Before transistors and integrated circuits revolutionized electronics, a humble mechanical switch called the relay provided the capability to process signals and data automatically. Developed based on pioneering discoveries in electromagnetism, the electromechanical relay ultimately proved indispensable both for long distance communications and for enabling programmable calculating machines that presaged today‘s computers.
What is a Relay?
In simple terms, a relay is a switch that opens or closes an electrical circuit remotely using a magnetic field rather than a manual movement. It consists of:
- An electromagnet made of a coil of insulated wire wrapped around an iron armature. Running electric current through the coil magnetizes the armature.
- A movable iron lever arm hinged near the electromagnet. This can be attracted by the magnetic field when energized.
- Electrical contacts connected to the lever arm and fixed contacts. These are pushed together or pulled apart as the arm moves to make or break a circuit.
By controlling power to the electromagnet remotely, one can toggle the relay‘s internal switch on and off. Crucially, only a small amount of current applied to the coil is required to operate high current capacity contacts. Next we‘ll see how discoveries in electromagnetism enabled inventors to unlock this capability.
How Experiments Unraveling Electromagnetism‘s Mysteries Led to the Relay
Magnetism seems on the surface to be an entirely distinct invisible force from the electric currents that flow through wires. However, some ingenious experimentation in early 1800s Europe uncovered an intimate connection between the two phenomena, forging a path that led to the relay.
| Year | Discovery | Contribution to Relays |
|---|---|---|
| 1819 | Hanson Christian Ørsted demonstrates electric current in a wire deflects compass needle | Reveals electricity creates circular magnetic force fields |
| 1825 | William Sturgeon invents the electromagnet by coiling bare wire around iron, magnetizing it when current flows through | Establishes method for concentrates magnetic force – later core principle of relay operation |
| 1831 | Joseph Henry greatly improves electromagnet strength by insulating wire with fabric wrapping | Allows stronger magnetic fields for reliable relay operation at low power |
Henry Demonstrates the First Electric Relay
Capitalizing on progress revealing links between electricity and magnetism, American scientist Joseph Henry constructed a pioneering electric relay in 1835 at Princeton University. His simple device consisted of:
- A large iron bar wrapped in insulated copper wire to serve as an electromagnet
- An iron armature positioned next to the bar‘s pole with a hinge to act as a lever arm
- Separate fixed contacts positioned so the armature‘s motion pushed contact points together
Henry‘s sketch of his initial relay using wire coils around an iron bar to control a switch via a hinged armature. [Public domain image]
Applying electric current to the coils induced a magnetic field in the iron bar. This magnetized the armature, causing it to tilt on its hinge and push the contacts closed, allowing separate battery power to flow through the contacts. Turning off the coil current removed the magnetic force, letting the armature spring arm pull back and separate the contacts again.
Later relay designs incorporated modifications like using a lightweight spring for the armature and adding multiple switch contacts. But the operating concept based on Henry‘s experiment remains fundamentally similar over 180 years later!
Next we‘ll see how the legendary Samuel Morse applied Henry‘s research to achieve long distance communications…
Extending Communications: Relay‘s Role Enabling Transcontinental Telegraph
Famed inventor Samuel Morse developed the electric telegraph in 1835 to transmit messages over wires using a code of short and long signal pulses. However his early telegraphs only worked over short distances – signals attenuated too rapidly from resistance in the wires.
Hearing of Henry‘s research, Morse met with him in 1837. Morse incorporated Henry‘s relay into his telegraphs, utilizing its ability to detect faint signals arriving in local wires and retransmit a strong boosted copy to the next station down the line. Adding relay amplifier nodes enabled messages sent in Morse code to traverse unprecedented distances across continents and oceans!
Revitalizing Communications
The early 19th century saw visionaries like Henry unveil amazing new technological capabilities through applied science. But often these remained laboratory curiosities failing to profoundly reshape society.
Morse‘s genius was recognizing how Henry‘s arcane electromagnetic relay could transform communications. Adoption of Morse Code and the electric telegraph then built momentum upgrading messaging from reliance on letters carried weeks overland to near instant transmission. The benefits over earlier optical telegraph networks were immeasurable.
Telephone Networks Drive Mass Adoption of Relays
Morse‘s telegraph famously connected America from coast to coast along railroad right of ways. But ironically, building transcontinental phone service lagged several decades behind, relying on operators manually connecting switchboard lines instead.
This hands-on approach faltered accommodating soaring subscriber growth following commercialization of Bell‘s telephone in 1878. By the early 1900s over 3 million phones demanded service!
Rotary Dial Interfaces
Bell company engineers like Almon Strowger addressed the overload by automating telephone exchanges using relays. Strowger‘s 1892 invention of the rotary dial enabling customer-directed call routing especially drove demand.
Previously an operator asked a caller whom they wished to contact then manually pulled cords to patch their lines together. Rotary dials instead generated sequences of hook fluctuations that pulsed through the exchange‘s electromechanical relays to electrically connect incoming and outgoing lines matching the dialed digit patterns.
Harnessing Relays‘ Potential for Flexible Programmable Circuits
While relays provided essential communications and switching capabilities, visionaries pushing boundaries of information processing recognized they could also form flexible computational circuits.
Charles Babbage devised mechanical gears performing programmable functions for his landmark Analytical Engine computer design in the 1830s. Spanish engineer Leonardo Torres Quevedo suggested in 1915 that relays could provide similar capabilities through reconfigurable electrical connections.
Compared to mechanical gears, relays offered key benefits:
- Simplicity: Relays were easier to manufacture than precision gearing
- Reliability: Electrical contacts had less wear over time than moving parts
- Reconfigurability: Relays could be unplugged and rewired easing modifications
By the 1930s, scientists like George Stibitz began demonstrating rudimentary computing circuits built from banks of relays. Combined with paper tape control mechanisms also made of relays, they constituted some of the earliest electrical computers!
The Triumph of Electromechanical Computation
Stibitz‘s early relay computers performed admirably, conducting complex mathematical calculations far exceeding mechanical adding machines of the era. Programs executing arithmetic operations were encoded into the paper tapes which directed associated relay modules.
Example Relay Computer Calculations
Proof of the relay‘s versatility came in 1943 when Bell Laboratories engineer George Stibitz used an a relay based complex number calculator to compute important data for the Manhattan Project. Stibitz remotely accessed the machine over telegraph lines from New York using an early modem to control calculations of implosion energies instrumental for the first nuclear weapons at Los Alamos in 1945.
Conclusion: The Relay‘s Enduring Legacy
From Henry‘s initial discovery that electricity could manipulate electromagnets to control mechanical motion, the relay persisted as an essential pillar of technological advancement for over a century. Refined into a robust industrial component, relays soon became ubiquitous allowing Bell‘s ambitious vision of reliable nationwide telephone service to scale up.
Beyond that, they found unexpected application enabling flexible information circuits paving the way for modern programmable computing. Today, transistors and microchips have long supplanted bulky mechanical contacts. But those tiny solid state gates modulating electrons to represent binary 0s and 1s are in a very real sense the spiritual descendants of venerable relays that provided the first glimpses of that conceptual power over 100 years earlier!
So while they seem antiquated now, relays hosted pivotal milestones spanning fundamental science, global communications, and computation that transformed civilization. Though the underlying technologies advanced, that persevering legacy continues opening new frontiers daily.
