  The reaction concept, introduced by Rumsey in 1954, describes interaction
between time-harmonic electromagnetic sources through the fields radiated by
the sources. In the original form the concept was a scalar quantity defined by
three-dimensional field and source vectors. In the present paper, the
representation is extended to four dimensions applying differential-form
formalism. It turns out that, in a coordinate-free form, the reaction concept
must actually be a one-form, whose temporal component yields Rumsey's scalar
reaction. The spatial one-form component corresponds to a three-dimensional
Gibbsian-vector reaction which consists of electromagnetic force terms.
