Il8 ADVANCED ELECTRICITY AND MAGNETISM. 



temporary magnetism of the ship is proportional to the vertical 

 component of the earth's field and it is therefore equal to aV 

 at the home port and equal to aV\ at the distant port. The 

 deviation of the compass (ship's head east or west) which is 

 produced by this field is proportional to its intensity and inversely 

 proportional to the horizontal intensity of the earth's field. 

 Therefore this deviation is equal to bV/H' at the home port and 

 equal to bVi/Hi at the distant port, where a and b are propor- 

 tionality factors. Therefore the observed compass deviation <j> 

 is equal to b(V/H' Vi/Hi), and the compass deviation $ t 

 which is due to the vertical temporary magnetism of the ship at 



V IT-f ' 



the distant port is equal to T7/ , T/ * ,,, , X <. With the 



Via. - KI//II 



ship's head east at the distant port (the condition under which 

 <t> was observed), put Flinders' bar into a vertical position in 

 front of, or behind the compass box, and move it towards or 

 away from the compass until the compass is turned through an 

 angle (f> t in a direction opposite to <, the angle <t> t being 

 reckoned from the deflected position of the compass. Then 

 eliminate the outstanding semicircular error by readjusting the 

 semicircular correctors. 



PROBLEMS. 



I. The semicircular error of a compass on board ship is found 

 to have a maximum value of 20 to the east when the ship heads 

 36 west of south. Make a sketch of the outline of the deck of 

 the vessel and draw a line on the deck showing the direction of 

 the horizontal component of the magnetic field at the compass 

 box which is due to the permanent magnetism of the ship, find 

 the value of this horizontal component and find the angle between 

 its direction and the direction of the keel, the earth's horizontal 

 field being equal to 0.2 gauss. Ans. (a) 0.068 gauss, (b) 106 

 from bow towards port side (left side). 



Note. When the semicircular error is a maximum, the field P at the compass 

 due to the ship's permanent magnetism is at right angles to the resultant of H 

 and P as shown in Fig. pi. 



