On this page of our guide to BioShock Infinite you will find the first part of a detailed walkthrough of Chapter 35 - Operating Theatre. Find Elizabeth, turn off the machines and release her.
After Chapter 34 is over, you will return to 1912. Walk through a long corridor to a large operating room. You will notice Elizabeth imprisoned here. The only way to rescue Elizabeth is to turn off two generators that power the siphon, because that will unlock her abilities. Both generators are located on an upper level behind the operating room.