A Romulan graviton field generator.
A graviton field generator is a piece of starship technology that generates gravitons.
When depolarized, a graviton generator may cause a failure to an engine core. In 2367, such an event occurred onboard a Romulan science vessel, which required the graviton generator to be replicated. Geordi La Forge and Ro Laren attempted to beam back to the USS Enterprise-D with the graviton generator for replication, however, were "phased" during the transport. (TNG: "The Next Phase")
Graviton generators are also capable of containing a residual charge, which, under normal circumstances, will bleed off on its own. The cause of such a residual charge can be attributed to the disabling of a tractor beam. Typically, a tractor beam will not again function until the residual charge is completely bled off, a process that can be hurried by increasing the flow to the EPS wave guides. In 2369, Benjamin Sisko performed this procedure after the Klaestrons disabled Deep Space 9's tractor beam and attempted to kidnap Jadzia Dax. (DS9: "Dax")