The spark at the break in the primary circuit is even larger than that in the secondary circuit, and as this primary spark serves no useful purpose, but, on the contrary, quickly burns away the contact points, such an arrangement is obviously defective. The vibrator-condenser coil is designed to overcome this trouble and also to give a series of sparks following in rapid succession instead of one. It should be noted that a series of sparks following each other with considerable rapidity may be obtained with a plain coil by placing a _mechanical vibrator_ in the primary circuit, as used on some motor cycle ignition circuits. The object of the vibrator, of a vibrator-condenser coil, is to rapidly make and break the primary circuit during the time the primary circuit is closed externally. It consists of a flat steel spring secured at one end, with the other free to vibrate. At a point about midway between its ends, contact is made with the point of an adjusting screw, from which it springs away and returns in vibrating. The points of contact of blade and screw are tipped with platinum. One wire of the primary circuit is connected to the blade and the other to the screw, hence, the circuit is made when the blade is in contact with the screw and broken when it springs away. [Illustration: FIG. 142.--Construction of condenser for an induction coil. The conducting material used is tinfoil, of which a large number of sheets are prepared, all cut to the same size. These are placed, one on top of the other, like the pages of a book, with a thin layer of insulating material between, usually two sheets of paraffined paper. Numbering the successive sheets of tinfoil serially, all sheets of even number are connected together and all sheets of odd number are connected together, these connections forming the terminals of the condenser. The condenser is then connected across the break in the primary circuit.] A condenser is used to absorb the self-induced current of the primary winding and thus prevent it opposing the rapid fall of the primary current. Every conductor of electricity forms a condenser and its capacity for absorbing a charge depends upon the extent of its surface. Hence, a condenser is constructed of conductive material so arranged as to present the greatest surface for a given amount of material. The usual form of condenser for induction coils as shown in figs. 141 and 142 is composed of a number of layers of tin foil separated by paraffin paper, each alternate layer being connected at the ends.