122 



EXPLORATION GEOPHYSICS 



instrument is being moved. This arrangement prevents "chipping" or 

 other injury to the edges and also provides a means for resetting the 

 system in the same place on the bearings. For transportation of the instru- 

 ment, the magnet system must be clamped in the arrested position. Oscil- 

 lations of the magnet system are dampened by eddy currents induced in 

 copper plates (removable) placed near the poles of the magnets. 



C D 



Fig. 49. — Magnetic systems of Askania magnetometers. 

 A — New-type, horizontal component, temperature-compensated system. 

 B — Uncompensated, horizontal component system. 

 C — New type vertical component, temperature-compensated system. 

 D — Uncompensated, vertical component system. 



1 — Temperature compensation weight. 5 — Quartz knife-edge. 



2 — Temperature compensation spindle (aluminum). 6 — Mirror. 



S — Latitude adjustment weight. 7 — Aluminum frame. 



4 — Latitude adjustment spindle (invar steel). 8 — Magnets. 



In order to provide for temperature corrections, two thermometers 

 (having ranges of —15° to 24° C. and 17° to 55° C.) are fastened inside 

 the case. The magnet system and thermometers are enclosed in a cork- 

 lined, aluminum casing to minimize rapid temperature changes. 



Small displacements of the magnet system from the horizontal are 

 directly proportional to the vertical intensity. They are measured by means 

 of a telescope with a gauss eyepiece using the optical arrangement shown 

 in Figure 50. Sky light is reflected from a mirror, passes through a ground 

 glass window in the side of the telescope and falls on a reflecting plate 

 which deflects it downwardly so that it illuminates a scale. The scale is 

 etched on a transparent glass plate situated at the focus point of the ob- 

 jective lens. (The latter is located vertically above the mirror attached 

 to the rnagnet system.) The light transmitted through the scale plate 



