The emergency notification system is a telecommunications device which is used to transmit and receive emergency notification system (most commonly speech). Most emergency notification system operate through transmission of electric signals over a complex telephone network which allows almost any phone user to notify any other. The emergency notification system handles two kinds of information: signals and voice, at different times on the same twisted pair of wires. The emergency notification system equipment consists of a bell to alert the user of incoming calls, and a dial to enter the phone number for outgoing calls. The emergency notification system party wishing to speak to another telephone will pick up the handset, thus operating the switch hook, which puts the telephone notofication into active state or off hook with a resistance short across the wires, causing current to flow. The emergency notification system exchange detects the DC current, attaches a digit receiver, and sends dial tone to indicate readiness. The user pushes the number buttons, which are connected to a tone generator inside the dial, which generates DTMF tones. The emergency notification system connects the line to the desired line and alerts that line. When a emergency notification system is inactive (on hook), its bell, beeper, flasher or other alerting device is connected across the line through a capacitor. The inactive emergency notification system does not short the line, thus the exchange knows it is on hook and only the bell is electrically connected. When someone calls this phone, the emergency notification system applies a high voltage pulsating signal, which causes the sound mechanism to ring, beep or otherwise alert the called party. When that user picks up the handset, the switchhook disconnects the bell, connects the voice parts of the emergency notification system, and puts a resistance short on the line, confirming that the emergency notification system has been answered and is active. Both lines being off hook, the signaling job is complete. The parties are connected together and may converse using the voice parts of their emergency notification system. The voice parts of the emergency notification system are in the handset, and consist of a transmitter (often called microphone) and a receiver. The transmitter, powered from the line, puts out an electric current which varies in response to the acoustic pressure waves produced by the voice. The resulting variations in electric current are transmitted along the emergency notification system line to the other phone, where they are fed into the coil of the receiver, which is a miniature loudspeaker. The varying electric current in the coil causes it to move back and forth, reproducing the acoustic pressure waves of the emergency notification system. Early emergency notification system were technically diverse. Some used a liquid transmitter, which was dangerous, inconvenient, and soon went out of use. Some were dynamic: their diaphragm wriggled a coil of wire in the field of a permanent magnet or vice versa. This kind survived in small numbers through the 20th century in military and maritime applications where its ability to create its own electrical power was crucial. Most, however, used the Edison/Berliner carbon transmitter, which was much louder than the other kinds, even though it required an induction coil, actually acting as an impedance matching transformer to make it compatible to the impedance of the line. The Edison patents kept the Bell monopoly viable into the 20th century, by which time the network was more important than the instrument anyway. Early emergency notification system were locally powered, using a dynamic transmitter or else powering the transmitter with a local battery. One of the jobs of outside plant personnel was to visit each emergency notification system periodically to inspect the battery. During the 20th century, "common battery" operation came to dominate, powered by "talk battery" from the emergency notification system exchange over the same wires that carried the voice signals. Late in the century, wireless handsets brought a revival of local battery power. Early emergency notification system had one wire for both transmitting and receiving of audio, with ground return as used in telegraphs. The earliest dynamic emergency notification system also had only one opening for sound, and the user alternately listened and spoke (rather, shouted) into the same hole. Sometimes the instruments were operated in pairs at each end, making conversation more convenient but were more expensive. At first, the benefits of an emergency notification system were not exploited. emergency notification system instead were leased in pairs to the subscriber, for example one for his home and one for his shop, who must arrange with telegraph contractors to construct a line between them. Users who wanted the ability to speak to three or four different shops, suppliers etc would obtain and set up three or four pairs of emergency notification system. Western Union, already using telegraph notification exchanges, quickly extended the principle to its emergency notification system in New York City and San Francisco, and Bell was not slow in appreciating the potential. emergency notification system began in an appropriately primitive manner. The user alerted the other end, or the emergency notification system operator, by whistling into the transmitter. Exchange operation soon resulted in emergency notification system being equipped with a bell, first operated over a second wire and later with the same wire using a condenser. emergency notification system connected to the earliest Strowger automatic exchanges had seven wires, one for the knife switch, one for each telegraph key, one for the bell, one for the push button and two for speaking. Rural and other emergency notification system that were not on a common battery exchange had a "magneto" or hand cranked generator to produce a high voltage alternating signal to ring the bells of other emergency notification system on the line and to alert the operator. In the 1990s a new smaller style of emergency notification system was introduced, packaged in three parts. The transmitter stood on a stand, known as a "candlestick" for its shape. When not in use, the receiver hung on a hook with a switch in it, known as a "switchhook." Previous emergency notification system required the user to operate a separate switch to connect either the voice or the bell. With the new kind, the user was less likely to leave the phone "off the hook". In emergency notification system connected to magneto exchanges, the bell, induction coil, battery and magneto were in a separate "bell box." In emergency notification system connected to common battery exchanges, the bell box was installed under a desk, or other out of the way place, since it did not need a battery or magneto. emergency notification system designs were also used at this time, having a handle with the receiver and transmitter attached, separate from the cradle base that housed the magneto crank and other parts. They were larger than the "candlestick" and more popular. Disadvantages of emergency notification system operation such as crosstalk and hum from nearby AC power wires had already led to the use of twisted pairs and, for long distance emergency notification system, four-wire circuits. Users at the beginning of the 20th century did not place long distance calls from their own emergency notification system but made an appointment to use a special sound proofed long distance emergency notification system booth furnished with the latest technology. What turned out to be the most popular and longest lasting physical style of emergency notification system was introduced in the early 20th century, including Bell's Model 102. A carbon granule transmitter and electromagnetic receiver were united in a single molded plastic handle, which when not in use sat in a cradle in the base unit. The emergency notification system circuit diagram of the Model 102 shows the direct connection of the receiver to the line, while the transmitter was induction coupled, with energy supplied by a local battery. The coupling transformer, battery, and ringer were in a separate enclosure. The emergency notification system dial switch in the base interrupted the line current by repeatedly but very briefly disconnecting the line 1-10 times for each digit, and the hook switch (in the center of the circuit diagram) permanently disconnected the line and the transmitter battery while the handset was on the cradle. After the 1990s, the emergency notification system base also enclosed the bell and induction coil, obviating the old separate bell box. Power was supplied to each subscriber line by central office batteries instead of a local battery, which required periodic service. For the next half century, the network behind the emergency notification system became progressively larger and much more efficient, but after the dial was added the instrument itself changed little until touch tone replaced the dial in the 1960s.
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