A mercury relay (mercury displacement relay, mercury contactor) is a relay that uses mercury as the switching element.
They are used as high-current switches or contactors, where contact erosion from constant cycling would be a problem for conventional relay contacts.
Owing to environmental considerations about the toxicity of mercury, mercury relays are mostly obsolete, though modern encapsulated units still have applications. They are generally being replaced by solid state relays.
Mercury relays consist of a vertical (usually glass) tube containing liquid mercury. They have isolated contacts at the bottom of the tube and partway up, usually in a side arm of the glass. The relay works by displacement. A pool of mercury fills the lower portion of the tube, but is insufficient to bridge the contacts. A magnetic [lower-roman 1] slug of iron or steel is placed in the tube, where it sinks by gravity so as to displace the mercury. The displaced mercury rises in the tube, [lower-roman 2] sufficiently to bridge the contacts and complete the circuit between them.
Around the top part of the tube is placed the coil. When energised, this coil attracts the slug, [lower-roman 3] lifting it upwards and out of the mercury pool. The mercury is no longer displaced, and thus flows downwards, away from the upper contact, and so the circuit opens. This allows for normally closed operation.
For the more traditional normally open relay operation, the side contact is arranged somewhat higher up (or the volume of mercury is reduced), so that contact is not made when the iron slug is freely floating on the pool of mercury. The control coil is mounted below the rest level of the slug, and when energised draws down the slug deeper into the pool, thereby displacing additional mercury and thus raising the level to the previously uncovered side contact and closing the circuit. [1]
The mercury relay thus allows for switching of higher currents with a small control current, for a large number of cycles. They are often installed into automatic controllers that required extended periods of unattended continuous switching operation. The mercury surface is self-restoring after an arc, and the contact resistance is low and stable. [1]
The glass tube of a mercury relay must be mounted near-vertically. The sensitivity of these relays can be altered by adjusting their angle relative to vertical. As sensitivity depends upon angle, they are unsuitable for use on mobile equipment or with conditions of high vibration.
Mercury relays have also been produced as latching or impulse relays. The Lenning design uses a horizontal glass tube with two axially isolated pools of mercury. [2] A conductive stirrup can bridge these to make the connection. The relay is controlled by the stirrup being rotated in and out of the pool along the horizontal axis of the tube. A weight on the stirrup's armature gives an over-centre action that provides the latching behaviour. A magnetic slug on the armature allows it to be rotated and controlled by an external electromagnet.
Owing to the mass of mercury moved during switching, compared to that of the armature and spring leaves of a conventional relay, the mercury relay is not a high-speed device. Despite this, the mercury relay does have a very low contact bounce time, in the sub-millisecond range. For some applications, particularly inductive loads, this alone may be a reason for their use – the timing of contact closure is not rapid, but the avoidance of bounce is valuable.
For high-speed use, the mercury-wetted relay is used instead. This combines the speed of a low-mass relay, together with the fast wetting of mercury contacts. A relay, usually a reed relay, has its contacts coated with a small quantity of mercury. This gives the low bounce advantage of mercury, although the current capacity is still limited to broadly that of the original reed relay.
A relay is an electrically operated switch. It consists of a set of input terminals for a single or multiple control signals, and a set of operating contact terminals. The switch may have any number of contacts in multiple contact forms, such as make contacts, break contacts, or combinations thereof.
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of switch is an electromechanical device consisting of one or more sets of movable electrical contacts connected to external circuits. When a pair of contacts is touching current can pass between them, while when the contacts are separated no current can flow.
A commutator is a rotary electrical switch in certain types of electric motors and electrical generators that periodically reverses the current direction between the rotor and the external circuit. It consists of a cylinder composed of multiple metal contact segments on the rotating armature of the machine. Two or more electrical contacts called "brushes" made of a soft conductive material like carbon press against the commutator, making sliding contact with successive segments of the commutator as it rotates. The windings on the armature are connected to the commutator segments.
An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature. Occasionally, a linear alternator or a rotating armature with a stationary magnetic field is used. In principle, any AC electrical generator can be called an alternator, but usually the term refers to small rotating machines driven by automotive and other internal combustion engines.
The stator is the stationary part of a rotary system, found in electric generators, electric motors, sirens, mud motors or biological rotors. Energy flows through a stator to or from the rotating component of the system. In an electric motor, the stator provides a magnetic field that drives the rotating armature; in a generator, the stator converts the rotating magnetic field to electric current. In fluid powered devices, the stator guides the flow of fluid to or from the rotating part of the system.
An induction coil or "spark coil" is a type of electrical transformer used to produce high-voltage pulses from a low-voltage direct current (DC) supply. To create the flux changes necessary to induce voltage in the secondary coil, the direct current in the primary coil is repeatedly interrupted by a vibrating mechanical contact called an interrupter. Invented in 1836 by Nicholas Callan, with additional research by Charles Grafton Page and others, the induction coil was the first type of transformer. It was widely used in x-ray machines, spark-gap radio transmitters, arc lighting and quack medical electrotherapy devices from the 1880s to the 1920s. Today its only common use is as the ignition coils in internal combustion engines and in physics education to demonstrate induction.
A coilgun is a type of mass driver consisting of one or more coils used as electromagnets in the configuration of a linear motor that accelerate a ferromagnetic or conducting projectile to high velocity. In almost all coilgun configurations, the coils and the gun barrel are arranged on a common axis. A coilgun is not a rifle as the barrel is smoothbore.
A mercury switch is an electrical switch that opens and closes a circuit when a small amount of the liquid metal mercury connects metal electrodes to close the circuit. There are several different basic designs but they all share the common design strength of non-eroding switch contacts.
The reed switch is an electromechanical switch operated by an applied magnetic field. It was invented in 1922 by professor Valentin Kovalenkov at the Petrograd Electrotechnical University, and later evolved at Bell Telephone Laboratories in 1936 by Walter B. Ellwood into the reed relay. In its simplest and most common form, it consists of a pair of ferromagnetic flexible metal contacts in a hermetically sealed glass envelope. The contacts are usually normally open, closing when a magnetic field is present, or they may be normally closed and open when a magnetic field is applied. The switch may be actuated by an electromagnetic coil, making a reed relay, or by bringing a permanent magnet near it. When the magnetic field is removed, the contacts in the reed switch return to their original position. The "reed" is the metal part inside the reed switch envelope that is relatively thin and wide to make it flexible. It somewhat resembles part of some reed plants. The term "reed" may also include the external wire lead as well as the internal part.
A DC motor is any of a class of rotary electrical motors that converts direct current (DC) electrical energy into mechanical energy. The most common types rely on the forces produced by induced magnetic fields due to flowing current in the coil. Nearly all types of DC motors have some internal mechanism, either electromechanical or electronic, to periodically change the direction of current in part of the motor.
A mercury-arc valve or mercury-vapor rectifier or (UK) mercury-arc rectifier is a type of electrical rectifier used for converting high-voltage or high-current alternating current (AC) into direct current (DC). It is a type of cold cathode gas-filled tube, but is unusual in that the cathode, instead of being solid, is made from a pool of liquid mercury and is therefore self-restoring. As a result mercury-arc valves, when used as intended, are far more robust and durable and can carry much higher currents than most other types of gas discharge tube. Some examples have been in continuous service, rectifying 50 ampere currents for decades.
In electrical engineering, the armature is the winding of an electric machine which carries alternating current. The armature windings conduct AC even on DC machines, due to the commutator action or due to electronic commutation, as in brushless DC motors. The armature can be on either the rotor or the stator, depending on the type of electric machine.
A contactor is an electrically-controlled switch used for switching an electrical power circuit. A contactor is typically controlled by a circuit which has a much lower power level than the switched circuit, such as a 24-volt coil electromagnet controlling a 230-volt motor switch.
A wire spring relay is a type of relay, that has springs made from drawn wires of nickel silver, rather than cut from flat sheet metal as in the flat-spring relay. This class of relays provided manufacturing and operating advantages over previous designs. Wire spring relays entered mass production in the early 1950s.
A repulsion motor is a type of electric motor which runs on alternating current (AC). It was formerly used as a traction motor for electric trains but has been superseded by other types of motors. Repulsion motors are classified as single phase motors.
A reed relay is a type of relay that uses an electromagnet to control one or more reed switches. The contacts are of magnetic material and the electromagnet acts directly on them without requiring an armature to move them. Sealed in a long, narrow glass tube, the contacts are protected from corrosion. The glass envelope may contain multiple reed switches or multiple reed switches can be inserted into a single bobbin and actuate simultaneously. Reed switches have been manufactured since the 1930s.
A brushed DC electric motor is an internally commutated electric motor designed to be run from a direct current power source and utilizing an electric brush for contact.
A shading coil or shading ring is a single turn of electrical conductor located in the face of the magnet assembly or armature of an alternating current solenoid. The alternating current in the energized primary coil induces an alternating current in the shading coil. This induced current creates an auxiliary magnetic flux which is 90 degrees out of phase from the magnetic flux created by the primary coil.
In engineering, a solenoid is a device that converts electrical energy to mechanical energy, using an electromagnet formed from a coil of wire. The device creates a magnetic field from electric current, and uses the magnetic field to create linear motion. In electromagnetic technology, a solenoid is an actuator assembly with a sliding ferromagnetic plunger inside the coil. Without power, the plunger extends for part of its length outside the coil; applying power pulls the plunger into the coil. Electromagnets with fixed cores are not considered solenoids. In simple terms, a solenoid converts electrical energy into mechanical work. Typically, it has a multiturn coil of magnet wire surrounded by a frame, which is also a magnetic flux carrier to enhance its efficiency. In engineering, the term may also refer to a variety of transducer devices that convert energy into linear motion, more sophisticated than simple two–position actuators. The term "solenoid" also often refers to a solenoid valve, an integrated device containing an electromechanical solenoid which actuates either a pneumatic or hydraulic valve, or a solenoid switch, which is a specific type of relay that internally uses an electromechanical solenoid to operate an electrical switch; for example, an automobile starter solenoid or linear solenoid. Solenoid bolts, a type of electromechanical locking mechanism, also exist.
