Quantum source of the back reaction on a classical field.

Pair creation in an external electric field is presented in terms of the localized events which contribute to the mean current that is produced. Each of these localized contributions is given by a nondiagonal matrix element of the current operator. The physical relevance of this matrix element is displayed. It can also be interpreted as the result of a weak measurement in the sense of Aharonov {ital et} {ital al}.