Electricity. 327 



paper were to be laid upon the edge at right angles to its former position, 

 similar lines of force would be obtained there. And the same is true of 

 a plane at any intermediate angle. If the lines of force could be seen 



FIG. 131. 



FIG. 132. FIG. 133. 



FIG. 131. Lines of Force of a permanent FIG. 132 Bar Magnet seen end-on 



Bar Magnet. showing radiating lines of force. 



FIG. 133. Whirls of Lines of Force around a wire carrying a current seen end-on. 



end-on they would appear to radiate in all directions from the pole of 

 the magnet. (See fig. 132.) If a piece of paper be pef orated in the 

 center and a wire conducting a current of electricity be passed through 

 the paper at right angles, fine iron dust sifted upon the paper will take 

 the form of concentric circles about the wire. ( See fig. 133. ) 



FIG. 134. Same as 133 seen sidewise. 



Around every direct current there is a field of magnetic 

 whirl something like that shown in fig. 134. There is a re- 

 ciprocity between the current through the wire and the whirls 

 of force around it. They belong together and either one be- 

 gets the other. The direct current gives rise to the whirls which 

 endure while the current lasts. And conversely if by any means 

 the whirls can be produced they will give rise to a current 

 which will continue as long as the whirling can be kept up. 

 When a wire ring is moved near a magnet across a space in 

 which there are lines of magnetic force, the resultant of this 

 motion of the wire upon the lines of force is the magnetic 

 whirl about the wire itself, and consequently a current in- 

 duced in the wire. And this current through the wire will 

 FIG. 134. continue as long as the motion of the wire through the field 

 of force continues. When the wire stops moving the whirls and current 

 through it stop. 



The foregoing statement is on this proviso ; that the motion of the 

 wire across the field of force ( in order to produce the current), must 

 cross the lines of magnetic force in such a way that the number of these 

 lines inclosed by the circuit must be constantly on the increase or de- 

 crease. If the wire hoop be moved through a uniform field, as in 

 Figs. 135 and 136, in such a way that the number of lines of force pass- 

 ing through the hoop is neither increased nor diminished, no current will 

 be induced. In Fig. 137 the ring is supposed to tilt as it moves and 

 thereby the number of lines enclosed decreases. In that case a current 



