Ans. An electric pressure is induced in the secondary circuit, but of not enough intensity to cause a spark to jump across the air gap. [Illustration: FIG. 139.--A Medical coil with armature and attachments consisting of electrodes, foot place, sponge, induction coil etc. A current of any degree of intensity may be obtained. The currents furnished are: 1, primary, 2, secondary; and 3, primary and secondary combined.] =Ques. What happens when the primary circuit is suddenly broken?= Ans. A spark is produced both at the point of break in the primary circuit and at the air gap in the secondary circuit. =Ques. Why is a spark produced at the air gap at break and not at make of the primary current?= Ans. Because when the current is flowing it cannot be stopped instantly on account of self-induction, that is, it acts as though it possessed weight. If the reader has charge of a gas engine with a make and break ignition system, he will often avoid vexatious delays in locating ignition troubles, if he remember that one of the most important conditions for obtaining a good spark is that the _break take place with great rapidity_. This, of course, involves that the ignition spring be adjusted to the proper tension. [Illustration: FIG. 140.--Rhumkorff induction coil. A secondary coil with vibrator and condenser; a type generally used in the laboratory. The name Rhumkorff was formerly very widely applied to induction coils for the reason that some of the earliest coils were constructed by Rhumkorff.] =Secondary Induction Coils with Vibrator and Condenser.=--A plain secondary coil, such as just described, will only give feeble sparks for its size for the following reasons: The inductive effect of the primary winding in the secondary depends as previously explained on the rate at which the current in the primary winding decreases or dies out. If a strong inductive effect is to be produced in the secondary, the current in the primary must stop suddenly. This is prevented by self-induction in the primary winding, which opposes any change in the current strength. The direct result is that, as the primary circuit is broken, a spark appears at the break, which means that the current continues to flow after the break has occurred, dying down comparatively slowly, hence, the inductive effect on the secondary winding is small. [Illustration: FIG. 141.--Conventional diagram of a condenser. A condenser is a device designed to absorb or hold an electric charge in about the same manner as a vessel will hold a liquid. Every conductor of electricity forms a condenser and its capacity for holding a charge depends upon the extent of its surface. A condenser is therefore made of conductive material formed into such shape as to present the maximum surface for a given amount of material.]