ELECTRICAL INVENTIONS 



243 



ment of the wire through the magnetic field. Then the thumb 

 held parallel to this wire will point in the direction of the flow 

 of the induced current. If, however, the same wire is considered 

 in the position indicated by the dotted line, the position of the 

 south pole is moving away from the north pole of the magnet, 

 and so the current in the loop is reversed. Rapidly rotating 

 such a loop would therefore produce an alternating current in 

 the wires connecting with its ends. 



Or suppose the armature of a dynamo is a wheel having a 

 number of cored coils set like cogs on its rim (Fig. 106). The 

 wire of the coils is continuous and wound in each coil in the same 

 direction. The two ends of the 

 wire each run to a circular metal- 

 lic band fixed to the axle of the 

 armature. The current gener- 

 ated is taken from these bands by 

 spring clips that are in contact 

 with them. If such a wheel re- 

 volves with its rim inside of a 

 circle of north magnetic poles, 

 every time a coil approaches a 

 pole it produces a current in 

 one direction, and as it leaves the pole it produces a current 

 in the opposite direction. This type of dynamo therefore gen- 

 erates on alternating current. 



When electric power is sent a long distance over wires from 

 a central generating plant to neighboring cities for running their 

 lights or factories, it is sent at high pressure. A long wire offers 

 much resistance, and it is found that less power is lost in leakage 

 to the air and to objects on the way when the current sent is of 

 high voltage. We have become familiar with these high-power 

 lines, as the distribution of electrical power has become common 

 (Fig. 107). The wires are usually supported on steel towers, 

 and huge porcelain insulators are used instead of the small glass 

 ones familiar on telegraph and telephone lines, which carry cur- 



FIG. 106. A dynamo with cored 

 coils set like cogs. 



