  We calculated the electron-positron pair production rate at the base of the
jet of 3C 120 by collisions of photons from the hot accretion flow using the
measurement of its average soft gamma-ray spectrum by Compton Gamma Ray
Observatory. We found that this rate approximately equals the flow rate of the
leptons emitting the observed synchrotron radio-to-IR spectrum of the jet core,
calculated using the extended jet model following Blandford \& Konigl. This
coincidence shows the jet composition is likely to be pair-dominated. We then
calculated the jet power in the bulk motion of ions, and found it greatly
exceeds that achievable by the magnetically arrested disk scenario for the
maximum black hole spin unless the jet contains mostly pairs. Next, we found
that the magnetic flux through the synchrotron-emitting jet equals the maximum
poloidal flux that can thread the black hole. Finally, we compared two
estimates of the magnetization parameter at the onset of the synchrotron
emission and found they are in agreement only if pairs dominate the jet
content.
