Coil ignition system (petrol engines) — purpose of the condenser across the contact-breaker In a battery-and-coil ignition system, a condenser (capacitor) is connected across the contact-breaker points. What is its primary combined function?

Introduction / Context:Older SI ignition systems use a mechanical contact-breaker and an induction coil. The rapid collapse of current in the primary winding is essential to induce a high voltage in the secondary. The condenser plays a crucial role in achieving a sharp current change and preserving the breaker points.

Given Data / Assumptions:

• Battery-and-coil Kettering ignition system.
• Contact-breaker actuated by cam on the distributor shaft.
• Capacitor connected in parallel with breaker points.

Concept / Approach:When the points open, the capacitor momentarily absorbs current, preventing an arc across the opening points. Simultaneously, together with the primary inductance, it forms an oscillatory circuit that produces a rapid fall (and brief oscillation) of current, increasing the rate of change of magnetic flux. This enhances the induced electromotive force in the secondary winding, yielding a strong spark.

Step-by-Step Solution:Points open → capacitor charges instead of an arc forming across the gap.Capacitor + inductor create a fast current collapse and transient.Faster dI/dt → larger induced voltage in the secondary (V ∝ dΦ/dt).Hence, the condenser both prevents arcing and boosts secondary voltage.

Verification / Alternative check:Without the condenser, contact points burn rapidly and secondary voltage is poor due to slow flux decay, a well-known maintenance symptom.

Why Other Options Are Wrong:“Only prevent sparking” omits the crucial role in voltage generation. “Increase dwell angle” relates to closed-time, not condenser function. “Clamp secondary voltage” is incorrect; the goal is to maximize it.

Common Pitfalls:Misunderstanding the condenser as a mere surge suppressor; it actively shapes the primary current waveform.

Final Answer:Both to prevent arcing at the points and to speed up the primary current collapse for a higher secondary voltage