MAGNETIC METHODS 



93 



Measurement of D, H, and I. 



Oscillations and Deflections. — To measure the horizontal magnetic 

 intensity at a station, it is necessary to perform two experiments which 

 require the use of a bar magnet of moment M, called the deflector, and a 

 compass. In one experiment the quantity M/H is determined where H 

 is the horizontal intensity of the earth's field at the point of observation. 

 In the other experiment the quantity MH is found. This gives two 

 simultaneous equations and two unknowns. 



Deflection experiment: See Figure 25. 



MAGNETIC NORTH 



PLAN VIEW 



Fig. 25. — Set-up for deflection experiment to 



measure M/H. 



Deflector magnet M 



r = cm. H = earth's field, horizontal component. 



6 = angle between magnetic A^ and compass needle at rest. 



M 

 With such a setup -^ = J^ r^ tan 6 



Oscillations experiment: see Figure 26. 



(39) 



SIDE VIEW 



Fig. 26. — Set-up for oscillations experiment to 

 measure MH. 



Using the deflector magnet, from the previous experiment its period 

 of oscillation To is determined when it is suspended on a fiber and swung 

 in the earth's magnetic field. In this case, 



