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electromechanical switchswitch,
electrical device having two states: on, or closed; and off, or open. Ideally a switch offers a zero impedance to a current when it is closed, and it offers infinite impedance when open.
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 operated by a flow of electricity in one circuit and controlling the flow of electricity in another circuit. A relay consists basically of an electromagnet with a soft iron bar, called an armature, held close to it. A movable contact is connected to the armature in such a way that the contact is held in its normal position by a spring. When the electromagnet is energized, it exerts a force on the armature that overcomes the pull of the spring and moves the contact so as to either complete or break a circuit. When the electromagnet is de-energized, the contact returns to its original position. Variations on this mechanism are possible: some relays have multiple contacts; some are encapsulated; some have built-in circuits that delay contact closure after actuation; some, as in early telephone circuits, advance through a series of positions step by step as they are energized and de-energized.



a device containing a relay element and used to cause a sudden change in conditions in an electric circuit as a result of specified inputs. The number of working states of the controlled circuit is usually limited to two or, less often, three. The term “relay” is also frequently applied to relay-action devices that effect a change in conditions in, for example, pneumatic or hydraulic systems. Sometimes a single relay element is called a relay. Historically, the term “relay” was applied first to electromagnetic relays. Electrical telegraph signals were attenuated in long transmission lines, and such relays were used in amplifying the signals to levels adequate for the operation of telegraph equipment.

Relays can be classified according to the field of technology in which they find application; we speak, for example, of telegraph, telephone, and aircraft relays. Relays are also classified according to the physical nature of the external phenomena that cause them to operate. Examples are (1) electrical relays, which include such subtypes as current, voltage, power, impedance, and frequency relays, (2) mechanical relays, such as displacement, speed, acceleration, pressure, and level relays, and (3) thermal, optical, acoustical, chemical, and magnetic relays. With respect to the functions performed, there are protective, monitoring, control, and signaling relays, among others. The name of a relay often conveys information about the characteristics of its main elements; examples are electromagnetic, mag-netoelectric, electrothermal, contact, contactless, bimetallic strip, and solenoid relays. Alternatively, the name of the relay, as in hermetically sealed and nonhermetically sealed relays, may convey information about the design of the relay as a whole. A relay may simultaneously controlseveral independent electric circuits. Contacts were long the only actuating elements used in relays. Since the 1950’s relay designs have also made use of magnetically saturated elements (magnetic amplifiers) and semiconductor devices (transistors, thyristors), which do not require mechanical displacements to control electric circuits.

Figure 1. Electromagnetic relay: (1) core, (2) coil, (3) heelpiece, (4) armature, (5) contacts, (6) restoring spring

In the mid-1970’s electromagnetic relays were still the most widely used type. Figure 1 shows a simple electromagnetic relay. It operates as a result of the interaction of a ferromagnetic armature with the magnetic field of the coil, through which a current flows. When a certain current strength is reached in the relay coil, the armature is attracted to the core, thereby closing the contacts in the controlled circuit.

Time-delay relays constitute a special group. They provide a time lag in relay equipment during the transmission of external actions outside or inside the equipment.


Sotskov, B. S. Osnovy rascheta i proektirovaniia elektromekhanicheskikh elementov avtomaticheskikh i telemekhanicheskikh ustroistv. Moscow-Leningrad, 1965.




a team competition in distance racing in which each team member covers part of the total distance. Each member of the team, after completing his own distance, passes a baton to the next, although in some races this is done only symbolically. The two types of relays are medley and non-medley. In non-medley relays, all parts of the total distance are completed in the same manner, for example, as a footrace. Medley relays may involve a single sport, for example, swimming in varied strokes, or several sports, for example, rowing and motorcycling.

The program of the modern Olympic Games includes relays in track and field, swimming, skiing, and biathlon. In track and field, men’s and women’s relays cover four distances of 100 m or 400 m each. In swimming, freestyle relays cover four distances of 100 m each for women and 200 m for men; medley relays, with four distances of 100 m each for both men and women, include alternating distances of backstroke, breaststroke, butterfly, and freestyle. Skiing relays have four distances of 5 km for women and 10 km for men. Relays in biathlon have four distances of 7.5 km.

The Olympic torch is carried to the city hosting the games in accordance with the principle of a medley relay.

In the first half of the 20th century, relays were widely held to encourage sociopolitical and cultural ties, for example, peace, friendship, and all-star relays.


A microwave or other radio system used for passing a signal from one radio communication link to another.
A device that is operated by a variation in the conditions in one electric circuit and serves to make or break one or more connections in the same or another electric circuit. Also known as electric relay.


An electromechanical or solid-state device operated by variations in the input which, in turn, operate or control other devices connected to the output. They are used in a wide variety of applications throughout industry, such as in telephone exchanges, digital computers, motor and sequencing controls, and automation systems. Highly sophisticated relays are utilized to protect electric power systems against trouble and power blackouts as well as to regulate and control the generation and distribution of power. In the home, relays are used in refrigerators, automatic washers and dishwashers, and heat and air-conditioning controls. Although relays are generally associated with electrical circuitry, there are many other types, such as pneumatic and hydraulic. Input may be electrical and output directly mechanical, or vice-versa.

Relays using discrete solid-state components, operational amplifiers, or microprocessors can provide more sophisticated designs. Their use is increasing, particularly in applications where the relay and associated equipment are packaged together. See Amplifier


An electromechanical device in which changes in the current flow in one circuit (that flows through the device) are used to open or close electric contacts in a second circuit.


1. an automatic device that controls the setting of a valve, switch, etc., by means of an electric motor, solenoid, or pneumatic mechanism
2. Electronics an electrical device in which a small change in current or voltage controls the switching on or off of circuits or other devices


An electrical switch that allows a low power to control a higher one. A small current energizes the relay, which closes a gate, allowing a large current to flow through.

In the radio world, a relay is a device that receives a signal from a low-power or distant transmitter and retransmits it on the same or different frequency in order to increase the coverage area. For example, the signal from a broadcast facility in a valley would only propagate within that valley. A relay site at the top of a nearby mountain would rebroadcast the original signal to a wider audience. In commercial TV and radio, these relay sites are known as "translators."