106 ADVANCED ELECTRICITY AND MAGNETISM. 



magnetism is not in general parallel to the earth's field, and in 

 the case of a long slim iron rod its temporary magnetism is 

 always in the direction of its length and proportional to the 

 component of the earth's field which is parallel to it, inasmuch 

 as that component of the earth's field which is at right angles to 

 a very slim rod produces no perceptible 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 

 magnetism 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. 



65. Compass error 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 component of this field com- 

 bines with the horizontal component of the 

 #' / R \ earth's field to give a resultant field, and the 

 compass needle points in the direction of this 

 resultant. Thus, H' in Fig. 70 represents the 

 . horizontal component of the earth's field, P 

 P *" represents the horizontal part of the magnetic 



Fig. 70. fi e ld at the compass which is due to the perma- 



nent magnetism of }he ship, R represents the re- 



sultant horizontal field at the compass, and represents the 



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



