The Telephone

ME240/106S: Product Dissection - Appliances

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)


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


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!


Simultaneous Invention ?

Bell’s drawing of the acidtransmitter, March 9, 1876 Gray’s version, Feb. 11


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.


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


Bell’s Invention

Models of the first electric telephone liquid transmitter (left) and tuned-reed receiver (right)


Basic principle: Voice input causes change in resistance which then modulates the current in the circuit.

Bell’s original microphone used acid.

Bell’s Microphone

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.

Cup

Wire

Funnel to concentrate sound

Diaphragm

Acid

Output Terminals


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) Loosely-packed

carbon granules

Metal cavity

Diaphragm

Flexible edges

Output

Incoming Sound Waves


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!)

range of normal voice levels (faithfully reproduced)

background room noises (are attenuated)

loud sounds (are attenuated)

Input Sound Level (dB SPL)

Ou

tpu

t Ele

ctr

ical Level (d

BV

)

-70

-55

-40

-25

70 100 130


Receiver

similar to a loudspeaker we still use Bell’s basic design

ref van derPuieA cross section of the moving iron telephone receiver

Magnetic Flux Path

Permanent Magnet

Iron Alloy Diaphragm Electromagnet Coil

Electromagnet alternately adds to and subtracts from the permanent magnet’s attraction of the diaphragm, causing it to oscillate

Sound OutputS

N

AC Electrical Signal Input


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

Cup

When finger wheel is released, wings rotate and fly apart due to centrifugal force, contact cup and generate friction, limiting the speed of rotation

Finger Plate Wheel

Gears

Clutch (only engages the governor when the finger wheel is recoiling)

Wings

Governor

Return Spring


Touchtone Dial

Pressing a button produces a Dual Tone, Multiple Frequency (DTMF) signal - so you can’t imitate it by whistling

1 2 3

4 5 6

7 8 9

* 0 #

897

770

852

941

1209 1336 1477

Column Frequencies (Hz)

Row

Fre

qu

en

cie

s (

Hz)

ABC DEF

GHI JKL MNO

PRS TUV WXY

OPER


NS

Permanent Magnet

Gong

Clapper Armature

ElectromagnetCoil

AC Current Input

S

NMagnetic flux pathfrom electromagnet

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)

OPERATION OF ELECTRO-MAGNETIC RINGER

NS

N

S


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


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


What happens when you call home?

HANDSET LIFTEDSWITCHOOK CONTACTS CLOSE

CALLER CENTRAL OFFICE A

DIRECT CURRENTFLOWS IN LINE

CURRENT DETECTED

DIAL TONE GENERATORCONNECTED TO LINE

DIAL TONE HEARD

NUMBER DIALED NUMBER STORED


Calling Home - continued

NUMBER STORED

FIRST THREE DIGITS INDICATE CENTRAL OFFICE DESTINATIONOR AREA CODE

FIND BEST AVAILABLEROUTE (TRUNK) ANDSEIZE IT

TRANSFER NUMBER(4 DIGITS)

STORE NUMBER

CONNECT TO MOM’S LINE

CHECK IF BUSY

SEND RINGINGSIGNAL PICK UP PHONE

CONNECT SWITCHHOOKDC CURRENT FLOWS

ANSWER DETECTED

STOP RINGING

CONNECT PARTIES

CENTRAL OFFICE A CENTRAL OFFICE B MOM’S HOUSE

“Hello mom? I need money”

The Telephone

Documents

Transcript of The Telephone