NORW. POL. EXP. 



side of the needle outwards, and as naturally no inversion of the poles could 

 be performed, since the needle was also to be employed for determinations 

 of the intensity, the mean value of the inclination found by the method 

 described above is only relative, and must be corrected for the index-error 

 of the needle in question. 



DETERMINATION OF THE INDEX-ERROR. 



If the inclination observed with the apparatus is called I', the actual 

 inclination J, and the index-error of the needle used J, we have 



7=7' + ^. 



This index-error is due to two different sources, namely, first, the cir- 

 cumstance that the magnetic axis of the needle does not coincide with its 

 geometrical axis, and secondly, that the needle's axis of rotation does not 

 pass exactly through its centre of gravity. 



Fig. 3. 



In fig. 3, NS indicates the needle's geometrical axis, N'S' its magnetic 

 axis, the angle between them being called c. Q is the needle's centre of 

 gravity, and the point of intersection of the axis of rotation with the 

 vertical plane. The line connecting these two points, r, then makes with 



