332 



MAGNETIC METHOD 



[Chap. 8 



prove the accuracy. Plotting or recording the deflection and temperature 

 curves separately has the advantage that the time lag between the reaction 

 of the magnetic system and temperature may be allowed for by shifting 

 them by the amount of the lag and calculating the temperature correction 

 from corresponding values of reading and temperature. 



The base correction allows for gradual or abrupt changes of the instru- 

 ment in the course of a survey. It is determined by checking back at a 

 base station once or twice a day. If the difference in readings on checking 



in on the base is small (1 or 2 scale divi- 

 sions), and if numerous stations have been 

 made between base checks, the base cor- 

 rection applied to any station p is 



(s2 - Si) 



(8-27) 



where Si is the first corrected reading, S2 

 the second reading, n the number of sta- 

 tions made between checks, and p the 

 number of the station after the first base 

 reading. The application of the formula 

 is based on the assumption that the change 

 takes place linearly. If there is cause to 

 assume an abrupt change in reading, the 

 base correction must be applied accord- 

 ingly. 



Magnetic variations are measured and 

 corrected for as in all other magnetic field 

 observations; procedures vary with accu- 

 racy requirements and are discussed on 

 page 367. A convenient recorder to use in 

 connection with Schmidt magnetometers is 

 illustrated in Fig. 8-22. 



The latitude correction is a correction for 

 the normal or planetary change of the in- 

 tensity and is obtainable from government magnetic maps. In the verti- 

 cal and horizontal intensities, the major components of planetary change 

 are in the N-S direction. As the vertical intensity increases toward north, 

 the correction in this direction is negative; for the horizontal intensity 

 which decreases toward north, the correction is positive. 



In the field operation of the instrument, the observer must exercise 

 sufl5cient care not to have about his person any iron or steel objects, such 

 as pocket knife, watch, keys and keytainers (if they contain iron), some 

 makes of belt buckles, pencil clips, some makes of fountain pens, ring-binder 



American Askania Corp. 



Fig. 8-22. Recording arrange- 

 ment with magnetic system. 



