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CHARACTERISTICS OF ALTERNATING CURRENT 467
type is called an "electrolytic condenser." (6) It cannot, however, be used with alternating current, since reversing the polarity of its connections destroys the insulating layer. It is used for certain purposes which will be described later in connection with direct current circuits. See Page 476.
The Induction Coil
(7) Remember that an electro-magnet consists of a wire carrying current. In its commercial form it is a coil of insulated wire. The coil is very often wound around a soft iron core (which of course is magnetized), since an iron core greatly increases the power of the magnet.
Such a winding is called an "induction coil," as well as by several other names, some of which are "inductive winding," "choke coil," (8) "retard coil" and "inductor." (9) When current is first applied to such a winding it immediately finds work to perform in moving the molecules of the iron into magnetic positions. Once this has been done the current flow increases greatly, since the haphazard magnetic fields of the iron molecules no longer resist its passage. When the switch is opened the molecules fall back into their original, unmagnetic positions, and in so doing the motion of their magnetic fields generates a reinforcing current in the wire.
Direct current flows through such a coil without difficulty as soon as it has moved the iron molecules into their magnetic positions, but alternating current causes the molecules to reverse their line-up periodically, hence meets much more opposition from them than would direct current. Consequently, if the condenser is a device that can be said to allow a. c. but not d. c. to flow through it, tht inductor is a device that allows d. c. but not a. c. to pass. As the condenser is sometimes used to pass a. c. but prevent the flow of d. c. the inductor is occasionally used for the reverse purpose.