from a tap on a field coil

A coil symbol with a tap (usually close to one end, but not always).

If you are working with a coil, you should have all parts of the coil and knowledge to work with them. In the related links box below, I posted a websited which has all this knowledge and another one about builduing the tesla coil.

Replace the coil pack. You can buy this part at any auto part store and have a nechanic put it in for you or do it yourself if yer feelin froggy but you need a new coil assembly or new wiring and connection to it. you can also probably find a good coil assembly from a junkyard as these usually outlast the cars the reason it works when you tap on it is the coils are burining out from the inside and losing their connection. When you tap on it you cause the vibration to make the coils hit a "hot spot" as electricians call it. anyway replace the coil assembly.

A tapped inductor is a coil to which electrical access is available at more points

than only the two ends. Connection points are provided at one or more points

on the coil that are between the ends. Each such point is called a 'tap'.