170 



APPLIED SCIENCE 



If, on the other hand, the current is not flowing, the filings will 

 not assume circular lines. This shows that there is a definite rela- 

 tion between magnetism and electricity. When a conductor passes 

 through a magnetic field in the proper direction, it produces a 

 current of electricity, or when a current passes through a conductor 

 it produces a magnetic field. 



201. Electromagnetic Force. Soft iron retains very 

 little magnetism and 3^et it can be magnetized to such an 

 extent that it can be utilized in lifting large bodies. When 

 a bar of soft iron, in the form of a horseshoe, is wrapped round 

 with copper wire and a current of 

 electricity is passed through the wire, 

 the iron becomes a powerful magnet 

 called an electromagnet (Fig. 72) and 

 may be constructed to support a 

 weight of many tons. By making 

 one magnet fixed and another mov- 

 able, and by causing one magnet to 

 FIG. 72. Electromagnet. 



revolve within the lines of force of 



another, an attraction and repulsion of great intensity can be 

 created, which will act as a great moving power. 



The strength or lifting power of a magnet is measured with 

 a lever and scales by noticing the number of pounds reg- 

 istered. The lifting weight is the pull exerted minus the 

 weight of the magnet. The magnetic flow is proportional 

 to the number of turns of wire of the conductor and the 

 current flowing around the turns. The magnetic flux is 

 inversely proportional to the resistance of the circuit. The 

 total resistance is the sum of the resistance of the iron path 

 and the air path. 



An electric bell (Fig. 73) depends upon the properties of electri- 

 city and magnetism for its action. When the button of the bell is 



