If another conductor or coil of wire is placed in the magnetic field of the first coil, but not directly connected to it, the movement of the magnetic field induces an alternating current in the second coil. If the second coil has a larger number of turns than the first, the voltage induced in the second coil will be larger than the voltage in the first, because the field is acting on a greater number of individual conductors. If the number of turns in the second coil is smaller, the secondary, or induced, voltage will be smaller than the primary voltage. This is the principle behind a transformer. A capacitor, or electrical condenser, is a device for storing an electrical charge.
Capacitors have capacitve reactance which is the opposition to current flow provided by a capacitor.
Capacitive reactance is measured in ohms and varies inversely with frequency. One of the earliest capacitors was the Leyden Jar. In its simplest form a capacitor consists of two metal plates separated by a non-conducting layer known as the dielectric or insulator. When one plate is charged with electricity the other plate will have an opposite charge induced in it. Current flows through a capacitor even if there is no direct electrical connection between its plates because the voltage on one plate induces an opposite charge on the other plate.