Telephone
The telephone is a telecommunications device that converts sound, typically the human voice, into electrical signals for transmission over a distance via wires or wirelessly, enabling real-time conversation between parties who are not in direct hearing range.[1] It operates by using a microphone to capture audio vibrations, converting them into signals that travel through a network, and a speaker at the receiving end to reconvert them into audible sound.[2] The invention of the practical telephone is credited to Alexander Graham Bell, who received the first U.S. patent for it on March 7, 1876, describing an apparatus for "transmitting vocal or other sounds telegraphically."[3] Three days later, on March 10, 1876, Bell made the device's first successful telephone call to his assistant Thomas Watson, famously saying, "Mr. Watson, come here—I want to see you."[4] Bell's work built on earlier experiments with sound transmission, including contributions from inventors like Antonio Meucci, who developed a basic voice communication device in 1849; in 2002, the U.S. House of Representatives passed a resolution acknowledging Meucci's work in the invention of the telephone, though Bell's patent secured the foundational rights.[5][6] Following its invention, the telephone rapidly transformed global communication, with the first commercial telephone line installed between Boston and Somerville, Massachusetts, in 1877, and nearly 600,000 telephones in use across the Bell system by 1900.[7] Early models featured a simple carbon microphone and battery-powered transmitter, evolving in the late 19th and early 20th centuries to include switchboards for connecting calls manually, which operators used to route connections in growing urban networks.[8] By the mid-20th century, innovations like the rotary dial in the 1890s and touch-tone dialing in the 1960s automated call placement, while the introduction of direct distance dialing in 1951 allowed users to connect long-distance calls without operator assistance.[7] The telephone's evolution continued into wireless forms, with Martin Cooper of Motorola making the first handheld mobile phone call on April 3, 1973, using a prototype device weighing about 2 pounds that operated on cellular networks.[9] This paved the way for commercial cell phones in 1983 and, by the 1990s, the integration of digital features like text messaging and internet access, culminating in smartphones that combine telephony with computing capabilities.[10] As of 2025, there are approximately 9.2 billion mobile-cellular subscriptions worldwide, with voice calls often supplemented by data-driven alternatives like video conferencing, fundamentally altering social, economic, and cultural interactions.[11]History
Invention and early development
The invention of the telephone emerged from mid-19th-century experiments in transmitting sound electrically, building on earlier telegraphy advancements. Italian inventor Antonio Meucci developed an early voice communication device known as the teletrofono around 1849 while working in Cuba, using a vibrating membrane connected to an electromagnet to transmit speech over wires between rooms in his residence.[12] By 1854, Meucci had refined this into a practical system for short-distance voice transmission, though financial constraints limited its further development after his immigration to the United States in 1850.[13] In 1871, Meucci filed a patent caveat with the U.S. Patent Office describing his "cave telegraph" for transmitting vocal sounds telegraphically, but it lapsed due to unpaid fees by 1874.[14] German inventor Johann Philipp Reis advanced these ideas with his Reis telephone, first demonstrated publicly on October 26, 1861, to the Physical Society of Frankfurt.[15] Reis's device featured a vibrating membrane attached to a platinum contact that interrupted an electrical circuit to transmit musical tones and some speech sounds over distances up to 100 meters, though it struggled with clear voice articulation.[16] Between 1861 and 1863, Reis built three iterative models, with the final version using a cork-lined platinum knife-edge contact to modulate current based on sound vibrations, earning recognition for pioneering electrical sound transmission.[17] Alexander Graham Bell, a Scottish-born inventor focused on harmonic telegraphs for multiple simultaneous signals, achieved the first practical telephone in 1876. Bell's early work included the 1875 harmonic telegraph, which used tuned reeds to transmit tones, leading to his liquid transmitter design filed under U.S. Patent 174,465 on February 14, 1876, for "Improvement in Telegraphy" that enabled vocal sound transmission via undulating currents.[18] On March 10, 1876, in Boston, Bell successfully transmitted the first intelligible sentence—"Mr. Watson, come here, I want to see you"—to his assistant Thomas Watson using an experimental gallows frame telephone, a sound-powered model with a taut membrane over an electromagnet.[19] This breakthrough followed Bell's prior gallows frame prototypes from late 1875, which produced indistinct voice-like sounds but paved the way for clearer speech.[20] That same day, American inventor Elisha Gray filed a patent caveat for a harmonic telegraph adapted for voice, describing a water-based variable resistance transmitter nearly identical to Bell's approach, submitted just hours after Bell's full patent application.[3] The U.S. Patent Office granted Bell's patent on March 7, 1876, sparking intense legal disputes over priority, including claims of caveat suppression and information sharing.[21] In 1879, a federal circuit court ruled in Bell's favor in the initial major case, affirming his invention claims against challengers like the Western Union Telegraph Company, though further appeals persisted.[22] The U.S. Supreme Court upheld Bell's patents in 1888 by a narrow 4-3 margin in the Telephone Cases, solidifying his legal recognition as the telephone's inventor despite ongoing debates involving Meucci and Reis. In 2002, the United States House of Representatives passed a resolution (H.Res. 269) acknowledging Antonio Meucci's pioneering work and honoring him for his role in the invention of the telephone.[23]Commercialization and expansion
Following the successful patenting of the telephone in 1876, Alexander Graham Bell, along with financial backers Gardiner Greene Hubbard and Thomas Sanders, formed the Bell Telephone Company on July 9, 1877, in Boston, Massachusetts, to commercialize the invention and manage its patents.[24] This entity initially operated without formal capital but quickly secured investments, including from Sanders, to support manufacturing and deployment.[25] The company's first marketable product was the 1877 magneto telephone, a wooden box-style instrument that combined transmitter and receiver functions, featuring a hand-crank generator to produce signaling current via electromagnetic induction, enabling reliable operator alerts without external batteries.[26] Commercial viability advanced significantly with Thomas Edison's carbon transmitter, patented as U.S. Patent 474,230 on May 3, 1892 (filed April 27, 1877), which used a carbon granule microphone to amplify voice volume and clarity, addressing the faint signals of Bell's original liquid-based design.[27] This improvement, licensed to Bell after a 1879 settlement with Western Union, made long-distance conversations practical and spurred widespread adoption.[28] In 1882, the American Bell Telephone Company acquired Western Electric Manufacturing Company, establishing it as the exclusive manufacturing arm for telephone equipment in the United States, including the production of the first manual switchboards that connected multiple subscribers through operators.[29] The telephone's global expansion accelerated in the 1880s, with the International Bell Telephone Company, founded in 1879 in Brussels, overseeing European operations. The first international telephone line in Europe opened between Paris and Brussels in January 1887, spanning 195 miles and using overhead wires to connect the cities' exchanges. By 1900, telephone networks had been established across Europe (e.g., over 900,000 subscribers in Germany), Asia (e.g., Japan's Tokyo exchange operational since 1883 and about 9,000 lines in India), and Latin America (e.g., Brazil's system introduced in 1881 with under 10,000 users).[28] In the United States, the Bell Telephone Company maintained dominance through its patent monopoly until the key patents expired between 1893 and 1894, prompting antitrust challenges and the emergence of independent telephone companies that built parallel networks.[30] By the early 1900s, these independents numbered over 6,000, serving more than 1 million subscribers and challenging Bell's control, though interconnection disputes persisted.[28]20th-century advancements
The invention of the rotary dial by Almon Brown Strowger, an undertaker from Kansas City, Missouri, marked a pivotal shift toward automated telephony. Strowger developed the device amid suspicions that telephone operators were diverting calls from his business, leading to his filing for a patent on an automatic telephone exchange in 1889, which was granted as U.S. Patent 447,918 on March 10, 1891. This stepping switch mechanism used finger holes on a dial to generate electrical pulses for routing calls without human intervention. A subsequent patent, U.S. 486,909 issued on November 29, 1892, refined the rotary dial interface, enabling callers to select connections directly. The first commercial automatic exchange using this technology opened in La Porte, Indiana, in 1892, with broader adoption by 1897 as Strowger Automatic Telephone Exchange expanded installations, reducing reliance on manual switchboards and improving efficiency in growing urban networks.[31][32][33] Early 20th-century telephone designs evolved from upright wall-mounted models to more user-friendly forms. The candlestick telephone, introduced by Western Electric in the 1910s, featured a tall, slender transmitter stand with a separate receiver, offering improved ergonomics over prior boxy units while maintaining compatibility with existing lines. This design dominated residential and business use through the 1920s. In 1927, Western Electric unveiled the first integrated handset desk set, combining the transmitter and receiver into a single ergonomic unit mounted on a rounded base, which streamlined manufacturing and user interaction. This innovation laid the groundwork for subsequent models like the 202 series in the 1930s, emphasizing durability and aesthetic appeal in mass-produced telephony. By the late 1920s, such desk sets became standard, facilitating wider household adoption.[34] The mid-20th century brought transformative electronic components to telephony. In December 1947, scientists John Bardeen, Walter Brattain, and William Shockley at Bell Laboratories demonstrated the first point-contact transistor, a solid-state semiconductor device that amplified signals without the fragility and heat of vacuum tubes. This breakthrough, announced publicly in 1948, enabled the miniaturization of telephone equipment, leading to more compact, reliable handsets and switching gear by the 1950s. Transistorized circuits reduced power consumption and maintenance needs, paving the way for portable and automated systems that enhanced network scalability.[35][36] Advancements in long-distance connectivity accelerated with automated dialing and undersea cabling. Direct Distance Dialing (DDD) debuted in the United States on November 10, 1951, allowing customers to place interstate calls without operator assistance using area codes and seven-digit numbers, initially connecting Englewood, New Jersey, to Alameda, California. This system expanded nationwide through the decade, cutting connection times from minutes to seconds. Internationally, the first transatlantic telephone cable, TAT-1, linked Scotland to Newfoundland and entered service on September 25, 1956, supporting 36 simultaneous voice circuits and enabling reliable, high-quality calls across the Atlantic for the first time via coaxial cable technology. These developments dramatically increased intercontinental communication capacity.[37][38] Dialing mechanisms progressed from pulse-based systems to electronic tones in the late 1950s. Bell Laboratories patented dual-tone multi-frequency (DTMF) signaling in 1960, as detailed in the Bell System Technical Journal, where each button press generated a unique pair of audio frequencies for faster, more accurate call routing. Marketed as Touch-Tone, it replaced rotary pulse dialing's mechanical interruptions with audible tones, allowing data transmission alongside voice and reducing errors in automated exchanges. Public rollout began in 1963, gradually supplanting rotary dials and enabling features like automated banking by the 1970s.[39] Switching technology transitioned from electromechanical relays to electronic controls during the 1960s, with stored program control (SPC) exchanges revolutionizing operations. Bell's No. 1 Electronic Switching System (1ESS), deployed in Succasunna, New Jersey, in January 1965, was the first large-scale SPC implementation, using a central processor to store and execute call-routing instructions in software rather than hardwired logic. This allowed flexible updates, fault diagnosis, and higher call volumes without physical rewiring, marking the shift to computer-controlled telephony. Subsequent systems worldwide adopted SPC, boosting efficiency and supporting traffic growth. These innovations drove substantial expansion in telephone access. Global teledensity, measured as main telephone lines per 100 inhabitants, rose from approximately 1% in 1900—concentrated in industrialized regions—to over 10% by 1980 in developed nations, reflecting infrastructure investments and economic integration. In the United States alone, household penetration exceeded 90% by 1980, underscoring the telephone's role as essential infrastructure.[40]Technical Principles
Acoustic-to-electrical conversion
In traditional telephones, the acoustic-to-electrical conversion process begins at the transmitter, where sound waves from the speaker's voice are captured and transformed into an electrical signal proportional to the acoustic variations. This signal is then transmitted over the telephone line to the receiver at the other end, where it is reconverted into audible sound waves. Early innovations in this mechanism were pioneered through patents by Alexander Graham Bell in 1876, which described a transmitter using a vibrating armature to induce undulatory currents mimicking vocal sounds, and by Thomas Edison, who introduced a more practical carbon-based design in 1877 (U.S. Patent 474,230, filed 1877, issued 1892).[18][41] The carbon granule microphone, a seminal design by Edison, serves as the core transmitter in early and many subsequent analog telephones. Sound pressure waves impinge on a thin diaphragm connected to a chamber containing loose carbon granules sandwiched between metal electrodes. As the diaphragm vibrates, it compresses or loosens the granules, varying the contact resistance R within the circuit. With a constant line current I supplied by the telephone exchange (typically 20-50 mA), the output voltage V modulates according to Ohm's law: V = I \times R, where changes in R (from approximately 100-1000 ohms) directly correspond to the acoustic input, producing a modulated current that encodes the voice signal. This variable-resistance principle provided higher sensitivity and output compared to prior electromagnetic designs, enabling clearer transmission over long distances.[41] At the receiving end, the earpiece employs an electromagnetic receiver to reverse the process. The incoming AC signal flows through coils wound on the poles of a permanent magnet, generating fluctuating magnetic fields that attract and vibrate a soft-iron diaphragm positioned nearby. These diaphragm vibrations displace surrounding air molecules, recreating pressure waves that match the original sound's frequency and amplitude. The permanent magnet ensures efficient operation by maintaining a steady bias field, with the AC variations superimposed to drive the diaphragm without requiring additional power. Typical designs maintain a small air gap (around 0.3-0.4 mm) between the diaphragm and magnet poles to optimize sensitivity while preventing distortion.[42] To ensure intelligible voice reproduction while conserving bandwidth and minimizing noise, telephone systems are tailored to the voice frequency band of 300 to 3400 Hz. This range captures the fundamental harmonics of human speech, excluding lower frequencies below 300 Hz (which convey little linguistic content) and higher ones above 3400 Hz (which add minimal intelligibility but increase susceptibility to interference). The limited bandwidth, spanning about 3.1 kHz, allows for efficient signal processing and transmission in early analog networks.[43] Full-duplex operation—allowing simultaneous speaking and listening over a single two-wire line—is facilitated by the hybrid transformer circuit at each telephone. This configuration uses a bridge circuit with transformers to isolate the transmit path (local microphone signal) from the receive path (incoming line signal), directing each appropriately while canceling echoes. A key component is the balance network, comprising inductors and capacitors tuned to mimic the line's impedance (typically 600-900 ohms with reactive elements like 2 µF capacitors), ensuring minimal sidetone (hearing one's own voice) and effective signal separation.[44] The electrical signals generated are low-level: the carbon granule microphone outputs typically 1-10 mV for normal speech input, reflecting its high sensitivity to pressure variations. This is amplified within the telephone or at the exchange to achieve line levels of approximately -10 dBm, providing sufficient power for transmission across the public switched telephone network without excessive attenuation.[45]Signal transmission and switching
In analog telephone systems, signals are transmitted over two-wire transmission lines consisting of twisted pair copper wires, which carry bidirectional voice signals in the frequency range of approximately 300 to 3400 Hz.[46] These pairs minimize electromagnetic interference through twisting, enabling reliable local loop connections from subscriber telephones to central offices.[47] Signal attenuation in such lines arises from resistive losses and increases with frequency and distance; the attenuation A in decibels per kilometer is given byA = 10 \log_{10} \left( \frac{P_{\text{in}}}{P_{\text{out}}} \right)
dB/km, where P_{\text{in}} and P_{\text{out}} are the input and output powers, respectively.[48] For typical 0.5 mm twisted pair cables used in telephony, maximum attenuation is around 8.25 dB/km, though values vary with conductor gauge and loading.[48] Early signal switching relied on manual switchboards introduced in 1878, where operators used cord plugs to physically connect incoming and outgoing lines at a central exchange, establishing circuits for voice transmission. This labor-intensive method handled growing call volumes but was limited by human speed and error rates.[49] To automate routing, Almon Strowger developed the step-by-step electromechanical switch in 1891, which used finger-like selectors driven by dial pulses to progressively route calls through banks of contacts without operator intervention.[50] Centralized exchanges employed pulse dialing, where a rotary dial generated interruptions in the loop current to encode digits; for example, the digit 0 produced 10 pulses per second by opening the circuit 10 times over a 1-second interval.[51] Automatic selectors in the exchange counted these pulses to advance switches and connect the called party.[50] By 1938, crossbar switching systems improved efficiency using electromagnetic grids of horizontal and vertical bars, where solenoids selected crosspoints to form paths, reducing wear and enabling faster, more reliable connections than step-by-step mechanisms.[52] Signaling protocols facilitated call setup and supervision: ringing was initiated by superimposing an AC voltage of 75-90 V at 16-25 Hz on the line to activate the called telephone's bell, while off-hook detection relied on a DC loop current of 20-50 mA flowing through the subscriber loop when the handset was lifted.[53] For long-distance transmission, loading coils—inductors spaced along lines to satisfy the Heaviside condition for distortionless propagation—were introduced by Mihajlo Pupin in 1899, extending clear voice reach by countering capacitive and inductive imbalances.[54] From the 1940s, microwave relay systems supplemented wire lines, using line-of-sight radio links at frequencies around 4-6 GHz to relay multiplexed voice channels over hundreds of kilometers with minimal latency.[55]