242 



OUR PHYSICAL WORLD 



anticlockwise direction. But since the strips on the armature 

 are in the reverse position from those of the coil about the core 

 whose free end is a, the current will flow from the machine in 

 the same direction as in the first case. The coil about the core 

 whose free end is c is now turning into the position occupied at 

 first by the coil about the core whose free end is a, and so the 

 process continues. Such a dynamo, therefore, gives rise to a 

 direct current. The core of the electromagnet remains mag- 

 netized sufficiently while the dynamo is idle to start the pro- 

 cess described when it is 

 again used, and then its 

 power is increased by 

 the current generated. 

 If a current is mov- 

 ing along a circular loop 

 of wire in the direction 

 indicated by the arrow 

 (Fig. 105), the south 

 pole of its magnetic field 

 is at the left of the loop, 

 the north pole at the 

 right. Recall the figure of the swimmer. If now the loop with- 

 out a current in it were rotated between the poles of a magnet 

 in a clockwise direction, the position previously occupied by 

 the south pole of the loop is approaching the north pole of the 

 magnet. The current generated in the wire will flow in such a 

 direction as would produce a south pole in this position if a 

 current were flowing: in other words, so as to develop a pole 

 opposite in character to the one which the wire is approaching. 

 Or you may use the "rule of thumb" to determine the direction 

 of the flow of the induced current. Hold the thumb and the 

 extended index and middle fingers of the left hand at right angles 

 to each other. Point the index finger in the direction of the lines 

 of magnetic force, the middle finger in the direction of the move- 



FIG. 105. Diagram of a simple dynamo 



