A Vorgatt defence field emitter was a device that create a force field around it. Sarah Jane Smith gave one to Rani Chandra on the night of 10 October 2009, and Rani's mother Gita walked into it the next morning when bringing her daughter some morning tea. After Rani quickly turned it off, Gita described the pain as akin to having walked into a brick wall. (TV: The Day of the Clown)
Rani kept the Vorgatt defence field emitter until Luke Smith needed it again. He wanted enclose the house at 39 Bannerman Road, that had swapped places with a Victorian house from 1884. He also wanted to stop the effect spreading. Sarah Jane replied that she was not sure that it had the range but at least it would keep people away.
Sarah Jane and the others later used the Vorgatt defence field emitter for another purpose. When they went to 39 Bannerman Road they were automatically sent to 1884. But an alien always sent them back to 2009, so that they couldn't foil the alien's plans. They used the defence field emitter, so that the alien couldn't send them back mmediately because they were protected by the device. Furthermore, no one could hurt them in 1884. When Charles Burden tried to attack Luke and Clyde, the force field was holding him back.
Then Rani used the Vorgatt defence field emitter on Skak's time manipulator. She put it onto the machine and Sarah Jane activated it with her sonic lipstick. Skak couldn't use the machine any more. (AUDIO: The Ghost House)