300 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



When a ball or rod of iron has a certain amount of permanent 

 magnetism, the effect of the earth's magnetic field upon it is to 

 produce an additional temporary magnetism, that is to say, the 

 magnetism of the ball or rod is the sum of two distinct parts, a 

 permanent magnetism and a temporary magnetism. Of course 

 the permanent magnetism of a rod may be changed by severe 

 mechanical shocks ; the word permanent here refers to that part 

 of the magnetism which does not change as the ball or rod is 

 slowly moved around in the earth's field. 



Similarly,an iron ship has a certain amount of permanent magnet- 

 ism which does not change as the ship moves around in the earth's 

 magnetic field and a certain amount of temporary magnetism 

 which is due to the magnetizing action of the earth's magnetic field. 

 9. Compass errors due to permanent magnetism of a ship. The 

 permanent magnetism of a ship produces at the compass box a 

 magnetic field which is constant in value and fixed in direction 

 with reference to the ship. The horizontal com - 

 ponent of this field combines with the horizontal 

 component of the earth's field to give a resultant 

 field in the direction in which the compass needle 

 points. Thus, H' in Fig. 9 represents the hori- 

 zontal component of the earth's field, P repre- 

 sents the horizontal part of the magnetic field at 

 the compass which is due to the permanent mag- 

 netism of the ship, R represents the resultant 

 horizontal field at the compass, and 6 represents 

 the compass error due to the ship's permanent magnetism. The 

 field P rotates with the ship and therefore the compass error 

 has a series of positive values (to the east) throughout a half 

 revolution of the ship, and a series of negative values (to the 

 west) throughout a half revolution of the ship. Therefore the 

 compass error due to the ship's permanent magnetism is called 

 the semicircular error* 



*The permanent magnetism of the ship contributes also to the heeling error 

 which is discussed in Art. 14, and the so-called semicircular error is due partly to 

 the temporary magnetism of the ship as explained in Art. 13. 



