Priniciples of Electromechanical Relay Operation
3.33 Mercury Displacement Relays
This type of relay, owing to its construction has many advantages, such as:
1. Self renewing contact. Mercury rewets the contact electrode providing
a new contact surface with each actuation, so the surface does not pit or
weld.
2. Hermetically sealed contacts. This provides internal and external
protection from arcing and environmental abuse.
3. Low contact resistance. Large electrodes and surrounding mercury
volume creates large contact mating areas.
4. Quiet operation. Acoustical noise from rebounding contacts is
eliminated.
5. Long life. One moving part an no pivots, hinges, pins or mechanical
linkages results in virtually no wear.
6. Bounce free operation. Mercury surface tension enable the mercury to
bridge the contacts during the plunger settling time.
The principle of operation can be understood by visualizing a glass tube
containing a pool of mercury upon which floats a metallic plunger. An
electrode is suspended some distance above the surface of mercury. When the
coil power is off, the mercury level is below the electrode tip. No current
path exists between the insulated center electrode and the mercury pool. When
coil power is applied the plunger is drawn down into the mercury pool by the
pull of the magnetic field and the plunger centers itself within the coil.
This action raises the mercury level, so it covers the end of the electrode
and thus completes the current path. Upon removing the coil power the
buoyancy force of the mercury causes the plunger assembly to again rise to the
starting position. This drops the level of the mercury and breaks the current
path through the center electrode and the mercury pool. A disadvantage of
this type of control is the relay must be mounted upright.