Jar

A percussion tool that operates on a mechanical or hydraulic principle and is designed to deliver a impact to objects in which it is attached within the borehole. Jars are typically used for the purposes of freeing stuck objects within the borehole during fishing operations or imparting a jarring motion to downhole devices. The design of the jar often permits impacts to be delivered in either a downward or upward direction, with control being effected at the surface.