Chap. 8] 



MAGNETIC METHOD 



341 



where (0.707AfZo — mgd)/2iM = ez , the "vertical-intensity scale value" 

 of the instrument. 



For measurements of horizontal intensity, the instrument is used in the 

 magnetic meridian and in two positions. The oblique deflector must be 

 turned about its center when the magnetic system is rotated from a = 

 to a = TT. For the two positions, from eq. (8-36a), 



is' - So)n ^ 0.707M(Z - H) dr mga 

 2f 0.707M(Z + H) ± tngd' 



is' - So)s 0.707ilf (Z + H) ± mga 



when a = 



when a = TT. 



2f 0.707M(Z - H) ± mgd ' 



With the following two scale values for the horizontal intensity ; 



0.707M(Zo + Ho) ± mgd 



(8-36e) 



and 



the readings 



€n 



6s 



2fM 



0.707M(Zo - Ho) zh mgd 

 2filf 



(8-36/) 



6NSN = 0.707(Z - H) ± "» 



esss = 0.707 (Z + H) ± 



mga 



(S-Sijg) 



with Sn = (s' — So)n and Ss = (s' — So)s • 



The difference of the two equations gives the horizontal intensity, so 

 that 



1 



1.414 



(esSs — eNSN) = H, 



while their sum gives the vertical intensity 



^(esSs + ^NSN) =0.707Z±'^'^. 



(8-36/i) 



(8-360 



The instrument may theoretically be used for the determination of the 

 magnetic meridian since the latter is situated midway between two direc- 

 tions near S-E and S-W in which the readings are equal. However, the 

 procedure is involved and has been rarely used. 



4. Miscellaneous modifications of Schmidt magnelometers. In Am- 

 bronn's modification of the Schmidt vertical magnetometer the scale is 

 engraved on the end of the magnetic blade and is read on a prism telescope. 



