168 



APPLIED SCIENCE 



The total number of lines of force which pass through a field 

 is called the magnetic flux. The magnetic flux always flows in 

 a complete circle or circuit. The material through which it 

 flows affects variously the resistance offered to the free pas- 

 sage of the flux. 



198. The Mariner's Compass. Experience shows that, 

 in all cases, like poles of magnets repel and unlike poles attract. 

 This principle, called the law of magnets, is utilized in the 



device known as the mariner's 

 compass (Fig. 70), which con- 

 sists of a magnetized steel 

 needle balanced on a point 

 so that it will turn freely; 

 the points of the horizon are 

 marked on a compass card. 

 The needle is acted upon by 

 the earth, which is a magnet. 

 The needle behaves the same 

 way in all parts of the earth. 

 FIG. 70. Mariner's Compass. The north magnetic pole is 

 near Hudson Bay, the south magnetic pole in the Antarctic 

 Ocean. As the true North Pole and the magnetic north pole 

 are not the same, allowance must be made for this variation. 

 Ships travel from point to point by the assistance of the 

 mariner's compass. 



199. Nature of Electricity. As we look about us we find 

 electricity moving the cars on which we ride and producing 

 the light by which we see at night, and we naturally ask, 

 "What is electricity?" That question cannot, as yet, be 

 answered definitely. Electricity is no doubt a form of energy 



