Phones Around the World

The PSTN was the earliest example of traffic engineering to deliver Quality of Service (QoS) guarantees. A.K. Erlang (1878–1929) is credited with establishing the mathematical foundations of methods required to determine the amount and configuration of equipment and the number of personnel required to deliver a specific level of service.

In the 1970s the telecommunications industry conceived that digital services would follow much the same pattern as voice services, and conceived a vision of end-to-end circuit switched services, known as the Broadband Integrated Services Digital Network (B-ISDN). The B-ISDN vision has been overtaken by the disruptive technology of the Internet. Only the oldest parts of the telephone network still use analog technology for anything other than the last mile loop to the end user, and in recent years digital services have been increasingly rolled out to end users using services such as DSL, ISDN, FTTP and cable modem systems.

Many observers believe that the long term future of the PSTN is to be just one application of the Internet – however, the Internet has some way to go before this transition can be made. The QoS guarantee is one aspect that needs to be improved in the Voice over IP (VoIP) technology.

There are a number of large private telephone networks which are not linked to the PSTN, usually for military purposes. There are also private networks run by large companies which are linked to the PSTN only through limited gateways, like a large private branch exchange (PBX).

Telephone exchange:

A telephone operator manually connecting calls with patch cables at a telephone switchboard. In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls. A central office is the physical building used to house inside plant equipment including telephone switches, which make phone calls “work” in the sense of making connections and relaying the speech information.

The term exchange can also be used to refer to an area served by a particular switch (typically known as a wire center in the US telecommunications industry). More narrowly, in some areas it can refer to the first three digits of the local number. In the three-digit sense of the word, other obsolete Bell System terms include office code and NXX. In the United States, the word exchange can also have the legal meaning of a local access and transport area under the Modification of Final Judgment (MFJ).

Electromechanical Network signaling:

Circuits connecting two switches are called trunks. Before Signalling System 7, Bell System electromechanical switches in the United States communicated with one another over trunks using a variety of DC voltages and signaling tones. It would be rare to see any of these in use today.

Some signalling communicated dialed digits. An early form called Panel Call Indicator Pulsing used quaternary pulses to set up calls between a Panel switch and a manual switchboard. Probably the most common form of communicating dialed digits between electromechanical switches was sending dial pulses, equivalent to a rotary dial’s pulsing, but sent over trunk circuits between switches. In Bell System trunks, it was common to use 20 pulse-per-second between crossbar switches and crossbar tandems. This was twice the rate of Western Electric/Bell System telephone dials. Using the faster pulsing rate made trunk utilization more efficient because the switch spent half as long listening to digits. DTMF was not used for trunk signaling. Multi-frequency (MF) was the last of the pre-digital methods. It used a different set of tones sent in pairs like DTMF. Dialing was preceded by a special keypulse (KP) signal and followed by a start (ST). Variations of the Bell System MF tone scheme became a CCITT standard. Similar schemes were used in the Americas and in some European countries including Spain. Digit strings between switches were often abbreviated to further improve utilization. For example, one switch might send only the last four or five digits of a telephone number. In one case, seven digit numbers were preceded by a digit 1 or 2 to differentiate between two area codes or office codes, (a two-digit-per-call savings). This improved revenue per trunk and reduced the number of digit receivers needed in a switch. Every task in electromechanical switches was done in big metallic pieces of hardware. Every fractional second cut off of call set up time meant fewer racks of equipment to handle call traffic.

Examples of signals communicating supervision or call progress include E and M signaling, SF signaling, and robbed-bit signaling. In physical (not carrier) E and M trunk circuits, trunks were four wire. Fifty trunks would require a hundred pair cable between switches, for example. Conductors in one common circuit configuration were named tip, ring, ear (E) and mouth (M). In two-way trunks with E and M signaling, a handshake took place to prevent both switches from colliding by dialing calls on the same trunk at the same time. By changing the state of these leads from ground to -48 volts, the switches stepped through a handshake protocol. Using DC voltage changes, the local switch would send a signal to get ready for a call and the remote switch would reply with an acknowledgment to go ahead with dial pulsing. This was done with relay logic and discrete electronics. These voltage changes on the trunk circuit would cause pops or clicks that were audible to the subscriber as the electrical handshaking stepped through its protocol. Another handshake, to start timing for billing purposes, caused a second set of clunks when the called party answered. A second common form of signaling for supervision was called single-frequency or SF signaling. The most common form of this used a steady 2,600 Hz tone to identify a trunk as idle. Trunk circuitry hearing a 2,600 Hz tone for a certain duration would go idle. (The duration requirement reduced falsing). Some systems used tone frequencies over 3,000 Hz, particularly on SSB frequency division multiplex microwave radio relays. On T-carrier digital transmission systems, bits within the T-1 data stream were used to transmit supervision. By careful design, the appropriated bits did not change voice quality appreciably. Robbed bits were translated to changes in contact states (opens and closures) by electronics in the channel bank hardware. This allowed direct current E and M signaling, or dial pulses, to be sent between electromechanical switches over a digital carrier which did not have DC continuity.

Problems listening to the file? See media help.A characteristic of electromechanical switching equipment is that the maintenance staff could hear the mechanical clattering of Strowgers or crossbar relays. Most Bell System central offices were housed in reinforced concrete buildings with concrete ceilings and floors. In rural areas, some smaller switching facilities, such as Community Dial Offices (CDOs), were sometimes housed in prefabricated metal buildings. These facilities almost always had concrete floors. The hard surfaces reflected sounds.

During heavy use periods, it could be hard to talk over the clatter of calls being processed in a large switch. For example, on Mother’s Day in the US, or on a Friday evening around 5pm, the metallic rattling could make raised voices necessary. For wire spring relay markers these noises resembled hail falling on a metallic roof.

On a pre-dawn Sunday morning, call processing might slow to the point that one might be able to hear individual calls being dialed and set up. There were also noises from whining power inverters and whirring ringing generators. Some systems had a continual, rhythmic “clack-clack-clack” from wire spring relays that made reorder (120 ipm) and busy (60 ipm) signals. In Bell System installations, there were typically alarm bells, gongs, or chimes. These would annunciate alarms calling attention to a failed switch element. Another noisemaker: a trouble reporting card system was connected to switch common control elements. These trouble reporting systems would puncture cardboard cards with a cryptic code that logged the nature of a failure. Remreed technology in Stored Program Control exchanges finally quieted the environment.

Maintenance Tasks:

The maintenance of electromechanical systems was partly DC electricity and partly mechanical adjustments. Unlike modern switches, a circuit connecting a dialed call through an electromechanical switch actually had DC continuity. The talking path was a physical, metallic one.

In all systems, subscribers were not supposed to notice changes in quality of service because of failures or maintenance work. A variety of tools referred to as make-busys were plugged into electromechanical switch elements during repairs or failures. A make-busy would identify the part being worked on as in-use, causing the switching logic to route around it. A similar tool was called a TD tool. Subscribers who got behind in payments would have their service temporarily denied (TDed). This was effected by plugging a tool into the subscriber’s office equipment (Crossbar) or line group (step). The subscriber could receive calls but could not dial out.

Strowger-based, step-by-step offices in the Bell System were under continual maintenance. They required constant cleaning. Indicator lights on equipment bays in step offices alerted staff to conditions such as blown fuses (usually white lamps) or a permanent signal (stuck off-hook condition, usually green indicators.) Step offices were more susceptible to single-point failures than newer technologies.

Crossbar offices used more shared, common control circuits. For example, a digit receiver (part of an element called an Originating Register) would be connected to a call just long enough to collect the subscriber’s dialed digits. Crossbar architecture was more flexible than step offices. Later crossbar systems had punch-card-based trouble reporting systems. By the 1970s, Automatic number identification had been retrofitted to nearly all step-by-step and crossbar switches in the Bell System.

Electronic Network switches:

The first Electronic Switching Systems were not entirely digital. The Western Electric 1ESS switch still had reed relay metallic paths. It was stored-program-controlled. Equipment testing, changes to phone numbers, circuit lockouts and similar tasks were accomplished by typing on a terminal. Northern Telecom SP1, Ericsson AKE, Philips PRX/A, ITT Metaconta, British Telecom TXE series and several other designs were similar. These systems could use the old electromechanical signaling methods inherited from crossbar and step-by-step switches. They also introduced a new form of data communications: two 1ESS exchanges could communicate with one another using a data link called Common Channel Interoffice Signaling, (CCIS). This data link was based on CCITT 6, a predecessor to SS7.

Digital Network switches:

A typical satellite PBX with front cover removed.Digital switches work by connecting two or more digital circuits together, according to a
dialed telephone number. Calls are set up between switches using the Signalling System 7 protocol, or one of its variants. In U.S. and military
telecommunication, a digital switch is a switch that performs time division switching of digitized signals. This was first done in a few
small and little used systems. The first product using a digital switch system was made by Amtelco. Prominent examples include Nortel DMS-100,
Lucent 5ESS switch, Siemens EWSD and Ericsson AXE telephone exchange. With few exceptions, most switches built since the 1980s are digital,
so for practical purposes this is a distinction without a difference. This article describes digital switches, including algorithms and equipment.

A digital exchange (Nortel DMS-100) used by an operator to offer local and long distance services in France. Each switch typically serves 10,000-100,000+ subscribers depending on the geographic areaDigital switches encode the speech going on, in 8000 time slices per second. At each time slice, a digital PCM representation of the tone is made. The digits are then sent to the receiving end of the line, where the reverse process occurs, to produce the sound for the receiving phone. In other words, when you use a telephone, you are generally having your voice “encoded” and then reconstructed for the person on the other end. Your voice is delayed in the process by a small fraction of one second — it is not “live”, it is reconstructed — delayed only minutely. (See below for more info.)

Individual local loop telephone lines are connected to a remote concentrator. In many cases, the concentrator is co-located in the same building as the switch. The interface between remote concentrators and telephone switches has been standardised by ETSI as the V5 protocol. Concentrators are used because most telephones are idle most of the day, hence the traffic from hundreds or thousands of them may be concentrated into only tens or hundreds of shared connections.

Some telephone switches do not have concentrators directly connected to them, but rather are used to connect calls between other telephone switches. These complex machine (or series of them) in a central exchange building are referred to as “carrier-level” switches or tandems.

Some telephone exchange buildings in small towns now house only remote or satellite switches, and are homed upon a “parent” switch, usually several kilometres away. The remote switch is dependent on the parent switch for routing and number plan information. Unlike a digital loop carrier, a remote switch can route calls between local phones itself, without using trunks to the parent switch.

Telephone switches are usually owned and operated by a telephone service provider or carrier and located in their premises, but sometimes individual businesses or private commercial buildings will house their own switch, called a PBX, or Private branch exchange.

Fully-connected mesh network:

One way is to have enough switching fabric to assure that the pairwise allocation will always succeed by building a fully-connected mesh network. This is the method usually used in central office switches, which have low utilization of their resources.

Telephone:

Before the invention of electromagnetic telephones, there were mechanical devices for transmitting spoken words over a greater distance than ordinary speech. The very earliest mechanical telephones were based on sound transmission through pipes or other physical media. According to a letter in the Peking Gazette, in 968, the Chinese inventor Kung-Foo-Whing invented the thumtsein, which probably transported the speech through pipes. Speaking tubes long remained common, and can still be found today. The lover’s telephone or string telephone has also been known for centuries, connecting two diaphragms with string or wire which transmits the sound from one to the other by mechanical vibrations along the string and not by electric current. The classic example is the children’s toy made by connecting the bottoms of two paper cups, metal cans, or plastic bottles with string.

Invention of the Telephone

The modern telephone is the result of work done by many people, all worthy of recognition of their contributions to the field. Alexander Graham Bell was the first to patent the telephone, an “apparatus for transmitting vocal or other sounds telegraphically” after experimenting with many primitive sound transmitters and receivers. However, the history of the invention of the telephone is a confusing mass of claims and counterclaims, further worsened by lawsuits which attempted to resolve the patent claims of several individuals.

According to a letter in the Peking Gazette, in 1968, the Chinese inventor Kung-Foo-Whing invented a speech transmitting device, which probably transported the speech through pipes. Speaking tubes remained common and can still be found today.

The lover’s telephone (or string telephone) has also been known for centuries, connecting two diaphragms with string or wire which transmits the
sound from one to the other by vibrations along the string and not through electric current. The classic example is the tin can telephone, a
children’s toy made by connecting the bottoms of two paper cups, metal cans, or plastic tubes with string.

Antonio Meucci:

An early version of the telephone was invented around 1860 by Antonio Meucci who called it teletrofono (telectrophone). Despite a public statement by the then Secretary of State that “there exists sufficient proof to give priority to Meucci in the invention of the telephone,” and despite the fact that the United States initiated prosecution for fraud against Bell’s patent, the trial was postponed from year to year until, in 1896, the case was dropped.

The first American demonstration of Meucci’s invention took place in NYC, USA in 1854. In 1860, a description of it was published in New York’s Italian language newspaper. Meucci invented a paired electro-magnetic transmitter and receiver, where the motion of a diaphragm modulated a signal in a coil by moving an electromagnet. This resulted in a good fidelity, but a very weak signal. Meucci is also credited with the early invention of inductive loading of telephone wires to increase long-distance signals. Unfortunately, serious burns, lack of English, and poor business abilities resulted in Meucci failing to develop his inventions commercially in America. Meucci demonstrated some sort of instrument in 1849 in Havana, Cuba, but the evidence is unclear if this was an electric telephone or a variant on the string telephone using wires.

(Meucci has been further credited with invention of an anti-sidetone circuit. However, examination shows that his solution to sidetone was to maintain two separate telephone circuits, and thus twice as many transmission wires. The anti-sidetone circuit later introduced by Bell Telephone instead cancelled sidetone with a feedback process.)Western Union laboratory reportedly lost Meucci’s working models, and Meucci, who at this point was living on public assistance, was unable to renew the patent after 1874.

In March 1876 Alexander Graham Bell, who conducted experiments in the same laboratory where Meucci’s materials had been stored, was granted a patent and was thereafter credited with inventing the telephone.

Meucci was recognized as the first inventor of the telephone by the United States House of Representatives in House Resolution 269 dated 11 June 2002. The resolution states that “if Meucci had been able to pay the $10 fee to maintain the caveat after 1874, no patent could have been issued to Bell.” However, this declaration is non-binding and has no legal effect.

Chronology of Meucci’s invention:

An Italian researcher in telecommunications Basilio Catania and the Italian Society of Electrotechnics “Federazione Italiana di Elettrotecnica” have devoted a Museum to Antonio Meucci making a chronology of his inventing the telephone and tracing the history of the two trials opposing Antonio Meucci and Alexander Graham Bell [1] [2]. They both support the claim that Antonio Meucci was the real inventor of the telephone. What follows, if not otherwise stated, is a résumé of their historic reconstruction.

In 1834 Meucci constructed a kind of acoustic telephone as a way to communicate between the stage and control room at the theatre “Teatro della Pergola” in Florence. This telephone is constructed on the model of pipe- telephones on ships and is still working. In 1848 Meucci developed a popular method of using electric shocks to treat rheumatism. He used to give his patients two conductors linked to 60 Bunsen batteries and ending with a cork. He also kept two conductors linked to the same Bunsen batteries. He used to sit in his laboratory, while the Bunsen batteries were placed in a second room and his patients in a third room. In 1849 while providing a treatment to a patient with a 114V electrical discharge, in his laboratory Meucci heard his patient’s scream through the piece of copper wire that was between them, from the conductors he was keeping near his ear. His intuition was that the “tongue” of copper wire was vibrating just like a leave of an electroscope;
which means that there was an electrostatic effect. In order to continue the experiment without hurting his patient, Meucci covered the copper wire with a piece of paper. Through this device he heard in articulated human voice. He called this device “telegrafo parlante” (litt. “talking telegraph”). On the basis of this prototype, Meucci worked on more than 30 kinds of telephone. At the beginning he got inspiration from the telegraph model. Differently from other pioneers of the telephone, such as Charles Bourseul, Philipp Reis, Innocenzo Manzetti and others, he did not think about transmitting voice by using the principle of the telegraph key (in scientific jargon, the “make-and-break” method), but he looked for a “continuous” solution, which means without interrupting the electric flux.

In 1856 Meucci constructed the first electromagnetic telephone, made of an electromagnet with a nucleus in the shape of a horseshoe bat, a diaphragm of animal skin, stiffened with potassium dichromate and keeping a metal disk stuck in the middle. The instrument was hosted in a cylindrical carton box. He constructed this as a way to connect his second-floor bedroom to his basement laboratory, and thus communicate with his wife who was an invalid.
Meucci separated the two directions of transmission in order to eliminate the so-called “local effect”, adopting what we would call today a 4-wire-circuit. He constructed a simple calling system with a telegraphic manipulator which short-circuited the instrument of the calling person, producing in the instrument of the called person a succession of impulses (clicks), much more intense than those of normal conversation. As he was aware that his device required a bigger band than a telegraph, he found some means to avoid the so- called “skin effect” through superficial treatment of the conductor or by acting on the material (copper instead of iron). He successfully used an insulated copper plait, thus anticipating the litz wire used by Nikola Tesla in RF coils.

In 1864 Meucci’s realized his “best device”, using an iron diaphragm with optimized thickness and tightly clamped along its rim. The instrument was housed in a shaving-soap box, whose cover clamped the diaphragm. In August 1870, Meucci obtained transmission of articulate human voice at a mile distance by using as a conductor a copper plait insulated by cotton. He called his device “teletrofono”. According to an Affidavit of lawyer Michael Lemmi drawings and notes by Antonio Meucci dated September 27, 1870 show that Meucci understood inductive loading on long distance telephone lines 30 years before any other scientists. The painting made by Nestore Corradi in 1858 mentions the sentence “Electric current from the inductor pipe” All this information has been published on the Scientific American Supplement No. 520, December 19, 1885.

Charles Bourseul:

In 1854 in the magazine L’Illustration (Paris) Charles Bourseul, a French telegraphist, published a plan for conveying sounds and even speech by electricity. Bourseul’s ideas were also published in Didaskalia (Frankfurt am Main) on September 28, 1854. “Suppose”, he explained, “that a man speaks near a movable disc sufficiently flexible to lose none of the vibrations of the voice; that this disc alternately makes and breaks the currents from a battery: you may have at a distance another disc which will simultaneously execute the same vibrations…. It is certain that, in a more or less distant future, speech will be transmitted by electricity. I have made experiments in this direction; they are delicate and demand time and patience, but the approximations obtained promise a favorable result.”

Johann Philipp Reis:

In 1860 Johann Philipp Reis produced a device which could transmit musical notes, and even a lisping sentence or two. The first sentence spoken on it was “Das Pferd frisst keinen Gurkensalat” (the horse doesn’t eat cucumber salad). See Reis’ telephone for a detailed description. The Reis transmitter was a make-break transmitter. That is, a needle attached to a diaphragm was alternately pressed against, and released from a contact as the sound moved the diaphragm. This make-or-break signaling was able to transmit tones, and some vowels, but since it did not follow the analog shape of the sound wave (the contact was pure digital, on or off) it could not transmit consonants, or complex sounds. The Reis transmitter was very difficult to operate, since the relative position of the needle and the contact were critical to the device’s operation at all. This can be called a “telephone”, since it did transmit sounds over distance, but is hardly a telephone in the modern sense, as it failed to transmit a good copy of any supplied sound. Reis’ invention is best known then as the “musical telephone”. In 1863 the device was tested by the British company Standard Telephones and Cables (STC). The results also confirmed it could faintly transmit and receive speech. At the time STC was bidding for a contract with Alexander Graham Bell’s American Telephone and Telegraph Company, and the results were covered up by STC’s chairman Sir Frank Gill to maintain Bell’s reputation.

Innocenzo Manzetti:

Innocenzo Manzetti mooted the idea of a telephone as early as 1844, and may have made one in 1864, as an enhancement to an automaton built by him in 1849.

Cromwell Varley:

Around 1870 Mr. C. F. Varley, F.R.S., a well-known English electrician, patented a number of variations on the audio telegraph based on Reis’ work. He never claimed or produced a device capable of transmitting speech, only pure tones.

Poul la Cour:

Around 1874 Poul la Cour, a Danish inventor, experimented with audio telegraphs on a line of telegraph between Copenhagen and Fredericia in Jutland. In this a vibrating tuning-fork interrupted the current, which, after traversing the line, passed through an electromagnet, and attracted the limbs of another fork, making it strike a note like the transmitting fork. Moreover, the hums were made to record themselves on paper by turning the electromagnetic receiver into a relay, which actuated a Morse code printer by means of a local battery. Again, la Cour made no claims of transmitting voice, only pure tones.

Elisha Gray:

Mr. Elisha Gray, of Chicago also devised a tone telegraph of this kind about the same time as Herr La Cour, and sought the patent for a telephone design. In the tone telegraph a vibrating steel reed interrupted the current, which at the other end of the line passed through an electromagnet and vibrated a matching steel reed near its poles. Gray’s ‘harmonic telegraph,’ with the vibrating reeds, was used by the Western Union Telegraph Company. Since more than one set of vibrations — that is to say, more than one note — can be sent over the same wire simultaneously, the harmonic telegraph can be utilized as a ‘multiplex’ or many-ply telegraph, conveying several messages through the same wire at once; and these can either be read by the operator by the sound, or a permanent record can be made by the marks drawn on a ribbon of travelling paper by a Morse recorder.

Interestingly, Gray filed his preliminary patent petition (caveat) for a telephone on the very same day in 1876 as did Bell, and the devices described were strikingly similar, a fact which has been said to imply that Bell (who knew Gray) was inspired by Gray’s design or vice versa.

Alexander Graham Bell:

Bell’s March 10, 1867 laboratory notebook entry describing his first successful experiment with the telephone.Alexander Graham Bell of Scotland is commonly credited as the first inventor of the telephone. The classic story of his crying out “Watson, come here! I want to see you!” is a well known part of American history. But Alexander Graham Bell was also an astute and articulate business man with influential and wealthy friends.

As Professor of Vocal Physiology at Boston University, Bell was engaged in training teachers in the art of instructing deaf mutes how to speak, and experimented with the Leon Scott phonautograph in recording the vibrations of speech. This apparatus consists essentially of a thin membrane vibrated by the voice and carrying a light stylus, which traces an undulatory line on a plate of smoked glass. The line is a graphic representation of the vibrations of the membrane and the waves of sound in the air.

This background prepared him for work with sound and electricity. He claimed to have began his researches in 1874 with a musical telegraph, in which he employed an on-off-on-off make-break circuit driven by a vibrating iron reed which created interrupted current to vibrate the receiver, which consisted of an electro-magnet causing an iron reed or tongue to vibrate, exactly the same as Bourseul, Reis, Gray, and Antonio Meucci. During a 2 June 1875 experiment by Bell and his assistant Watson, a reed failed to respond to the intermittent current supplied by an electric battery. Bell told Watson, who was at the other end of the line, to pluck the reed, thinking it had stuck to the pole of the magnet. Mr. Watson complied, and to his astonishment Bell heard a reed at his end of the line vibrate and emit the same overtones of a plucked reed, although there was no interrupted on-off-on-off current to make it vibrate. A few experiments soon showed that his reed had been set in vibration by the magneto-electric currents induced in the line by the mere motion of the distant reed in the neighborhood of its magnet. The battery current was not causing the vibration but was needed only to supply the magnetic field in which the reeds vibrated. Moreover, when Bell heard the rich overtones of the plucked reed, it occurred to him that since the circuit was never broken, all the complex vibrations of speech might be converted into undulating (alternating) currents, which in turn would reproduce the complex frequencies of speech at a distance.

After Bell and Watson discovered on June 2, 1875 that movements of the reed alone in a magnetic field could reproduce the frequencies of spoken sound waves, Bell reasoned by analogy with the mechanical phonautograph that a skin diaphragm would reproduce sounds like the human ear when connected to a steel or iron reed or hinged armature. On 1 July 1875, he instructed Watson to build a receiver consisting of a stretched diaphragm or drum of goldbeater’s skin with an armature of magnetized iron attached to its middle, and free to vibrate in front of the pole of an electromagnet in circuit with the line. A second membrane-device was built for use as a transmitter. This was the “gallows” phone. A few days later they were tried together, one at each end of the line, which ran from a room in the inventor’s house in Boston to the cellar underneath. Bell, in the work room, held one instrument in his hands, while Watson in the cellar listened at the other. Bell spoke into his instrument, “Do you understand what I say?” and Mr. Watson answered “Yes”. However, the voice sounds were not distinct and the armature tended to stick to the electromagnet pole and tear the membrane.

Elisha Gray:

Gray’s harmonic telegraph apparatus followed in the track of Reis and Bourseul — that is to say, the interruption of the current by a vibrating contact. But Gray recognized the lack of fidelity of the make-break transmitter, and reasoned by analogy with the lovers telegraph that if the current could be made to model more closely the movements of the diaphragm rather than simply turning the circuit on and off, a greater fidelity might be achieved. Gray filed a patent caveat with the US patent office on February 14, 1876 for a liquid microphone, where a metal needle or rod was placed just barely in contact with a liquid conductor such as a water/acid mixture, and as the diaphragm vibrated,the needle dipped more-or-less into the liquid, resulting in more-or-less current passing to the receiver. Gray did not convert his caveat into a patent until after the caveat had expired and hence left the field open to Bell.

Bell’s success:

Alexander Graham Bell’s telephone patent drawing, 7 March 1876.The first successful bi-directional transmission of clear speech by Bell and Watson was made on 10 March 1876 when Bell spoke into his device, “Mr. Watson, come here, I want to see you.” and Watson answered. Bell used Gray’s liquid transmitter design[9] in his famous 10 March 1876 experiment, but avoided describing the liquid transmitter in his public demonstrations. The liquid transmitter had the problem that waves formed on the surface of the liquid, resulting in interference.

The first long distance telephone call was made on 10 August 1876 by Bell from the family homestead in Brantford, Ontario, to his assistant located in Paris, Ontario, some 10 miles (16 km) distant.

A finished instrument was then made, having a transmitter formed of a double electromagnet, in front of which a membrane, stretched on a ring, carried an oblong piece of soft iron cemented to its middle. A mouthpiece before the diaphragm directed the sounds upon it, and as it vibrated with them, the soft iron “armature” induced corresponding currents in the coils of the electromagnet. These currents after traversing the line were passed through the receiver, which consisted of a tubular electromagnet, having one end partially closed by a thin circular disc of soft iron fixed at one point to the end of the tube. This receiver bore a resemblance to a cylindrical metal box with thick sides, having a thin iron lid fastened to its mouth by a single screw. When the undulatory current passed through the coil of this magnet, the disc, or armature-lid, was put into vibration and the sounds evolved from it.

The primitive telephone was rapidly improved, the double electromagnet being replaced by a single bar magnet having a small coil or bobbin of fine wire surrounding one pole, in front of which a thin disc of ferrotype is fixed in a circular mouthpiece, and serves as a combined membrane and armature. On speaking into the mouthpiece, the iron diaphragm vibrates with the voice in the magnetic field of the pole, and thereby excites the undulatory currents in the coil, which, after traveling through the wire to the distant place, are received in an identical apparatus. [This form was patented 30 January 1877.] In traversing the coil of the latter they reinforce or weaken the magnetism of the pole, and thus make the disc armature vibrate so as to give out a mimesis of the original voice. The sounds are small and elfin, a minim of speech, and only to be heard when the ear is close to the mouthpiece, but they are remarkably distinct, and, in spite of a disguising twang, due to the fundamental note of the disc itself, it is easy to recognize the speaker.

Thomas Edison:

Thomas Alva Edison took the next step in developing telephonic fidelity with his invention of the carbon grain transmitter. Edison discovered that carbon grains, squeezed between two metal plates, had a resistance that was related to the pressure. Thus, the grains could vary their resistance as the plates moved in response to sound waves, and reproduce sound with good fidelity, without the problems associated with a liquid contact. This transmitter produced a strong signal that could be used for long-distances and the carbon microphone remained standard in telephony until the 1980s, and is still produced.

Later developments:

Bell had overcome the difficulty which baffled Reis, and succeeded in making the undulations of the current fit the vibrations of the voice as a glove will fit the hand. But the articulation, though distinct, was feeble, and it remained for Edison (by inventing a transmitter that provided for independent power on the transmitting circuit) and David E. Hughes (by inventing the carbon microphone in 1878) to render the telephone the useful and widespread apparatus which we see it now.

Source References:

• (fr) Bourseul, Charles, Transmission électrique de la parole, L’Illustration (Paris), 26.08.1854
• Thompson, Sylvanus P. (1883), Philipp Reis, Inventor of the Telephone, London: E. & F. N. Spon, 1883.
• Scientific American Supplement No. 520, December 19, 1885
• Coe, Lewis (1995), The Telephone and Its Several Inventors: A History, McFarland, North Carolina, 1995. ISBN 0-7864-0138-9
• Evenson, A. Edward (2000), The Telephone Patent Conspiracy of 1876: The Elisha Gray – Alexander Bell Controversy, McFarland, North Carolina, 2000. ISBN 0-7864-0883-9
• Baker, Burton H. (2000), The Gray Matter: The Forgotten Story of the Telephone, Telepress, St. Joseph, MI, 2000. ISBN 0-615-11329-X
• Shulman, Seth (2008), The Telephone Gambit, Norton & Company, New York, 2008. ISBN 978-0-393-06206-9
• Josephson, Matthew (1992), Edison: A Biography, Wiley, ISBN 0-471-54806-5
• Bruce, Robert V. (1990), Bell: Alexander Bell and the Conquest of Solitude, Cornell University Press, 1990. ISBN 0-80149691-8
• Heroes of the Telegraph by John Munro at Project Gutenberg
• American Treasures of the Library of Congress, Alexander Graham Bell – Lab notebook I, pages 40-41 (image 22)
  
• Wikipedia.org, the free encyclopedia