Oil filled power bushing with piston

An insulating liquid filled power bushing for a transformer or other high voltage apparatus comprising an insulator having a chamber therein for receiving insulating liquid, an expansion chamber communicating with said insulator chamber for receiving insulating liquid and gas, a movable piston within said expansion chamber for dividing said expansion chamber into a liquid filled section and a gas filled section, a conductor extending through said insulator, said piston and said expansion chamber, and means for sealing said piston with respect to said conductor and said expansion chamber so as to prevent the gas in said gas filled section from communicating with the liquid in the liquid filled section thereby allowing the liquid to expand and contract due to thermal changes while preventing the liquid from absorbing the gas and making gas bubble evolution impossible.