METHODS OF DEFLECTION. 573 



It is to be observed that, for deflections near 90, the oscillations 

 only depend on the rigid magnetism M . 



1144. We may still arrange the auxiliary field in a direction at 

 right angles to the terrestrial field, and determine the torsion neces- 

 sary for bringing the needle into its original direction. According to 

 the mode of suspension, we shall have 



Cw = M F, or Csino> = M F. 



The experiment is the same as for measuring currents by the method 

 of torsion (830), and the defects of adjustment will be eliminated by 

 observations made in both directions, right and left. 



1145. METHODS OF DEFLECTION. When the field F makes a 

 considerable angle a with the magnetic meridian, and the needle is 

 suspended by a wire without torsion, the condition of equilibrium 

 relative to the deflection 8 is 



F sin (a - 8) = H sin 5, 







and, if the two fields are rectangular, 



F = Htan8; 



the defects of adjustment are eliminated as in the tangent galvano- 

 meter. 



We may also work as with the sine galvanometer. The needle 

 being in the magnetic meridian, the field is made to act at right 

 angles to the original direction of the needle, and it is then turned 

 until it resumes the same position in respect of the needle that is 

 to say, that it is perpendicular to its final direction. If 8 is the 

 rotation observed, we have 



F = HsinS. 



These two methods, which were first utilised by Gauss for ob- 

 serving the action of a bar magnet, are equivalent to the use of 

 galvanometers in observing currents. Here we need not allow for 

 variations which the magnetism of the needle experiences, for its 

 magnetic action is parallel to the direction of the resultant field. 



In this case, as in the galvanometer, the dimensions of the 

 needle would have to be allowed for when the field in question 

 is not uniform in the space it occupies. 



