308 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



be explained as follows : When V changes in value the mag- 

 netism of Flinders' bar and the vertical temporary magnetism of 

 the ship change together, and Flinders' bar being once for all 

 adjusted to compensate the effect of the vertical temporary mag- 

 netism of the ship, the compensation holds, whatever the value of V 

 may be. Flinders' bar is usually about three inches in diameter 

 and from 6 to 24 inches long, according to the amount of iron 

 in the vessel, and it is usually * placed forward or aft of the 

 binnacle. 



* Figure 19^ shows the north polarity NNNN, etc., on the deck of an iron vessel 

 due to the vertical component of the earth's field. This north polarity is distributed 

 symmetrically with respect to the compass box C (ship's iron being symmetrical with 



-Flinders' bar 



N 



N 



A 



i 



Fig. 19a. 



Fig. 19b. 



respect to the compass box), and it produces, at the compass, a magnetic field of which 

 the horizontal component is represented by the arrow a which is parallel to the keel. 

 Flinders' bar is placed in the position shown, and its north pole N f (upper end 

 of bar), which is on a level with the compass box, produces at the compass box a 

 field b which is equal and opposite to a. Figure igb shows a side view of Flinders' 

 bar F. (the compass box is supposed to be placed at the point p). Flinders' bar 

 is magnetized by the vertical component. V of the earth's magnetic field, a is the 

 horizontal part of the field which is produced at the compass box by the vertical 



