<P> The mercury - wetted relay has one particular advantage, in that the contact closure appears to be virtually instantaneous, as the mercury globules on each contact coalesce . The current rise time through the contacts is generally considered to be a few picoseconds, however in a practical circuit it will be limited by the inductance of the contacts and wiring . It was quite common, before the restrictions on the use of mercury, to use a mercury - wetted relay in the laboratory as a convenient means of generating fast rise time pulses, however although the rise time may be picoseconds, the exact timing of the event is, like all other types of relay, subject to considerable jitter, possibly milliseconds, due to mechanical imperfections . </P> <P> The same coalescence process causes another effect, which is a nuisance in some applications . The contact resistance is not stable immediately after contact closure, and drifts, mostly downwards, for several seconds after closure, the change perhaps being 0.5 ohm . </P> <P> A mercury relay is a relay that uses mercury as the switching element . They are used where contact erosion would be a problem for conventional relay contacts . Owing to environmental considerations about significant amount of mercury used and modern alternatives, they are now comparatively uncommon . </P> <P> A polarized relay places the armature between the poles of a permanent magnet to increase sensitivity . Polarized relays were used in middle 20th Century telephone exchanges to detect faint pulses and correct telegraphic distortion . The poles were on screws, so a technician could first adjust them for maximum sensitivity and then apply a bias spring to set the critical current that would operate the relay . </P>

What is an electrical relay and how does it work