How to stop sparks flying

QMy 1980 Ford Capri has been getting through condensers at a rate of one every 5000 miles. I’ve just fitted two new ones with a switch so I can quickly go from one to the other at the side of the road. But surely there’s a better solution to this unnecessary problem? Chris Morris, via email

ACurrent flows through the coil’s primary winding, through the contact breakers and to earth. When current stops, the magnetic field set up in the primary winding collapses through the secondary winding. The more instantaneous the collapse, the higher the voltage induced in the secondary winding. With no condenser present, some electrons jump the points gap as the contacts start to open. This disorderly stop to the coil current induces a much lower voltage.

The condenser is a storage device for electricity, connected between the coil’s negative terminal and earth. Now when the contacts open, the electrons have two choices: to jump the gap (difficult) or to fill up the empty condenser (easy).

Thus, they all flow into the condenser as the contacts open.

The condenser is quickly filled, bringing the coil current to a rapid and orderly halt. By the time it’s full, the points are too far open to jump. This induces the largest possible voltage in the secondary winding, which is then delivered to the spark plugs.

The two windings are usually connected, which allows the primary to assist with spark formation. This is why, if you open the contact breakers with a screwdriver, they give a small spark. If you remove the condenser, there’s neither a spark at the breaker nor at the plugs. A condenser is known in electronics as a capacitor: typically 0.22μf and 400V. They’re generally a bit crude by normal electronics standards. You can replace it with a longer-lived ‘polyester film’ capacitor, which are widely available on ebay. Make sure you buy one with long leads.