140 EXPLORATION GEOPHYSICS 



system toward the knife edge, or axis of rotation, which causes a decrease 

 in the pull of gravity on the system. This, in turn, counteracts the decrease 

 in reading due to the lesser magnetic moment. When properly designed 

 and adjusted, the temperature-compensated systems have zero temperature 

 coefficients. 



In these systems temperature changes do not affect the position of the 

 weight Wz on the latitude spindle in relation to the axis of rotation. The 

 reason for this is that this spindle is made of invar metal which, for all 

 practical purposes, has a negligible coefficient of expansion for changes 

 in temperature. 



In the earlier Askania systems an attempt was made to minimize tem- 

 perature effects by controlling the relationship of the magnetic blades to the 

 aluminum block to which they were attached. The knife edge, the latitude 

 adjustment and sensitivity screws, and the resulting center of gravity of 

 the system as a whole, were designed so as to compensate for temperature 

 changes in the manner explained above. The compensation was, however, 

 not complete. 



Various means of temperature compensation for the old Askania sys- 

 tems have been proposed, utilizing the difference in the thermal coefficients 

 of the different materials in the magnet system.f 



With the new, or compensated, systems, it is still necessary to check 

 the temperature coefficient from time to time to find out if it is truly zero. 

 If it is not zero, the position of the weight Wi on the temperature spindle, 

 as shown in the figure previously referred to, can be adjusted and locked in 

 position by its small set screw so as to bring the coefficient to zero. 



The distance a between the center of gravity of the system and the 

 axis of rotation (as shown in Figure 54) is 0.1 mm., measured along the 

 magnetic axis of the system. The distance h or the displacement of the 

 center of gravity below the axis of rotation is even smaller, being 0.08 mm. 

 These minute distances are quite critical in affecting the temperature 

 coefficient and scale value of a particular system. It is apparent therefore 

 that a magnetometer, although it is a field instrument, must be handled 

 with great care and protected from jolts and jars which might change the 

 gravity center to rotation axis relationships. In cases where an instrument 

 is accidentally dropped or jolted, the scale value and temperature coefficient 

 should be checked. Magnetometers should also be kept away from strong 

 magnets or high amperage direct current sources, as the magnetic moment 

 of the system might be affected. 



Temperature Correction. — Where the temperature coefficient of a 

 magnetometer is not zero, a correction must be applied to the instrument 

 readings. As will be seen later, in the section on the calculation of field 

 notes, the readings at a series of stations are reduced to a common temper- 

 ature. The reading at a station, in such a series, after the application of 



t T. Koulomzine, "Temperature Compensation of Old Type Askania Magnetometers," A.I.M.E. 

 Mining Transactions, pp. 133-136, Vol. 184, May, 1949. 



