A Carbon Transmitter
"...is used in some telephone sets to this very day.
The motion of an iron diaphragm varies the pressure on carbon granules confined between two electrodes. The variation in pressure causes the electric resistance between the electrodes to vary, which in turn varies the current."
(Pierce, 1990, p. 34)
These early telephones were large and clumsy, and sounds made by them were unclear and very soft. Later telephones combined receiver and transmitter into one unit. They were smaller and worked better. Telephone receivers and transmitters of today are more efficient. Microphones are either the condensor or electret type, and receivers are moving-coil type.
A single-piece transmitter and receiver
(Pierce, 1990, p. 32)
Beside the problem of making telephone reception clear, sending a signal a long distance is a problem. A recent National Geographic article, "The Future is Calling," describes the problem of running cables around the world to meet the increasing demand for more telephones:
"The first wiring of the world began in 1850, only six years after Samuel Morse demonstrated the reality of telegraphy. British engineers made a copper-wire cable, insulated it with gutta-percha (a rubberlike Malayan tree sap), and laid it across the English Channel. ... Soon came a cable across the Atlantic. ...Queen Victoria sent a message to President James Buchanan. Some of the words reached Washington that day; the rest came through on the (next day). Agonizingly slow and chronically unreliable, the cable went dead after three weeks. ... The problem was the behavior of electricity in cables. Convinced that they had found the solution, engineers tried again, this time with the world's largest ship. ... In July 1865 she set out from England with a crew of 500, a dozen oxen for hauling, a cow for fresh milk, a herd of pigs for bacon -- and a thickly insulated 2,800-mile cable that weighed 5,000 tons. They had almost finished laying it when the cable snapped. The next year they succeeded." (Allen, 2001, p. 83)
The article continues, "It took a hundred years to connect a billion people by wire. It has taken only ten years to connect the next billion people." The first hundred years saw copper wire slowly lose favor to radio telephony, microwaves, and satellite signals. In the 1990's cable finally made a comeback with fiber optics. Bell saw this as a posibility with his photophone in the 1870's, but it did not become a reality until over 100 years later.
Alexander Graham Bell's Photophone, About 1880
"Bell contended that this was his greatest discovery, outranking in potential importance even the telephone."
(Eber, 1982, p. 20)
Bell Laboratory Archives
Fiber Optic Cables
"A fiber optic cable is actually a center cable of strong wire, six optical fibers, and copper wires to supply power to repeaters. Thousands of calls per minute can be made over one cable at a faster rate than standard copper wire."
(Parker, 1995, p. 26)
Local Area Repeaters are used to boost the electrical signals,
but they are inefficient for long distance communication.
They are used in local areas about every 1/4 mile.
The long black "sausages" on the
bottom line are the repeaters
(Leah Schoelles, 23 Nov. 2001
Sutter Telephone Exchange,
San Francisco 1914
Telephone Pioneer Communications Museum
(Fischer, 1992, p. 155)
Telephone wires in a large city about 1900
Telephone Pioneer Communications Museum
(Fischer, 1992, p. 156)
Almon Strowger was the first to put the Connolly & McTigthe automated switching system to practical use with the Strowger Automatic Telephone Exchange in 1891. Calls became faster because they no longer relied on manual switching of calls by groups of operators. Strowger also had to make a rotary dial telephone that allowed callers to dial a telephone number which sent the call to the correct switchboard. Until then, the operator manually connected the call and checked the status of the telephone line. Strowger's system meant that people needed to know if the line were busy or available themselves. He created tones that each sounded different which told the caller the status: the dial tone - the line is available for calls; the busy signal - the number being called is in use; the line unavailable tone - the circuit is busy; and the ring tone - lets the caller know the connection was made.
|Line Un-available Tone|
All tones provided by
"The Strowger Telecomms Page", 1996
A telephone switching system
(Pierce, 1990, p. 188)
As the number of telephones grew, a way had to be found to identify each telephone. To switch between systems, a code was dialed to direct that call to a different exchange, then the caller's number was dialed. This system is used today with the 3-digit exchange number + 4-digit subscriber number. Area codes were added in 1947. Area codes were originally for large cities, then they were applied to entire states. Today, there are 9,999,999,999 possible combinations of area codes and telephone numbers available.