MAGNETIC METHODS 



235 



In one type of instrument measuring the vertical component of the 

 total intensity, the "sensing element" is mounted so that its axis of sensi- 

 tivity lies normally in the vertical plane. The sensing element is kept vertical 

 by a gyroscope which in turn is stabilized in order to minimize errors 

 normally caused by the precession of the gyro. With the vertical orienta- 

 tion, the greatest error per unit of deviation from the vertical occurs when 

 the deviation takes place along the magnetic meridian and the minimum 

 error when the deviation is normal to the plane of the magnetic meridian. 



*H, sin a, 



-H, sin a — ^^ 



SENSING ELEMENT --.^ \ 



HORIZONTAL PLANE 



-Vo cos /3, 



/-v 



ANGLES OF DEVIATION /i Kli, 

 ALONG MAGNETIC MERIDIAN 



DEVIATION « FROM VERTICAL PLANE 

 NORMAL TO THE MAGNETIC MERIDIAN 



(a) 



(b) 



Fig. 116. — (a) Angles of deviation R and Qi along magnetic meridian; (b), 

 deviation a from vertical plane normal to the magnetic meridian. (Q)urtesy 

 of Hans Lundberg.) 



Referring to Figure 116, 



Let 6 = Angle of "dip" of total magnetic field intensity vector 

 To = Total magnetic field intensity vector 

 Vo = Vertical component of To 

 Ho = Horizontal component of To 

 /? = Deviation of sensing element from the vertical plane along 



magnetic meridian North 

 /?i = Deviation of sensing element from the vertical plane along 

 magnetic meridian South 

 a = Deviation of sensing element from the vertical plane 

 normal to the magnetic meridian. 



V = Magnetic field intensity acting on the sensing element 

 Vo — To sin 6 



Ho — To cos 9 



V = Vo=To sin 6 when the axis of sensitivity of the sensing element 



lies in the vertical plane. 



