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Wire-ended resistors.

Surface mount resistors.

In many circuits, resistors are the commonest components. They are also generally the cheapest. Their purpose is to resist the flow of electricity, either to limit the current for a given applied voltage (electrical pressure), or to allow a certain voltage (pressure) to build up when a given current flows. Most consist of a thin film of oxide or carbon deposited on a ceramic base. Heat is always generated in a resistor as the current flows, often only a tiny amount but sometimes it can be quite large.