  Recent radio emission, polarization, and Faraday rotation maps of the radio
jet of the galaxy 3C 303 have shown that one knot of this jet has a {\it
galactic}-scale electric current of $\sim 3\times 10^{18}$ Amp\`ere flowing
along the jet axis (Kronberg et al. 2011). We develop the theory of
relativistic Poynting-flux jets which are modeled as a transmission line
carrying a DC current $I_0$, having a potential drop $V_0$, and a definite
impedance ${\cal Z}_0 =90(u_z/c)\Omega$, where $u_z$ is the bulk velocity of
the jet plasma. The electromagnetic energy flow in the jet is ${\cal Z}_0
I_0^2$. The observed current in 3C 303 can be used to calculate the
electromagnetic energy flow in this magnetically dominated jet. Time-dependent
but not necessarily small perturbations of a Poynting-flux jet - possibly
triggered by a gas cloud penetrating the jet - are described by "telegrapher's
equations," which predict the propagation speed of disturbances and the
effective wave impedance ${\cal Z}$. The disturbance of a Poynting jet by the
cloud gives rise to localized dissipation in the jet which may explain the
enhanced synchrotron radiation in the knots of the 3C 303 jet, and in the
apparently stationary knot HST-1 in the jet from the nucleus of the galaxy M87
(Biretta et al. 1999).
