The quadrantal spheres are used to（）.

单项选择题The principal purpose of magnetic compass adjustment is to（）.

A.reduce the variation as much as possible
B.reduce the deviation as much as possible
C.reduce the magnetic dip as much as possible
D.allow the compass bowl to swing freely on its gimbals

单项选择题The permanent magnetism of a vessel may change in strength due to（）.

A.a collision with another vessel
B.being moored on a constant heading for a long period of time
C.being struck by lightning
D.All of the above

单项选择题The permanent magnetism of a vessel may change in polarity due to（）.

A.being moored for a long time on one heading
B.being struck by lighting
C.steaming from the north magnetic hemisphere to the south magnetic hemisphere
D.loading a homogenous magnetic cargo such as steel plate，iron bars，etc

单项选择题The MOST important feature of the material used for making the binnacle of a standard magnetic compass is that it is（）.

A.nonmagnetic
B.weatherproof
C.corrosion resistant
D.capable of being permanently affixed to the vessel

单项选择题The magnetic compass operates on the principle that（）.

A.like magnetic poles attract
B.unlike magnetic poles repel
C.unlike poles attract
D.the poles of the compass line up with the geographic poles of the earth