132 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



flux thus produced through the rod may be utilized to induce 

 electromotive force in an auxiliary coil of wire wound on the rod. 

 Such an arrangement is called an induction coil. The winding 

 through which the magnetizing current from the battery flows is 

 called the primary coil and the auxiliary winding in which the 

 desired electromotive force is induced is called the secondary coil. 

 The iron rod is always made of a bundle of fine iron wires to pre- 

 vent the flow of eddy currents as explained in Art. 74. 



When the iron rod or core is magnetized a pulse of electromo- 

 tive force is induced in the secondary coil, and when the iron core 

 is demagnetized, a reversed pulse of electromotive force is induced 

 in the secondary coil. These impulsive electromotive forces may 

 be made very large in value, hundreds of thousands of volts, by 

 making the secondary coil of many turns of wire and by provid- 

 ing for the quickest possible magnetization and demagnetization 

 of the core. 



A battery or any ordinary current generator does not magnetize 

 a core very quickly when connected to a winding of wire ; in 

 fact, a very considerable fraction of a second is usually required 

 for the core to become magnetized. Therefore, during the mag- 

 netization of the iron core of an induction coil the electromotive 

 force induced in the secondary coil is a comparatively weak pulse 

 of long duration. 



On the other hand, proper arrangements permit of an extremely 

 quick demagnetization of the iron core of an induction coil when 

 the battery is disconnected from the primary winding, and this 

 quick demagnetization induces in the secondary coil an intense 

 pulse of electromotive force of short duration. 



The quick demagnetization of the iron core of an induction 

 coil is accomplished as follows : Figure 92*2 shows the connections 

 of a battery to the primary coil. The battery is connected and 

 disconnected by making and breaking contact between the metal 

 terminals tt. Two large metal plates separated by an insulator 

 (a condenser) are connected to the terminals tt as shown. 

 When the points // are connected together the core is slowly 



