604 MAGNETIC FIELD. 



adopted. The comparison of the magnetic field of the magnet with 

 the terrestrial field is made by a method of deflection (1145). The 

 auxiliary needle of the declinometer is placed in a principal position 

 in respect of the bar, either on its direction (first position), or in the 

 plane of the equator (second position). 



Let us consider the first case, for instance, and suppose that the 

 deflecting bar is perpendicular to the magnetic meridian; its moment 

 is not modified by the action of the earth, and the deflection a of the 

 declinometer satisfies the equation 



F 2M 

 (23) ta 



At the distance R', the deflection a! will give, in like manner, 



2 M 



(24) 



1176. It is important to investigate how we ought to choose the 

 ratio of the distances R and R', to determine as exactly as possible 

 the term of correction p. Putting 



R 



we deduce, from equations (23) and (24), 



V 



i tan a! 

 p 3 tan a 



(25) *TT~ 



-- P 



p 3 tan a . 



As the value of/ is less than o - o2 when the distances are at least 

 four times the length of the deflecting magnet, and the lengths of 



the bars in the ratio of 2 to i (1155), the fraction - - is near 



p 3 tana 



unity ; if the deflections a and a are themselves small, which is the 

 ordinary case, we get as approximate value 



I p' 2 p :3 (l p 2 ) a 



