Touch-tone telephone

The invention: 



A push-button dialing system for telephones that

replaced the earlier rotary-dial phone.



The person behind the invention:



Bell Labs, the research and development arm of the American

Telephone and Telegraph Company







Dialing Systems



A person who wishes to make a telephone call must inform the

telephone switching office which number he or she wishes to reach.

A telephone call begins with the customer picking up the receiver

and listening for a dial tone. The action of picking up the telephone

causes a switch in the telephone to close, allowing electric current to

flow between the telephone and the switching office. This signals

the telephone office that the user is preparing to dial a number. To

acknowledge its readiness to receive the digits of the desired number,

the telephone office sends a dial tone to the user. Two methods

have been used to send telephone numbers to the telephone office:

dial pulsing and touch-tone dialing.

“Dial pulsing” is the method used by telephones that have rotary

dials. In this method, the dial is turned until it stops, after which it is

released and allowed to return to its resting position. When the dial

is returning to its resting position, the telephone breaks the current

between the telephone and the switching office. The switching office

counts the number of times that current flow is interrupted,

which indicates the number that had been dialed.



Introduction of Touch-tone Dialing



The dial-pulsing technique was particularly appropriate for use

in the first electromechanical telephone switching offices, because

the dial pulses actually moved mechanical switches in the switching

office to set up the telephone connection. The introduction of

touch-tone dialing into electromechanical systems was made possi-

ble by a special device that converted the touch-tones into rotary

dial pulses that controlled the switches. At the American Telephone

and Telegraph Company’s Bell Labs, experimental studies were

pursued that explored the use of “multifrequency key pulsing” (in

other words, using keys that emitted tones of various frequencies)

by both operators and customers. Initially, plucked tuned reeds

were proposed. These were, however, replaced with “electronic

transistor oscillators,” which produced the required signals electronically.

The introduction of “crossbar switching” made dial pulse signaling

of the desired number obsolete. The dial pulses of the telephone

were no longer needed to control the mechanical switching process

at the switching office. When electronic control was introduced into

switching offices, telephone numbers could be assigned by computer

rather than set up mechanically. This meant that a single

touch-tone receiver at the switching office could be shared by a

large number of telephone customers.

Before 1963, telephone switching offices relied upon rotary dial

pulses to move electromechanical switching elements. Touch-tone

dialing was difficult to use in systems that were not computer controlled,

such as the electromechanical step-by-step method. In about

1963, however, it became economically feasible to implement centralized

computer control and touch-tone dialing in switching offices.

Computerized switching offices use a central touch-tone receiver

to detect dialed numbers, after which the receiver sends the

number to a call processor so that a voice connection can be established.

Touch-tone dialing transmits two tones simultaneously to represent

a digit. The tones that are transmitted are divided into two

groups: a high-band group and a low-band group. For each digit

that is dialed, one tone from the low-frequency (low-band) group

and one tone from the high-frequency (high-band) group are transmitted.

The two frequencies of a tone are selected so that they are

not too closely related harmonically. In addition, touch-tone receivers

must be designed so that false digits cannot be generated when

people are speaking into the telephone.

For a call to be completed, the first digit dialed must be detected

in the presence of a dial tone, and the receiver must not interpret

background noise or speech as valid digits. In order to avoid such

misinterpretation, the touch-tone receiver uses both the relative and

the absolute strength of the two simultaneous tones of the first digit

dialed to determine what that digit is.

A system similar to the touch-tone system is used to send telephone

numbers between telephone switching offices. This system,

which is called “multifrequency signaling,” also uses two tones to

indicate a single digit, but the frequencies used are not the same frequencies

that are used in the touch-tone system. Multifrequency

signaling is currently being phased out; new computer-based systems

are being introduced to replace it.



Impact



Touch-tone dialing has made new caller features available. The

touch-tone system can be used not only to signal the desired number

to the switching office but also to interact with voice-response

systems. This means that touch-tone dialing can be used in conjunction

with such devices as bank teller machines. Acustomer can also

dial many more digits per second with a touch-tone telephone than

with a rotary dial telephone.

Touch-tone dialing has not been implemented in Europe, and

one reason may be that the economics of touch-tone dialing change

as a function of technology. In the most modern electronic switching

offices, rotary signaling can be performed at no additional cost,

whereas the addition of touch-tone dialing requires a centralized

touch-tone receiver at the switching office. Touch-tone signaling

was developed in an era of analog telephone switching offices, and

since that time, switching offices have become overwhelmingly digital.

When the switching network becomes entirely digital, as will

be the case when the integrated services digital network (ISDN) is

implemented, touch-tone dialing will become unnecessary. In the

future, ISDN telephone lines will use digital signaling methods exclusively.



See also: Cell phone; Rotary dial telephone; Telephone switching.

Tidal power plant

The invention:



Plant that converts the natural ocean tidal forces

into electrical power.



The people behind the invention:



Mariano di Jacopo detto Taccola (Mariano of Siena, 1381-1453),

an Italian notary, artist, and engineer

Bernard Forest de Bélidor (1697 or 1698-1761), a French engineer

Franklin D. Roosevelt (1882-1945), president of the United States







Tidal Energy



Ocean tides have long been harnessed to perform useful work.

Ancient Greeks, Romans, and medieval Europeans all left records

and ruins of tidal mills, and Mariano di Jacopo included tidal power

in his treatise De Ingeneis (1433; on engines). Some mills consisted of

water wheels suspended in tidal currents, others lifted weights that

powered machinery as they fell, and still others trapped the high

tide to run a mill.

Bernard Forest de Bélidor’s Architecture hydraulique (1737; hydraulic

architecture) is often cited as initiating the modern era of

tidal power exploitation. Bélidor was an instructor in the French

École d’Artillerie et du Génie (School of Artillery and Engineering).

Industrial expansion between 1700 and 1800 led to the construction

of many tidal mills. In these mills, waterwheels or simple turbines

rotated shafts that drove machinery by means of gears or

belts. They powered small enterprises located on the seashore.

Steam engines, however, soon began to replace tidal mills. Steam

could be generated wherever it was needed, and steam mills were

not dependent upon the tides or limited in their production capacity

by the amount of tidal flow. Thus, tidal mills gradually were abandoned,

although a few still operate in New England, Great Britain,

France, and elsewhere.



Electric Power from Tides



Modern society requires tremendous amounts of electric energy

generated by large power stations. This need was first met by

using coal and by damming rivers. Later, oil and nuclear power became

important. Although small mechanical tidal mills are inadequate

for modern needs, tidal power itself remains an attractive

source of energy. Periodic alarms about coal or oil supplies and

concern about the negative effects on the environment of using

coal, oil, or nuclear energy continue to stimulate efforts to develop

renewable energy sources with fewer negative effects. Every crisis—

for example, the perceived European coal shortages in the

early 1900’s, oil shortages in the 1920’s and 1970’s, and growing

anxiety about nuclear power—revives interest in tidal power.

In 1912, a tidal power plant was proposed at Busum, Germany.

The English, in 1918 and more recently, promoted elaborate schemes

for the Severn Estuary. In 1928, the French planned a plant at Aber-

Wrach in Brittany. In 1935, under the leadership of Franklin Delano

Roosevelt, the United States began construction of a tidal power

plant at Passamaquoddy, Maine. These plants, however, were never

built. All of them had to be located at sites where tides were extremely

high, and such sites are often far from power users. So

much electricity was lost in transmission that profitable quantities

of power could not be sent where they were needed. Also, large

tidal power stations were too expensive to compete with existing

steam plants and river dams. In addition, turbines and generators

capable of using the large volumes of slow-moving tidal water that

reversed flow had not been invented. Finally, large tidal plants inevitably

hampered navigation, fisheries, recreation, and other uses

of the sea and shore.

French engineers, especially Robert Gibrat, the father of the La

Rance project, have made the most progress in solving the problems

of tidal power plants. France, a highly industrialized country, is

short of coal and petroleum, which has brought about an intense

search by the French for alternative energy supplies.

La Rance, which was completed in December, 1967, is the first

full-scale tidal electric power plant in the world. The Chinese, however,

have built more than a hundred small tidal electric stations about the size of the old mechanical tidal mills, and the Canadians

and the Russians have both operated plants of pilot-plant size.

La Rance, which was selected from more than twenty competing

localities in France, is one of a few places in the world where the

tides are extremely high. It also has a large reservoir that is located

above a narrow constriction in the estuary. Finally, interference with

navigation, fisheries, and recreational activities is minimal at La

Rance.

Submersible “bulbs” containing generators and mounting propeller

turbines were specially designed for the La Rance project.

These turbines operate using both incoming and outgoing tides,

and they can pump water either into or out of the reservoir. These

features allow daily and seasonal changes in power generation to be

“smoothed out.” These turbines also deliver electricity most economically.

Many engineering problems had to be solved, however,

before the dam could be built in the tidal estuary.

The La Rance plant produces 240 megawatts of electricity. Its

twenty-four highly reliable turbine generator sets operate about 95

percent of the time. Output is coordinated with twenty-four other

hydroelectric plants by means of a computer program. In this system,

pump-storage stations use excess La Rance power during periods

of low demand to pump water into elevated reservoirs. Later,

during peak demand, this water is fed through a power plant, thus

“saving” the excess generated at La Rance when it was not immediately

needed. In this way, tidal energy, which must be used or lost as

the tides continue to flow, can be saved.



Consequences



The operation of La Rance proved the practicality of tide-generated

electricity. The equipment, engineering practices, and operating

procedures invented for La Rance have been widely applied. Submersible,

low-head, high-flow reversible generators of the La Rance

type are now used in Austria, Switzerland, Sweden, Russia, Canada,

the United States, and elsewhere.

Economic problems have prevented the building of more large

tidal power plants. With technological advances, the inexorable

depletion of oil and coal resources, and the increasing cost of nu-

clear power, tidal power may be used more widely in the future.

Construction costs may be significantly lowered by using preconstructed

power units and dam segments that are floated into place

and submerged, thus making unnecessary expensive dams and reducing

pumping costs.







See also : Compressed-air-accumulating power plant; Geothermal power; Nuclear power plant; Nuclear reactor; Solar thermal engine; Thermal cracking process.