52 THE LAST CRUISE OF THE CARNEGIE 



which furnishes a correct reading for magnetic north. The 

 "earth-inductor" in the forward dome, and the "deflector" in 

 the after observatory, both use this compass for standard magnetic 

 readings. 



Visitors have often expressed surprise that such a well-equipped 

 vessel had no gyroscopic compass, or "metal mike," as it is re- 

 ferred to by sailors. The apparatus may be employed to actuate 

 an auxiliary device, which is fast becoming standard equipment 

 on ocean-liners, and steers the ship automatically on any desired 

 heading. But on a sailing ship the course must be constantly 

 changed to take advantage of wind and squalls. The gyroscope 

 would have required precious power for operation, and would 

 have introduced magnetic materials on board. For these reasons 

 it was out of the question. Besides this, we were seldom trying 

 to make a bee-line from one port to another. 



We shall now climb into the forward observatory-dome to 

 inspect the marine earth-inductor. It determines the "dip" of 

 the magnetic needle, or inclination. It is essentially a rotating 

 coil of wire which is connected to current or potential meters in 

 the control-room. Any coil rotating in a magnetic field, with 

 its axis perpendicular to the lines of force, will generate a current 

 in the circuit in which it is placed. It is on this principle that 

 ordinary dynamos operate, except that they use either permanent 

 magnets or electromagnets, whereas we use the feeble magnetic 

 field of the Earth. 



If we move the coil around to such a position that its rotation 

 axis is parallel to the lines of force (pointing exactly to the mag- 

 netic pole), no current will be generated. This is true because 

 the magnetic field is being cut so that the effect of one half of the 

 coil exactly neutralizes the effect of the other. So when the 

 observer in the control-room signals that no current is being 

 produced, the man in the dome reads off the angle of inclination. 

 In actual practice the procedure is somewhat more complicated 

 than this. 



In the after dome is the "deflector" which gives us the strength 

 of the magnetic field acting on the compass-needle. Briefly, we 

 balance the effect on the compass of a small magnet of known 



