Just a bit of clarification on the post about DC.
The feed through capacitor is exactly that, a feed through!
If you put an ohm meter to both ends of the feed through capacitor you will find zero resistance. The center post is a solid piece of metal, the capacitance if formed between the outer, grounded shell, and the center conductor. They are wrapped internally with two layers of foil, with an insulator, dialectic, between the foil layers. In aircraft design they are used primarily to feed 12 or 28 VDC through a firewall, leaving any spiking noise pulses being shunted to ground through the "Capacitance" of the device. The AC impedance of the capacitance is very low to the low level AC signals that are the noises pulses that you want to remove due to their high repetition rate. when the 60 Hertz (Cycle) AC comes along the small value of capacitance does not shunt the line voltage to ground, but any "High Frequency" noise riding along on the AC Power will be removed because it is so much higher a frequency rate.
Kind of hard to explain and understand if you are not into electronics. Hope it makes sense to all who read it.
George