[Illustration: Fig. 92. Vibratory Coil.] The diagram, Fig. 92 will show how this type of secondary is made and operated. The induction coil has a core A of soft iron, and at one end is an armature B, mounted on the end of a spring finger C, this finger being attached to a binding post D. The spring C holds the armature B normally out of contact with the end of the core A, and in contact with the end of an adjusting screw E which screws through a post F. The primary coil has one of its ends connected up with the binding post D, by a wire G; and the other terminal of the primary, has a wire H which goes to the battery I, and from the battery to the post F, through wire J. A condenser K is placed intermediate the two wires G J, by the connections L M. The wire H has a switch N in its line, as shown, and the secondary coil O is wound around the primary in the usual manner. OPERATION OF VIBRATORY COIL.--The operation is as follows: When the switch N is closed the current from the battery goes through the primary coil, wire G, spring finger C, and wire J back to the battery which originated the energy. The result of this current is to magnetize the core A, and thus draw the armature B away from the adjusting screw E, thereby breaking the primary circuit, which demagnetizes the core, and the spring finger returns and again establishes a circuit. This action of the vibrating armature is exactly similar to the electric bell, but there is one important addition, and that is the condenser K which is added to the familiar mechanism, and the uses of which should be explained in connection with this apparatus. SURGING MOVEMENT OF CURRENT.--Whenever a primary current is broken, a surging effect takes place. When the break occurs the strength of the field or force in the armature winding rapidly decreases, and when the connection is again made this force rapidly increases. This objection of the current to constantly change its current strength, produces what is called _self inductance_. TIMING DEVICE.--The current in the secondary, which makes the spark, at the time the break occurs, depends for its strength on the rapidity with which the strength of the primary goes down, so that a timing device is used on a plain or ordinary coil to effect this. [Illustration: Fig. 93. Contact Maker.] In the vibratory coil, however, the object is to make the break with exceeding rapidity so there will be a series of sparks, instead of only a single one at each break. CONTACT MAKERS.--This device is designed to afford a means whereby a circuit is closed, and broken only at the time a spark is made. A type of this device is shown in Fig. 93.