An intrinsic field subtractor is a machine that utilizes radiation to remove "intrinsic" fields from solid objects. Typically, the object subjected to the machine's process is disintegrated.
The machine is first introduced in Watchmen's Chapter IV: Watchmaker, when Jon Osterman reports to Gila Flats research center after completing his doctoral degree in 1959. He runs and studies many experiments using the subtractor until, one day, he is accidentally locked in the main chamber and obliterated by the radiation when the program cannot be overriden. Over the next few months, Osterman is able to reassemble himself into his new form, a blue-skinned superhuman being with extraordinary powers, one being the ability to control matter at a subatomic level, reflective of the nuclear energy that created him.