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The Telephone

The Telephone. Telephono - Greek word for “far speaking” Some Famous Quotes: “an instrument for the transmission of articulate speech by the electric current” - (definition of telephone from Bell’s 1876 patent application)

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The Telephone

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  1. The Telephone Telephono - Greek word for “far speaking” Some Famous Quotes: “an instrument for the transmission of articulate speech by the electric current” - (definition of telephone from Bell’s 1876 patent application) “As to Bell’s talking telegraph, it only creates interest in scientific circles . . . its commercial values will be limited . . . “ (Elisha Gray, telephone inventor runner-up, June 1876)

  2. A Brief History • Two patents were filed on same day (February 14, 1876) by Alexander Bell, and Elisha Gray (Bell beat him by two hours). It would turn out to be the most valuable patent ever issued. • Bell was an elocution teacher, amateur tinkerer and immigrant from Scotland, Gray was a professional inventor from Chicago • The immortal words: “Mr. Watson -- come here -- I want to see you” were yelled on March 10, 1876, after many years of frustration, and after the patent application • Both Bell and Gray were trying to improve the telegraph (a pair of wires could only carry one Morse-code transmission at a time), both thought that many musical tones could be simultaneously transmitted - the harmonic telegraph

  3. The Mystery of Bell’s Patent • The critical element of Bell’s patent - a description of the variable resistance transmitter - was added to the patent later and was written in the margin. • Gray’s patent application featured a variable resistance transmitter • Up to that time, Bell had never even experimented with a variable resistance transmitter • Gray contended that Bell’s friends in the Patent Office told him of Gray’s patent and illegally let him look at it, and that Bell copied the idea from him. • But, it was never proven in court - Moral: Get a good lawyer!

  4. Simultaneous Invention ? Bell’s drawing of the acid transmitter, March 9, 1876 Gray’s version, Feb. 11

  5. It Was Almost “Ma Gray” • Gray could have patented telephone earlier, but he (and his colleagues, patent attorney and investors) could not see how it could be used for anything more than a toy. He thought the multiplexing telegraph was more important. • Bell ignored his investors. He was convinced that voice transmission was the future “the day is coming when telegraph wire will be laid on to houses just like water and gas - and friends converse with each other without leaving home” • Bell offered to sell the patent to Western Union (the telegraph behemoth) in 1876 for $100,000 but they were not interested. The next year, Western Union backed Gray in a lawsuit to strip Bell of his patent (they lost). In 1878, they would gladly have bought the patent rights for $25 million.

  6. The Telephone Wars • Western Union launched its own telephone business using its existing telegraph lines and began setting up local exchanges across the country • Elisha Gray founded Western Electric, which supplied the rival Western Union with telephone equipment during the great wars for telephone supremacy • From 1877 - 1888 there were numerous challenges to Bell’s patent and countersuits of patent infringement • In 1879 Western Union gave up all its patents, claims and facilities in the telephone business - the Edison transmitter, various other technical improvements, and a network of 56,000 telephones in 55 cities - in return for 20% of telephone receipts over the 17-year life of the Bell patents • In 1882 Western Electric became the equipment manufacturing subsidiary for what was then known as American Bell

  7. Bell’s Invention Models of the first electric telephone liquid transmitter (left) and tuned-reed receiver (right)

  8. Bell’s Microphone • Basic principle: Voice input causes change in resistance which then modulates the current in the circuit. • Bell’s original microphone used acid. Bell’s liquid transmitter consisted of a metal cup filled with a mixture of acid and water. A sound wave causes the diaphragm to move thereby forcing the wire to move up and down in the liquid. The electrical resistance between the wire and the cup is inversely proportional to the amount of wire submerged. Therefore the resistance across the two terminals varies in response to the sound. Funnel to concentrate sound Diaphragm Wire Output Terminals Cup Acid

  9. The Edison Microphone Most common type (invented by Edison and still in use) uses carbon granules - when vibrated by attached diaphragm, the overall resistance alternately decreases (when particles are compacted) and increases (when particles are loosened) Diaphragm Loosely-packed carbon granules Metal cavity Incoming Sound Waves Output Flexible edges

  10. Microphone - continued • nonlinear input-output characteristics • suppress background noise (microphone does not respond to low level sounds) • limits maximum output (so you can’t hurt someone’s ears by screaming!) loud sounds (are attenuated) -25 -40 Output Electrical Level (dBV) range of normal voice levels (faithfully reproduced) -55 -70 background room noises (are attenuated) 70 100 130 Input Sound Level (dB SPL)

  11. Receiver Electromagnet alternately adds to and subtracts from the permanent magnet’s attraction of the diaphragm, causing it to oscillate • similar to a loudspeaker • we still use Bell’s basic design AC Electrical Signal Input S Sound Output Magnetic Flux Path Permanent Magnet N Iron Alloy Diaphragm Electromagnet Coil ref van derPuie A cross section of the moving iron telephone receiver

  12. Rotary Dial • produces 10 pulses/sec (pause longer than 1/10 sec is interpreted by central office as inter-digit separator) • ingenious governor and clutch mechanism regulates the rotational speed Governor Gears When finger wheel is released, wings rotate and fly apart due to centrifugal force, contact cup and generate friction, limiting the speed of rotation Return Spring Wings Finger Plate Wheel Cup Clutch (only engages the governor when the finger wheel is recoiling)

  13. Touchtone Dial • Pressing a button produces a Dual Tone, Multiple Frequency (DTMF) signal - so you can’t imitate it by whistling DEF 897 ABC 1 2 3 770 JKL GHI MNO 5 4 6 Row Frequencies (Hz) 852 PRS TUV WXY 7 8 9 OPER 941 # * 0 1336 1209 1477 Column Frequencies (Hz)

  14. AC Current Input Gong Electromagnet Coil S Armature Clapper N S Magnetic flux path from electromagnet N Permanent Magnet Electro-Mechanical Ringer • Brass gongs struck by a clapper • Mechanism resonates at 20 Hz • Ringing signal is ~75V at 20 Hz • Modern phones use an electronic ringer (similar to a small speaker) N S N S OPERATION OF ELECTRO-MAGNETIC RINGER

  15. How Telephones WERE Designed • Reliability was the most important factor (30 year life goal), manufactured cost was secondary, because the cost to service a phone was so high (if something ever failed, they had to send a union repairman out with donuts in a green truck) • The design philosophy then : “The telephone is designed to meet functional requirements and is then cost-reduced to maximize the cost saving associated with the large production volumes... Cost considerations are, however, not limited to manufacturing. Since operating companies own the sets, reliability, maintenance, and ease of installation are important factors in the design” - Engineering and Operations in the Bell System, 1977

  16. Telephone as a Consumer Product • Obvious things which consumers base their purchases on: • low cost • good looking • lots of features • low cost • Hidden qualities of a good design (and things the designer cares about) • good voice quality • reliability (must survive “drop test”) • ease of use • easy to cradle handset • filters out room noise

  17. What happens when you call home? CALLER CENTRAL OFFICE A HANDSET LIFTED SWITCHOOK CONTACTS CLOSE DIRECT CURRENT FLOWS IN LINE CURRENT DETECTED DIAL TONE GENERATOR CONNECTED TO LINE DIAL TONE HEARD NUMBER STORED NUMBER DIALED

  18. Calling Home - continued CENTRAL OFFICE A CENTRAL OFFICE B MOM’S HOUSE NUMBER STORED STORE NUMBER CONNECT TO MOM’S LINE FIRST THREE DIGITS INDICATE CENTRAL OFFICE DESTINATION OR AREA CODE CHECK IF BUSY SEND RINGING SIGNAL FIND BEST AVAILABLE ROUTE (TRUNK) AND SEIZE IT PICK UP PHONE CONNECT SWITCHHOOK DC CURRENT FLOWS ANSWER DETECTED TRANSFER NUMBER (4 DIGITS) STOP RINGING “Hello mom? I need money” CONNECT PARTIES

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