MAGNETIC METHODS 



109 



Referring to Figure 35,* it will be seen that the north-seeking end of 

 a compass needle at rest is acted upon by the total intensity force T. This 

 force may be resolved into its horizontal component H and its vertical 

 component Z. On the south-seeking end of the needle a similar condition 

 exists, except that there is repulsion instead of attraction. The vertical 

 components of both the attractive and the repulsive forces tend to rotate the 

 needle in a vertical plane, about its pivot point. To counteract this moment, 

 it is necessary to introduce an opposite and equal efifect by using a small 

 counterweight W. The magnitude of this counter moment is Wd. The 

 distance d must therefore be adjusted when going from one area to another 

 having an appreciable change in value of T. This same principle is utilized 

 in the magnetic system of the vertical field magnetometer. 



Fig. 35. — Diagram showing magnetic components in 

 northern hemisphere. 



When properly adjusted, a compass needle rotates in a horizontal plane 

 and is acted upon only by the horizontal component of the earth's magnetic 

 field ; the torque due to the vertical magnetic component is balanced by 

 the torque due to the counterweight and does not produce a rotative effect 

 on the needle. For use in the southern hemisphere, the counterweight 

 would be on the north side of the pivot. 



The Wilson Attachment for a Brunton Compass 



An attachment for a Brunton compass has been devised by Mr, John 

 Wilson so that horizontal magnetic intensity can be measured. The attach- 

 ment permits the determination of the absolute value of the horizontal 



* See also Figure 24. 



