MAGNETIC METHODS 125 



table (Figure 51 A) contains guide holes so that either the compass or the 

 magnetometer may be placed in the same relative position on the turntable 

 at a series of stations. In the center of the turntable is the level bubble 

 used in preliminary leveling of the tripod head for initial orientation. The 

 stop-ring is so adjusted that the turntable may be turned exactly 180° 

 without reading the scale or compass, thereby facilitating check readings. 



The turntable is oriented by placing the compass on it. (See Figure 

 5 IB.) Orientation of the instrument must be correct to within one-half 

 degree of arc. This requires care and is one of the sources of errors mili- 

 tating against accuracy. Misalignment of the vertical instrument causes an 

 error which is chiefly due to two factors : (a) a small portion of the 

 horizontal magnetic component of the earth's field will act on the magnet 

 system, and (b) the scale constant of the instrument will be altered. Mis- 

 alignment of the horizontal instrument introduces a small portion of the 

 vertical component which decreases the true reading. However, the sen- 

 sitivity of the horizontal instrument is not affected by misalignment of a 

 few degrees. 



Leveling. — Initial leveling of the tripod head is accomplished by the 

 level bubble. This leveling is sufficiently accurate for orientation of the 

 turntable by the compass. After the orientation step is completed, the 

 compass is removed from the turntable and the magnetometer is placed 

 on it in the proper orientation for that station. The instrument is now 

 accurately leveled by adjusting the three leveling screws on the tripod 

 head and observing the two levels fastened to the magnetometer itself. 

 The leveling should be accurate to within a half-division of the level 

 bubbles (15 seconds of arc) ; under these conditions, errors due to in- 

 correct leveling will be less than 0.1 of a scale division. If the leveling is 

 changed by one division, due to clamping, unclamping, or rotation of the 

 turntable, the instrument should be releveled and the reading repeated. 



One prevalent cause of shifting of the instrument is due to improper 

 placing of the tripod when first set up at the station. The tripod legs 

 should make an angle of about 30° with the vertical and should be firmly 

 pressed into the ground when initially set up. Apparent shifting of the 

 instrument may be due to unequal expansion of the level glasses and 

 supports when in the direct rays of the sun. The operator should stand 

 between the instrument and the sun or employ an umbrella. Instruments 

 at base stations should be shielded from direct sunlight and protected from 

 wind and rain by suitable shelter. 



Instrument Readings. — The optical system in the Askania magneto- 

 meters is shown in schematic form in Figure 50. Deflections are read by 

 observing the relative displacement of two scale images. In the magnet- 

 ometers manufactured prior to about 1935, the scale had 40 equal divisions, 



