CHAPTER XVI. USE OF THE HORIZONTAL FORCE 

 INSTRUMENT 



137. Occasionally it will be necessary to determine the actual 

 strength of the magnetic field at some compass location. This problem 

 may arise for one of the following reasons : 



(1) It may be desired to determine accurately the horizontal 

 shielding factor, A, for : 



(a) A complete mathematical analysis. 



(b) Accurate Flinders bar adjustment. 



(c) Accurate heeling adjustment. 



(d) Calculations on a dockside magnetic adjustment. 



(e) Determining the best compass location on board ship. 



(2) It may be desired to make a docksido magnetic adjustment, 

 and hence determine the existing directive force at the magnetic 

 compass both for its magnitude and direction. (See Kielhorn's, 

 A Treatise on Compass Compensation, for details of one heading 

 adjustments.) 



Lambda, A, is the horizontal shielding factor or ratio of the reduced 

 earth's directive force, fl', on the compass to tlie horizontal earth's 

 field. H, Sis: 



^ E 

 From this it is apparent that A may easily be determined for a 

 compass location by making a measurement of the reduced earth's 

 directive force, W . On a corrected compass, this value H' may be 

 measured with the ship on any heading, since this reduced earth's 

 directive force is the only force acting on the compass. If the compass 

 is not corrected for the ship's magnetism and the deviations are 

 large, H' is determined from the several resultant directive forces 

 observed with equally spaced headings of the ship, as indicated later. 

 Lambda, A, should be determined for every compass location on every 

 ship, as indicated on form NBS 1104 and NBS 1106. 



138. The actual measurement of such magnetic fields may be made 

 by use of a suitable magnetometer, or by the use of a horizontal force 

 instrument. The magnetometer method is a direct reading method, 

 which needs no calculation. The force instrument is by far the simpler 



119 



