VOLTAIC ELECTRICITY. 



259 



wire about 2 mm. in diameter. Upon the primary coil, but 

 carefully insulated from it, is wound the secondary coil. 

 The wire of the secondary coil is very fine (about J mm.) 

 and many times longer than that of the primary; two hun- 

 dred and eighty miles of wire has been put into a secondary 

 coil. 



(a.) The wire bundle becomes magnetized ( 392) by the action of 

 current in the primary coil, and then adds its inductive effect upon 

 the secondary coil to that of the primary itself. The circuit is 

 broken and closed by an automatic interrupter, represented at the 

 left hand of the coil, Fig. 205. One of the posts there seen carries 

 a metallic vibrating plate with an iron disc at its end. This plate 

 vibrates back and forth between the end of the iron core of the coils 

 and the end of the metal adjusting screw which is carried by the 

 other post seen in the figure. These posts are in the circuit of the 



FIG. 206. 



current passing through the primary coil. When the vibrating 

 plate rests against the end of the adjusting screw, the circuit is 

 closed and the iron core is magnetized. As soon as the core is 

 magnetized, it attracts the iron disc at the end of the vibrating 

 plate, thus drawing it away from the end of the screw and break- 

 ing the circuit. As soon as the circuit is broken, the bar is de- 

 magnetized and the plate, by virtue of its elasticity, springs back to 



