A spatial vector quantity equal to a potential gradient, produced by charged bodies or a time-varying magnetic field. Unit is volts per meter. The electric field induced in a loop equals the negative time derivative of the magnetic flux cutting the loop ( is a length element of the loop):
It is also expressed in terms of the change in the magnetic induction with time :
See a classic text, such as Jackson (1962) 
- Jackson, J. D. (1962). Classical electrodynamics (Vol. 3). New York etc.: Wiley.