324 POPULAR SCIENCE MONTHLY. 



through which the current enters and leaves are held stationary; 

 keeping this fact in mind, it can be seen at once that in Fig. 12 the 

 current will flow from the brush a through the two sides of the ar- 

 mature wire to brush h, hence all the coils on the right of the ver- 

 tical line will be traversed by the current in the same direction — 

 that is, either to or from the center of the shaft — and in the coils 

 on the left the direction will be opposite, which is just the same 

 order as was explained in connection with Fig. 11. 



An electric motor can be turned into an electric generator by 

 simply reversing the direction in which the armature rotates — 

 that is, any electric machine is either a generator or a motor. This 

 fact can be illustrated by means of Figs. 13 and 14, both of which 

 show the armature and the poles of the field magnet. The first 

 figure represents an electric motor, and, as can be seen, the pull 

 between the N pole of the armature and the P pole of the field is 

 in the direction of arrow &, hence the armature will rotate in 

 the same direction, as indicated by arrow a. To obtain the 

 polarity of the armature and field it is necessary to pass an elec- 

 tric current through both — that is to say, we must expend elec- 

 trical energy to obtain power from the machine. As soon as the 

 current ceases to flow, the polarity of the armature and field dies 

 out, and the rotation of the former comes to an end. The mag- 

 netism, however, does not die out entirely; a small residue is al- 

 ways left, although it is never sufiicient to produce rotation, and 

 even if it were it could only cause the armature to revolve through 

 one quarter of a turn. If, after the current has been shut off, 

 the armature shaft is rotated in the reverse direction, as indicated 

 by arrow a in Fig. 14, the motion will be against the pull of the 

 magnetism; therefore, although the poles may be very weak, an 

 amount of power sufiicient to overcome their attraction must be 

 applied to the pulley, otherwise rotation can not be accomplished. 



In consequence of the back- 

 ward rotation a current is 

 generated in the armature 

 coils, and this current, as it 

 traverses the field coils as 

 well as those of the armature, 



Figs 13, 14.— Diagrams illustratino the Dif- CaUSeS the polarity of both 



FERENCE BETWEEN AN El-ECTRIC MoTOR AND p^j.^g ^^ iuCrCaSC. As a TCSUlt 

 A (lENEKATOK. , , . 1 1 • 



of the increased polarity the 

 resistance to rotation is increased, and more power has to be applied 

 to the pulley. The increase in the strength of the poles results in 

 increasing the current generated, and this in turn further increases 

 the pole strength, so that one effect helps the other, the result being 



