Inside a source of emf (such as a battery) that is open-circuited, a charge separation occurs between the negative terminal N and the positive terminal P. This leads to an electrostatic field E o p e n c i r c u i t that points from P to N, whereas the emf of the source must be able to drive current from N to P when connected to a circuit. This led Max Abraham to introduce the concept of a nonelectrostatic field E ′ that exists only inside the source of emf. In the open-circuit case, E ′ = − − E o p e n c i r c u i t , while when the source is connected to a circuit the electric field E inside the source changes but E ′ remains essentially the same. In the open-circuit case, the conservative electrostatic field created by separation of charge exactly cancels the forces producing the emf. Mathematically: