[Chap. 8 



MAGNETIC METHOD 



331 



where Sn is the reading in the north, Se the reading in the east, and H 

 the horizontal intensity taken with its approximate value from govern- 

 ment maps. 



The temperature correction of a magnetometer is found by measuring 

 the variation of reading with temperature. Temperature variations may 

 be produced by setting up the instrument outdoors during periods of 

 temperature rise or fall, by placing it in a box heated by alternating cur- 

 rent or hot water, or by inserting a small coil-type heater in the case. Dry 



American Askania Corp. 



Fig. 8-21. Helmholtz coil for scale-value determination. 



ice may be used to chill the instrument but should be applied with caution, 

 since too rapid cooling results in a condensation of water vapor on the 

 knife edge and thus in erratic readings. 



A second instrument should be used to observe or record the daily varia- 

 tion and should be kept at constant temperature. Observations on both 

 are taken with magnets permanently released. The instrument to be 

 calibrated is read every five minutes and the corresponding temperature 

 is noted. The temperature coefficient is then given by the mean slope 

 of the curve s = /(O). A least-square adjustment may be applied to im~ 



