Chap. 8] MAGNETIC METHOD 349 



as in a dip ileedle; however, the comparison force is not gravity but the 

 torsion of a hehcal spring. 



The Tiberg inclinato/^ is essentially a compass. The needle rotates 

 on a steel pivot in agate bearings and may be used in either horizontal or 

 vertical plane; the sides of the compass case carry two studs which fit into 

 the standards of the Thal^n-Tiberg magnetometer. 



The combined ThaUn-Tiberg magnetometer consists of a tripod head with 

 two uprights to receive the Tiberg inclinator and a horizontal arm with 

 deflecting magnet for the determination of the horizontal or vertical in- 

 tensity by the tangent method of Gauss or the sine method of Lament.' 

 In the tangent method, the deflection at a base station is given by tan ao = 

 F/Ho , where ao is a normal angle corresponding to the horizontal intensity 

 Ho at the base, and F is 2Afk/r\ At another station, Hi tan ai = F; 

 therefore, 



Hx = 5°J^^° = ^_?_ . (8-40a) 



tan ai tan ai 



In the sine method, the needle is deflected and the deflecting arm is 

 rotated until it is at right angles to the needle. Then, sin ao = F/Ho; 

 at another station, Hi sin ai = F; thus 



^^^Hosinco^ ^^ (8-406) 



sm «i sin ori 



Because of the proximity of the deflecting magnet, F is usually large, and 

 conditions may be encountered were Hi is small compared with Ho sin ao • 

 Then the needle cannot be made to occupy a position at right angles to 

 the deflection arm. This defect can be overcome by using the "Dahlblom 

 sine arm," which makes an angle of 30° with the original arm. On the 

 arm the distance of the magnet (and therefore F) may be varied by a 

 pulley arrangement. The angle of deflection is kept constant at 30°. 

 Then, Ho sin 30° = Fo , Hi sin 30° = Fi , or Ho = 2Fo and Hi = 2Fi . 

 Therefore, Hi = Ho(ro/ri)^, where fo and n are the distances of the deflector 

 corresponding to the fields Fo and Fi . Hence, in principle, the Dahlblom 

 sine arm operates like the Thomson-ThaMn magnetometer. 



7. Other prospecting magnetometers based on balance or compass principle 

 include those designed by Kohlrausch, Schmidt, De Collongue, Ostermeier, 

 Wilson, and the author. In the Kohlrausch deflection magnetometer, a 

 compass is fastened to a vertical shaft^^ which may be moved up and down 

 by rack and pinion in a tubular extension of a pedestal and may be clamped 



3" Illustrated in Figa. 505, 506 s, and 507 s of Berg's catalogue, on pp. 9, 11, 12. 

 " Illustrated in Figs. 507 s, p. 12, and 507 s.d., p. 18, of Berg's catalogue. 

 '2 See illustration in H. Haalck, op. cit., Fig. 25, p. 29. 



