148 EXPLORATION GEOPHYSICS 



Determination of the Scale Value. — Before conducting a magnetic 

 survey, it is desirable to determine the scale value of the instrument. The 

 differential magnetic intensity per scale division, generally expressed in 

 gammas per scale division, is obtained by observing the deflection of the 

 magnetic system produced by a magnet of known magnetic moment placed 

 in one of two positions, and applying the required mathematical formulae 

 involving the magnetic constants and the distances. 



Instead of using a magnet of known magnetic moment, the scale value 

 may also be determined with the aid of a Helmholtz coil arrangement, as 

 discussed previously. 



The steps outlined below illustrate the procedure at each station when 

 using a vertical component Schmidt field balance after the instrument has 

 been removed from the box and has acquired the temperature of the air. 



(1) Remove all iron objects from the observer's person and from the 

 immediate vicinity of the instrument. Place the carrying-box with its set 

 of auxiliary magnets and other magnetic materials at a distance of 30 to 

 50 feet from the station. 



(2) Set up and level the tripod. Carefully orient the tripod head at 

 270° on the compass, and clamp it. (With the tripod head oriented in 

 this direction, the magnetometer will set in the head at right angles to the 

 magnetic meridian.) Remove the compass to a safe distance from the 

 instrument. 



(3) Clamp the magnetometer on the turntable. Level the instrument 

 accurately by means of the level screws. 



(4) Release the magnetic system by means of the arresting device 

 and note whether the reflected scale remains in the field of vision. If the 

 instrument has been properly adjusted for latitude, the scale will be in view. 

 If the scale is not in view and it disappears because of a large anomaly, 

 employ an auxiliary magnet to bring it into the field of vision, recording 

 which magnet is used and its position and distance. 



(5) Read and record the scale reading. Revolve the instrument 

 through 180°* and again read and record the position of the scale. Repeat 

 these readings two or more times to obtain representative readings. 



If instrumental trouble is not encountered, careful manipulation will 

 give readings which check within one or two tenths of a scale division. f 



(6) Read and note the temperature and time. 



* Some tripods have an automatic reversing stop for this operation. Old-style tri- 

 pods carry graduations in degrees on the lower and fixed portion of the tripod head and 

 two index marks on the moving portion of the turntable or head. The appropriate index 

 is used for the 180° reversal. 



t Poor check readings may be due to the following causes : a. mis-orientation ; 

 b. misleveling of tripod ; c. misleveling of the instrument when placed on the tripod ; 

 d. poor adjustment of the tripod or instrument levels; e. one quartz bearing, on which 

 the knife edge rests, being higher than the other ; f . chipped knife edge or bearing ; 

 g. dust or other foreign matter inside the instrument and on knife edge or bearings; 

 h. improper releasing of system and letting it down on bearings too rapidly. 



