  We propose a scheme to measure both the current-phase relation and
differential conductance $dI/dV$ of a superconducting junction, in the normal
and the superconducting states. This is done using a dc Superconducting Quantum
Interference Device (dc SQUID) with two Josephson junctions in parallel with
the device under investigation and three contacts. As a demonstration we
measure the current-phase relation and $dI/dV$ of a small Josephson junction
and a carbon nanotube junction. In this latter case, in a regime where the
nanotube is well conducting, we show that the non-sinusoidal current phase
relation we find is consistent with the theory for a weak link, using the
transmission extracted from the differential conductance in the normal state.
This method holds great promise for future investigations of the current-phase
relation of more exotic junctions.
