INDUCED ELECTROMOTIVE FORCE. 



133 



magnetized by the current from the battery. When the points 

 tt are separated, the current persists in flowing for a short in- 



sparfc 



battery 



Fig. 92a. 



terval of time, this persisting current flows into the condenser 

 plates, and the electric charge which thus accumulates on the 

 plates surges back through 

 the circuit as a reversed cur- 

 rent and demagnetizes the 

 iron core. 



Figure 92^ shows a com- 

 plete induction coil. The 

 condenser is mounted inside 

 of the box-like base. Fig 92b . 



72. The alternating-current transformer consists of two coils of 

 wire, a primary coil and a secondary coil, wound upon an iron 

 core. This iron core is built up of strips of sheet iron, and it 

 usually forms a complete magnetic circuit as shown in Fig. 93. 

 Figure 94 shows a sectional view of Fig. 93 ; the primary coil is 

 represented by PP and a secondary coil by 55. Either coil 

 of a transformer may be the primary coil according to the way in 

 which the transformer is used as explained later. The induction 

 coil and the alternating-current transformer/afeTdentical, except 



