Publisher Summary Filters are generally built from three kinds of electronic devices: resistors, capacitors, and inductors. These devices are collectively known as “passive devices.” These devices are acted upon by voltages or currents already existing in the circuit and are not able to initiate any activity on their own. This chapter describes passive devices and their properties when acted on by voltages or currents. The current through the resistor depends on its resistance and the potential difference (pd), or voltage across, produced by the voltage generator. It is noted that the current flowing through the resistor is at all times proportional to the pd across it. The opposition to current flow is due to fixed resistivity. The addition of a capacitor to a circuit can ensure that the current flowing through the resistor flows to the capacitor. Because the dielectric of the capacitor is a non-conductor, current is unable to flow through the capacitor itself, so the capacitor becomes charged. Adding a capacitor to the circuit has a significant effect on the current–pd relationship.