A relay is a switch that activates a current that turns on a separate circuit. A relay is utilized when a separate circuit is needed or if multiple circuits need to be controlled by an individual signal. An example of a relay in action is starting a car. A car key doesn’t power a car battery directly, but instead, the ignition signals the relay, which then transfers the signal for the car to start.
The start of these electric switches was first used in long distance telegraph circuits. Relays were introduced during an educational demonstration by Joseph Henry an “electromagnetism pioneer” in 1835. He showed how his electromagnetic switch could turn on a much larger switch.
Relays are mainly used with either electromagnetic relays (EMR) or solid-state relays (SSR). The main difference is that the solid state doesn’t have any moving parts, and electromagnetic does contain moving parts. EMRs work through magnetic force while SSRs are entirely electronic.
Solid state relays are faster than electromagnetic relays, but they are semiconductors as opposed to mechanical contacts. These relays use less voltage and have no moving parts which makes them faster. In contrast to their speed, when closed, SSRs suffer from high resistance. When open, SSRs suffer from low resistance. These relays use light-emitting diodes (LED) and light sensitive semiconductors to switch power off or on. Since there are no moving parts as there are in electromagnetic relays, SSRs are able to operate with minimal to no delay. These types of relays are specifically used in areas were switching on and off is done frequently.
Electromagnetic relays use mechanical operations to open and close the switch using magnetic force. These relays are used in devices like alarms and sensors. At the center of this device is an electromagnet which is constructed by a twisted coil around an iron core. Induction is also used in an electromagnetic relay to control the electronic circuits. The advantage with these relays is being able to switch between circuits. This function has made the modern computer and microprocessor better and faster. Switching between circuits is one of the core elements in electronic devices. These types of 
relays come in an array of sizes and are found anywhere from small devices to industrial machinery.
Overall relays, in general, are used to provide a link between different electrical points. They are suitable in a range of various applications and come in a variety of forms. For example, they can be used in automobile devices, industrial equipment, security devices, and household appliances. A relay is an essential the switch that makes the electrical component of these devices functional.
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