THE HISTORY OF TELEPHONE
The Wire Telegraph Era began in the mid 19th century and continued in the 20th century. Sending voice signals over the telegraph wire lines, was a dream of inventors in the 1870’s.
In 1876, two inventors applied for a patent for the telephone. Elisha Gray was the first inventor of the Liquid Transmitter. It consisted of a membrane attached to an electrode inside a small jar with acid solution. The receiver was made of an electromagnet with iron membrane. Alexander Graham Bell applied for a patent for a similar Liquid Transmitter on the same day as Elisha Gray, February 14, 1876.
There is a dispute about the reason that the Telephone Patent was granted to Bell and not to Gray. The historic voice message of Bell "Mr. Watson - come here - I want to see you" , was the first time that a human voice was transmitted over an electric wire line.
Elisha Gray's first sketch of Liquid Transmitter and a receiver dated February 11, 1876
Part of a page from the notebook of Alexander Graham Bell, describing the Liquid Transmitter and electromagnet receiver, dated March 9, 1876
The liquid transmitter was not practical at all. Three different persons searched for a better kind of Voice Transmitter. David Edward Hughes (England), Emile Berliner and Thomas Edison (USA) found the solution separately, by using Carbon Granules pressed by a metal membrane. Hughes demonstrated his idea in 1870 and granted it to the public without patent registration. Hughes was the first person that called the carbon transmitter MICROPHONE. Berliner and Edison applied for a patent and it was finally granted to Edison in 1877.
Diagram of the Bell's patent application
The Carbon Transmitter was so successful, that it was used in telephones for a period of a century. With the development of electronic and digital telephones In the 1970-80’s, the carbon transmitter was replaced by Magnetic and later on by Condenser Microphone (ELECTRET).
Installation of telegraph lines began in 1846. The long experience with installation of telegraph lines, enabled instant installation of telephone lines on the same poles and along the same routes.
TELEPHONE INSTRUMENT TYPES
The most common type at the beginning of the telephone era, was the MAGNETO type. It uses a small hand operated Alternative Current Generator that sends a ringing voltage of 70-100 VAC, 17-20 Hz to the second party telephone. An electromagnetic bell ringer at the receiving side, rings at the same rate of 17-20 Hz. The advantage of the Magneto Type Telephone is the simplicity and the ability to send a ringing signal over long distance lines.
Wall Mounted MAGNETO Telephone
It is a Local Battery type for feeding the Carbon Transmitter, with a Hook Switch at the receiver hanger.
Electrical circuit of a Local Battery Magneto Telephone.
The Magneto Switch disconnects the magneto from the talking circuit, when the hand crank starts to turn.
The Hook Switch connects the Receiver and Transmitter to the line, when the receiver is lifted from the hanger.
The lines of magneto telephone subscribers are connected to the Central Office Switchboard. A ringing voltage that comes from a subscriber, actuates the Indicator at the switchboard panel. Later on, the Local Battery telephone was improved by designing the Common or Central Battery System.
The Common Battery Telephone, does not need the Magneto for signaling to the Central Office Switchboard. The Central office Switchboard Panel detects a call from a subscriber, when the receiver or handset is lifted. The central battery at the switchboard, supplies also the DC current for the Carbon Transmitter through the telephone line, so no local battery is required.
TYPICAL COMMON BATTERY TELEPHONES
In the electrical circuit, the ringing signal is connected to the ringer through a condenser, to block the DC voltage from the line.
When the Hook-Switch is closed, it creates a loop on the llne, which operates the indicator at the switchboard panel.
The next telephone system development, was the Automatic Rotary Dial Telephone. It was invented by Almon Strowger In 1889 and received a patent in 1891. The idea came to Strowger, due to problems with a Switchboard Operator that did not connect calls that were intended for his business. He decided to look for a way to enable the subscriber to route his call to a second party, without the Switchboard Operator.
AUTOMATIC TELEPHONE EXCHANGE
The drawing is one of the illustrations in Strowger's patent application. It shows a crude version of the rotary selector at the central office, with the control relays.
The picture at the right, shows a modern
dial selector unit. The finger dialing disc on the subscriber telephone sends a string of On-Off pulses to the line. The number of pulses mark the number dialed. The Rotary Dial Selector at the Automatic Switchboard rotates in steps and the number of steps, are determined by the number of pulses that reached from the line.
Electrical Circuit of a Typical Rotary Dial Telephone
It is quite similar to the Common Battery Telephone, with the addition of the Rotary Dialer Assembly in series with the talking circuit.
During the dialing process, the receiver is shorted, to avoid the hearing of the square wave pulse clicks.
The original design of Strowger finger dialing disc was with rotating angle of 170 degrees. The picture at the left, shows a Candle Stick version of such telephone. Some decades after Strowger's patent expired, AT&T introduced their first Dial Telephone System. The Finger holes on the dial in the AT&T telephones rotate almost entirely around the dial disc. The picture at the right is one of the first AT&T dial telephones.
Dial telephone systems began to be installed in large cities in the 1930s and dominated all the global telephone systems.
In 1961, AT&T introduced a new method of telephone "Dialing", the Touch-Tone System. It enabled "dialing" numbers with push-button switches, instead of the Rotary Dial. The Touch-Tone System uses Dual-Tone Multi Frequency (DTMF) signaling, in which a pair of tones are transmitted simutaneously on the line for every number or digit. The table shows the DTMF frequency matrix.
AT&T Early DTMF Telephone
After the introduction of the Touch-Tone Push Button telephones in the 1960's, DTMF telephone systems were installed from the 1970's. This is the system that still dominates the world's telephone networks.
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