SHIP'S MAGNETISM. 30 1 



10. The semicircular correctors. The ideal compensation for 

 the compass errors due to a ship's permanent magnetism would 

 be to place a permanent steel magnet in such a position that it 

 would produce, at the compass box, a magnetic field equal and 

 opposite to the field produced at the compass box by the ship's 

 permanent magnetism. As long as the ship remains on an even 

 keel, however, it is only the horizontal part P t Fig. 9, of the 

 field which is produced at the compass box by the permanent 

 magnetism of the ship, which causes the deflection of the compass. 

 Therefore it is sufficient to neutralize this horizontal field P. For 

 this purpose, one or more horizontal magnets are placed in trays 

 in the pedestal of the binnacle and adjusted until they produce a 

 field at the compass box which is equal and opposite to P. 

 Usually, two such trays are employed, in one of which, magnets 

 are placed parallel to the line of the keel of the ship so as to annul 

 the bow component of P, and in the other of which, magnets are 

 placed at right angles to the line of the keel so as to annul the 

 athwart-ship component of P. These two trays with their per- 

 manent magnets are called the semicircular correctors. 



11. Compass errors due to temporary magnetism of a ship. 



An idea of the general character of the compass errors which are 

 due to the temporary magnetism of a ship may be obtained by 

 imagining the ship to be a long slim bar AB t Fig. 10, with a 

 compass box at the point C. The earth's horizontal field H* 

 may be resolved into two components, one parallel to AB and 

 the other at right angles to AB. The component which is at 

 right angles to AB has no perceptible magnetizing action on 

 AB, the component which is parallel to AB causes the end 

 B to become a north pole and the end A to become a south 

 pole, and the magnetic field at C due to these magnet poles is 

 parallel to BA and towards A. The magnetic field at the 

 compass box which is due to the temporary magnetism of the 

 bar AB in Fig. i o is represented by the arrow T in Fig. 1 1 , 

 the earth's field at the compass box is represented by H' , and 

 the arrow R represents the resultant field at the compass box 



