308 MAGNETISM 



Suppose that thedip circle is in a plane making an unknown aii^lc 

 with the magnetic meridian. If H is tin- hori/ontal intensity in 

 the meridian, the effective horizontal intensity in the piano con- 

 sidered is H cos 6 9 and if S x is the dip observed when the needle is 

 free to move in this plane, and if D is the true dip, 



The circle is now turned through 90 about the vertical axis and 

 the dip (5 a in the new position is observed. Then 



cot 3, = S cos (0-90) = cot D sin 



and cot 2 ^ 1 +cot*^ 1 = cot 8 D, whence D is obtained from observation 



of S l and S r 



The earth inductor. There is a totally different method of 

 obtaining the dip the earth-inductor method. It is cmpl. 

 at some observatories and appears likely to come into 

 use. The principle may be given thus: Suppose that a coil 

 in the form of a large ring has total area A, counting the 

 areas of all the turns. Let it be laid Hat on a hoi 

 and be connected to a ballistic galvanometer. The total flux 

 of induction through it is AV, where V is th \crtieal ii 

 Now let it be turned over suddenly so that the oilier face of the 

 coil is on the table. The change in induction is .AV. and the 

 throw of the galvanometer will be due to a flow <>: 

 proportional to 2AV. Say that the i meter ^JM-S thi- Mow 



as Qi coulombs. Now let the coil be raised into the \ rtical plane 

 perpendicular to the magnetic meridian. The total flux thr 

 it is now All, where II is the hori/ontal intensity. Now let 

 turned suddenly round a vertical axis through 180. Tip 

 in induction is 2AH. Say that the galvan ..... t r indicates a 

 of Q 2 coulombs. Then 



2AV V 



The intensity of the field. The total intensity I is 

 determined directly. If H is the horizontal component and 1) is 

 the dip, H = I cos D. The value of H can be found much more 

 easily and much more exactly than I, and if D is known I may In- 

 deduced if needed. But in almost all CHM I ire want to know H. 



To determine the horizontal intensity H. The 

 horizontal intensity is determined by a combination of two 

 separate experiments. By one, known as the vibration experi- 

 ment, we find the product of H and of M the magnetic moment of 

 the magnet used. By the other, known as the deflection experi- 



