MAGNETIC METHODS 147 



to change auxiliary magnets or the distance of a magnet on the extension rod. 



If precise work involving an accuracy of 5 gammas or better is neces- 

 sary, auxiliary magnets should not ])e employed for extending the scale 

 of the instrument. At such times, the only procedure is to open the case 

 of the instrument and adjust the latitude screw until the desired scale 

 reading is obtained. The sensitivity adjustment should not be changed 

 unless the work clearly shows the need for such a change. After shifting 

 the scale reading by the latitude adjustment weight, repeat readings should 

 be made at one or two prior stations to effect a proper tie-in of data. 



Adjustment of Instrument Before Field Work 



Latitude Adjustment. — This adjustment is accomplished by rotating 

 the lateral screws until the zero point of the reflected scale is approximately 

 at the middle of the scale.* In regions where the magnetic gradient is 

 small, no other adjustment will be required to keep the scale in the field of 

 vision. However, in regions where igneous rocks outcrop at or near the 

 surface (e.g., in many mining regions), the magnetic anomalies are often 

 large and show rapid variations. Under these conditions, it is often neces- 

 sary to use auxiliary magnets to keep the scale within the field of vision. 

 (The auxiliary magnets are so mounted that their effect on the magnetic 

 system opposes that of the geologic anomaly). 



Conditions of the Knife-Edge, — Sensitivity and accuracy of the 

 instrument depend in large measure on the condition of the knife-edge. 

 The operator must take every precaution, therefore, to protect the fragile 

 quartz-edges on which the system pivots. The magnetic system must 

 always be clamped, except during the time when a reading is being made. 

 If the knife-edge is lowered upon the quartz bearings with a slow move- 

 ment, and if the instrument is not jarred while the knife-edge is resting 

 on the bearings, the edges and bearings should last as long as the 

 instrument. 



Special precautions must be taken to protect the instrument from dust 

 and lint when the case is open for adjustment of the magnet system. The 

 instrument should never be opened in windy or dusty places, A minute 

 particle of grit between the knife-edge and the bearings will cause erratic 

 readings and changes in sensitivity. Do not touch the fingers to the knife 

 edges or the bearings. To do so deposits a film of organic matter that 

 causes irregular operation of the balance. To remove such foreign material, 

 the bearing surfaces should be wiped carefully, just prior to closing the 

 instrument case, with a linen cloth dampened with pure ether or 

 chloroform. 



* In old-style magnetic systems the two latitude screws are on the ends of the block 

 to which the magnetic blades are attached. In the compensated systems the latitude 

 adjustment weight is on the small diameter spindle on the south end of the system. 



