364 MAGNETIC METHOD [Chap. 8 



horizontal intensity. Therefore, the E-W gradient of the vertical intensity 

 is measured. When the frame is now turned into the N-S direction, the 

 axes of rotation are at right angles to the horizontal intensity so that both 

 H and Z (that is, the total intensity T) are now effective. Roman actually 

 measures T by setting the brushes at right angles to the field (which 

 requires a knowledge of the inclination) and calculates Z from it. This 

 makes the method laborious, but it can be avoided if provision is made to 

 turn the coils in their frame in such a manner that their axes of rotation 

 always point north. 



3. (Iron) induction instruments are designed to measure the inclination, 

 the vertical intensity, and the magnetic meridian. Induction inclinom- 

 eters consist of a magnetic needle suspended from a thread or wire, and 

 two vertical iron bars mounted opposite each end of the magnetic needle. 

 One bar is up from the plane of the needle; the other is down. There M^ill 

 be a south pole induced in the upper end of the lower bar and a north pole 

 in the lower end of the upper bar. This deflects the needle from the mag- 

 netic meridian. Naturally, opposite poles are also produced in the bars 

 by the poles of the needle. This effect can be kept down and is neglected 

 in the following formulas. If F is the force produced on the suspended 

 needle from the induction in the bars, the deflection in the Lamont method 

 is given by sin (p = F/H, where F = 2Zk"/(f, k" is an induction factor, 

 and d is the distance between needle poles and poles induced in the bars. 

 Hence, 



If an astatic magnetic system instead of the magnetic needle is used, 

 the effect of the horizontal intensity is eliminated, and the torsion of the 

 suspension wire furnishes the restoring force that opposes the effect of 

 induction in the bars. In the vertical intensity magnetometer designed 

 by McNish,^^ the astatic system is represented by an armature consisting 

 of two vertical bars mounted on the ends of a nonmagnetic vane (see 

 Fig. 8-39). 



For magnetic meridian determinations, Rieber^^ has designed a micro- 

 magnetometer consisting of two iron bars mounted end-to-end with a small 

 gap and the fiber of a string galvanometer between them. When alter- 

 nating current of about 100 cycles is passed through the string, and if the 

 bars are in the magnetic prime vertical, the induction and string deflection 

 are zero. 



" Terr. Mag., 41(2), 161 (June, 1936). 



" A.I.M.E. Geophysical Prospecting, 410 (1929). 



