250 



ELECTRICITY 



ample, if the coil in Figure 394 is moved downward 

 over the magnet, the lines of force are cut in the order 

 as numbered, 1-2-3-4-5-6, and the throw of the gal- 

 vanometer needle is to the right. Now if the coil is 

 taken from the magnet as shown in the figure, the 

 lines of force are cut in the reverse order of 6-5-4-3-2-1, 

 and the galvanometer needle throws to the left, show- 

 ing that the current is now flowing around the circuit 

 in the opposite direction. If the coil is moved very 

 rapidly up and down over the magnet, the needle will 

 vibrate back and forth, showing that current is flow- 



FIG. 394. HOW ELECTRICITY IS GENERATED 



ing first in one direction around the circuit and then 

 in the other. This is called an alternating current. Mod- 

 ern generators produce current which alternates sixty 

 times per second. This is so rapid that we do not no- 

 tice it in lights or heating devices. 



The discovery made by Faraday remained a labora- 

 tory experiment for more than forty years until the 

 genius of Thomas A. Edison saw the possibilities of 

 producing electricity on a large scale by making an 

 electric generator which used coils and magnets. 



The simplest form of electric generator is the mag- 

 neto ; it is used in rural telephone systems and in 

 certain automobiles for generating the current used 

 to charge the storage battery. In the magneto the mag- 

 netic field is supplied by permanent magnets. The 

 armature is made of many turns of wire and revolves 

 rapidly in the magnetic field where the lines of force 

 are cut many times per second. In the modern elec- 

 tric generator the permanent magnets in the field 

 of the magneto are replaced by electromagnets. In 

 small generators current is taken directly from the 

 armature and run through the field electromagnets. 

 In the larger ones it is necessary to have a separate 

 generator, called an "exciter," to furnish current for 

 the field magnets. 



FIG. 



395. GENERATION OF 

 NATING CURRENT 



What are the two types of electric currents. When 

 you repeated Faraday's experiment you observed that 

 if the lines of force were cut in one order the current 

 flowed in one direction around the circuit while if 

 they were cut in reverse order the current flowed in 

 the opposite direction. The coils of wire in the arma- 

 ture of an electric generator cut the lines of force in 

 the magnetic field in one order during a half revolu- 

 tion and in the reverse 

 order in the next half 

 revolution. This is 

 shown in Figure 395, 

 where a single coil in a 

 magnetic field is illus- 

 trated. If the coil is ro- 

 tated in the magnetic 

 field in the direction in- 

 dicated by the arrow, 

 while the section of the coil AB is cutting the lines of 

 force in the order 1-2-3-4-5 as numbered, the section 

 CD is cutting them in the order 5-4-3-2-1. In the next 

 half revolution the conditions are reversed and the 

 current in each section of the coil reverses. If each 

 end of the coil were attached to two rings which turn 

 with the coil, the current thus generated could be 

 taken to outside lights, electric irons, and other house- 

 hold devices by means of brushes which rub on rings. 

 In this way the current in the outside circuit of lights 

 would change its direction each time the current in 

 the armature reversed, once each half revolution. This 

 type of current is called alternating. 



It is sometimes necessary to make the alternating 

 current generated in the armature one which always 

 flows in the same direction outside the generator. To 

 do this a slight change is made in the device on which 

 the brushes rub. Instead of two continuous rings as in 

 the A.C. generator, the ends of the coil are now con- 

 nected to a split ring or commutator as shown in Figure 

 396. In this generator, just as the current reverses in 

 the armature coil, the commutator segment attached 

 to it slips over to the 

 other brush. This 

 means that brush B will 

 always be in contact 

 with the coils which are 

 cutting the lines of 

 force in one direction 

 and brush C in contact 

 with those coils cutting 

 lines of force in the op- 

 posite direction. Thus 

 current will always 

 flow in one direction, 

 out of one brush, 

 around the outside cir- FIG. 396. USE OF THE COMMUTATOR 



