Contact resistance of relay

__I consider there are “contact resistance” and “contact electric potential” between leads for all relays.
But it seems wrong. If the materials of leads are the same, there is no such “contact electric potential” thing.
Or there is, but less than 1uV and I can’t observe it use 34401 multimeter.
__All relays have their contact resistance. This resistance is in the scale of mOhm level.
__I measure ten power-relays (IDEC RJ1V-C-D24, power relay, 12A), results are bellow:
____Always-On ports: 105mOhm->37mOhm (when sudden off, shows 105mOhm, then turn small slow)
____Always-Off ports: 6mOhm+/-3mOhm (stable in one second)
__At the same time, not only there are differences between relays on alway-on ports resistance, also unstable for one same relay.
Summery:
1. In general, contact resistance of power relay is small than signal relays because power relays are used under high current.
2. If you want to control the resistance, use always-off ports of relays.
3. Different relays may have different contact resistance, read the data-sheet and measure yourself.